Inducible expression systems

Abstract

Provided is an rtTA and single chain rtTA variants and uses thereof for inducible expression of a nucleic acid of interest. Nucleic acid molecules comprising an improved rtTA and/or sc rtTA sequence according to the invention are also provided, as well as vectors, replicons and cells comprising such nucleic acid molecules.

The invention claimed is: 
     
       1. A method for inducibly expressing a nucleic acid sequence of interest, the method comprising:
 providing a nucleic acid construct comprising said nucleic acid sequence of interest operably linked to an inducible gene expression system that comprises a reverse tetracycline-controlled transactivator (rtTA) encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprising a mutation in a codon at rtTA amino acid position 9, and/or 19, and/or 37, and/or 56, and/or 67, and/or 68, and/or 138, and/or 157, and/or 171, and/or 177, and/or 195; 
 introducing said nucleic acid construct to a suitable expression system; and 
 allowing for inducible expression of said nucleic acid sequence of interest. 
 
     
     
       2. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence further comprise a mutation in a codon at rtTA amino acid position 12, and/or 86, and/or 209. 
     
     
       3. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon, and/or a codon at rtTA position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon, and/or a glutamine or lysine codon at rtTA amino acid position 56. 
     
     
       4. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprise a glycine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon. 
     
     
       5. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprise an alanine, cysteine, phenylalanine, histidine, isoleucine, leucine, methionine, asparagine, arginine, serine, threonine, valine, tryptophan or tyrosine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon. 
     
     
       6. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprise a histidine, a leucine or an arginine codon at rtTA amino acid position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon. 
     
     
       7. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 9 encoding isoleucine, and/or a codon at rtTA amino acid position 19 encoding alanine, cysteine, aspartate, phenylalanine, histidine, isoleucine, lysine, leucine, methionine, asparagine, glutamine, arginine, serine, threonine, valine, tryptophan or tyrosine, and/or a codon at rtTA amino acid position 37 encoding cysteine, methionine, glutamine, threonine, histidine, leucine or arginine, and/or a codon at rtTA amino acid position 56 encoding lysine or glutamine, and/or a codon at rtTA amino acid position 67 encoding serine, and/or a codon at rtTA amino acid position 68 encoding arginine, and/or a codon at rtTA amino acid position 86 encoding tyrosine, and/or a codon at rtTA amino acid position 138 encoding aspartate or serine, and/or a codon at rtTA amino acid position 157 encoding lysine, and/or a codon at rtTA amino acid position 171 encoding lysine, and/or a codon at rtTA amino acid position 177 encoding leucine, and/or a codon at rtTA amino acid position 195 encoding serine, and/or a codon at rtTA amino acid position 209 encoding threonine. 
     
     
       8. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprise at least one mutation as depicted in  FIG.  14 B  or  FIG.  14 C . 
     
     
       9. The method according to  claim 1 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprise at least one codon mutation as compared to a rtTA encoding nucleic acid sequence depicted in  FIG.  19   . 
     
     
       10. The method according to  claim 1 , wherein said nucleic acid of interest is expressed in a higher eukaryotic expression system. 
     
     
       11. The method according to  claim 10 , wherein said nucleic acid of interest is expressed in a mammalian cell. 
     
     
       12. The method according to  claim 1 , wherein said nucleic acid of interest comprises a viral sequence essential for replication. 
     
     
       13. The method according to  claim 1 , wherein said nucleic acid of interest comprises at least part of an HIV genome essential for replication. 
     
     
       14. A synthetic or recombinant nucleic acid sequence comprising a rtTA encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, which rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a mutated codon at rtTA amino acid position 9, and/or 19, and/or 37, and/or 56, and/or 67, and/or 68, and/or 138, and/or 157, and/or 171, and/or 177, and/or 195. 
     
     
       15. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence further comprises a mutation in a codon at rtTA amino acid position 12, and/or 86, and/or 209. 
     
     
       16. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon and/or a codon at rtTA position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon, and/or a glutamine or lysine codon at rtTA amino acid position 56. 
     
     
       17. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a glycine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon. 
     
     
       18. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises an alanine, cysteine, phenylalanine, histidine, isoleucine, leucine, methionine, asparagine, arginine, serine, threonine, valine, tryptophan or tyrosine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon. 
     
     
       19. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a histidine, a leucine or an arginine codon at rtTA amino acid position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon. 
     
     
       20. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 9 encoding isoleucine, and/or a codon at rtTA amino acid position 19 encoding alanine, cysteine, aspartate, phenylalanine, histidine, isoleucine, lysine, leucine, methionine, asparagine, glutamine, arginine, serine, threonine, valine, tryptophan or tyrosine, and/or a codon at rtTA amino acid position 37 encoding cysteine, methionine, glutamine, threonine, histidine, leucine or arginine, and/or a codon at rtTA amino acid position 56 encoding lysine or glutamine, and/or a codon at rtTA amino acid position 67 encoding serine, and/or a codon at rtTA amino acid position 68 encoding arginine, and/or a codon at rtTA amino acid position 86 encoding tyrosine, and/or a codon at rtTA amino acid position 138 encoding aspartate or serine, and/or a codon at rtTA amino acid position 157 encoding lysine, and/or a codon at rtTA amino acid position 171 encoding lysine, and/or a codon at rtTA amino acid position 177 encoding leucine, and/or a codon at rtTA amino acid position 195 encoding serine, and/or a codon at rtTA amino acid position 209 encoding threonine. 
     
     
       21. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises at least one mutation as depicted in  FIG.  14 B  or  FIG.  14 C . 
     
     
       22. The synthetic or recombinant nucleic acid sequence according to  claim 14 , wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises at least one mutation as compared to an rtTA encoding nucleic acid sequence depicted in  FIG.  19   . 
     
     
       23. A synthetic or recombinant amino acid sequence encoded by the nucleic acid sequence according to  claim 14 . 
     
     
       24. A synthetic or recombinant amino acid sequence comprising a rtTA sequence and/or a single chain rtTA sequence, which rtTA sequence and/or single chain rtTA sequence comprises an isoleucine at position 9, and/or an alanine, cysteine, aspartate, phenylalanine, histidine, isoleucine, lysine, leucine, methionine, asparagine, glutamine, arginine, serine, threonine, valine, tryptophan or tyrosine at position 19, and/or a cysteine, methionine, glutamine, threonine, histidine, leucine or arginine at position 37, and/or a lysine or glutamine at position 56, and/or a serine at position 67, and/or an arginine at position 68, and/or a tyrosine at position 86, and/or an aspartate or serine at position  138 , and/or a lysine at position 157, and/or a lysine at position 171, and/or a leucine at position 177, and/or a serine at position 195, and/or a threonine at position 209. 
     
     
       25. In a method of inducing expression of a nucleic acid sequence of interest, the improvement comprising:
 utilizing the synthetic or recombinant nucleic acid sequence of  claim 14  for inducible expression of a nucleic acid sequence of interest. 
 
     
     
       26. In a method of inducing expression of a nucleic acid sequence of interest, the improvement comprising:
 utilizing the amino acid sequence encoded by any one of the nucleic acid sequences of  claim 24  for inducible expression of a nucleic acid sequence of interest. 
 
     
     
       27. In a method of tetracycline-inducible and/or minocycline-inducible expression of a nucleic acid sequence of interest, the improvement comprising:
 utilizing the recombinant nucleic acid sequence comprising an rtTA encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, which rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a mutation or a combination of mutations as depicted in  FIG.  15   , except for the wild type rtTA and the F86Y A209T variant, for tetracycline-inducible and/or minocycline-inducible expression of a nucleic acid of interest. 
 
     
     
       28. A vector comprising the nucleic acid sequence of  claim 14 . 
     
     
       29. An inducible viral replicon, comprising:
 the nucleic acid sequence of  claim 14 , and 
 at least one viral sequence that is essential for replication under direct or indirect control of said nucleic acid sequence. 
 
     
     
       30. The inducible viral replicon according to  claim 29 , comprising all viral sequences essential for replication under direct or indirect control of said nucleic acid sequence. 
     
     
       31. The inducible viral replicon according to  claim 29 , which is derived from a human immunodeficiency virus. 
     
     
       32. The inducible viral replicon of  claim 29 , wherein the nucleic acid sequence is inserted into the nef gene. 
     
     
       33. The inducible viral replicon of  claim 29 , further comprising at least one tetO motif in at least one functional LTR. 
     
     
       34. The inducible viral replicon of  claim 33 , further comprising at least 2, 4, 6, or 8 such elements in at least one functional LTR. 
     
     
       35. The inducible viral replicon of  claim 29 , wherein at least one LTR is modified to avoid reversion to wild type virus. 
     
     
       36. A method for producing a virus dependent upon an inducing agent for replication, the method comprising:
 providing a permissive cell with the inducible viral replicon of  claim 29 , 
 culturing said cell in the presence of said inducing agent, and 
 harvesting said dependent virus from said culture. 
 
     
     
       37. The method according to  claim 36 , in which said dependent virus is a human immunodeficiency virus. 
     
     
       38. The method according to  claim 36 , in which said virus is an attenuated virus. 
     
     
       39. A virus dependent on an inducing agent for replication obtainable by the method according to  claim 36 . 
     
     
       40. The virus according to  claim 39 , which is a human immunodeficiency virus. 
     
     
       41. A method for the controlled replication of a virus or a viral replicon, the method comprising:
 providing a permissive cell with the inducible viral replicon of  claim 29 ; 
 culturing said cell in the presence of said inducing agent; and 
 manipulating the amount of inducing agent present. 
 
     
     
       42. An isolated cell comprising the nucleic acid sequence of  claim 14 . 
     
     
       43. A method for inducibly expressing a nucleic acid sequence of interest, the method comprising the steps of:
 providing a nucleic acid construct comprising said nucleic acid sequence of interest operably linked to an inducible gene expression system that comprises a reverse tetracycline-controlled transactivator (rtTA) encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprising a mutation in a codon at rtTA amino acid position 67, optionally with one or more additional mutations in a codon at rtTA amino acid position 9, 12, 19, 37, 56, 68, 86, 138, 157, 171, 177, 195 or 209;   introducing said nucleic acid construct to a suitable expression system; and   allowing for inducible expression of said nucleic acid sequence of interest.    
     
     
       44. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon, and/or a codon at rtTA position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon, and/or a glutamine or lysine codon at rtTA amino acid position 56.  
     
     
       45. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a glycine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon.  
     
     
       46. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises an alanine, cysteine, phenylalanine, histidine, isoleucine, leucine, methionine, asparagine, arginine, serine, threonine, valine, tryptophan or tyrosine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon.  
     
     
       47. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a histidine, a leucine or an arginine codon at rtTA amino acid position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon.  
     
     
       48. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 9 encoding isoleucine, and/or a codon at rtTA amino acid position 19 encoding alanine, cysteine, aspartate, phenylalanine, histidine, isoleucine, lysine, leucine, methionine, asparagine, glutamine, arginine, serine, threonine, valine, tryptophan or tyrosine, and/or a codon at rtTA amino acid position 37 encoding cysteine, methionine, glutamine, threonine, histidine, leucine or arginine, and/or a codon at rtTA amino acid position 56 encoding lysine or glutamine, and/or a codon at rtTA amino acid position 67 encoding serine, and/or a codon at rtTA amino acid position 68 encoding arginine, and/or a codon at rtTA amino acid position 86 encoding tyrosine, and/or a codon at rtTA amino acid position 138 encoding aspartate or serine, and/or a codon at rtTA amino acid position 157 encoding lysine, and/or a codon at rtTA amino acid position 171 encoding lysine, and/or a codon at rtTA amino acid position 177 encoding leucine, and/or a codon at rtTA amino acid position 195 encoding serine, and/or a codon at rtTA amino acid position 209 encoding threonine.  
     
     
       49. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises at least one mutation as depicted in FIG. 14B or FIG. 14C.  
     
     
       50. The method according to claim 43, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises at least one codon mutation as compared to a rtTA encoding nucleic acid sequence depicted in FIG. 19.  
     
     
       51. The method according to claim 43, wherein said nucleic acid of interest is expressed in a higher eukaryotic expression system.  
     
     
       52. The method according to claim 51, wherein said nucleic acid of interest is expressed in a mammalian cell.  
     
     
       53. The method according to claim 43, wherein said nucleic acid of interest comprises a viral sequence essential for replication.  
     
     
       54. The method according to claim 43, wherein said nucleic acid of interest comprises at least part of an HIV genome essential for replication.  
     
     
       55. A synthetic or recombinant nucleic acid sequence comprising a rtTA encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, which rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a mutated codon at rtTA amino acid position 67, optionally with one or more additional mutations in a codon at rtTA amino acid position 9, 12, 19, 37, 56, 68, 86, 138, 157, 171, 177, 195 or 209.  
     
     
       56. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon and/or a codon at rtTA position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon, and/or a glutamine or lysine codon at rtTA amino acid position 56.  
     
     
       57. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a glycine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon.  
     
     
       58. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises an alanine, cysteine, phenylalanine, histidine, isoleucine, leucine, methionine, asparagine, arginine, serine, threonine, valine, tryptophan or tyrosine codon at rtTA amino acid position 19 that differs in at least two nucleotides from a glutamate codon.  
     
     
       59. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a histidine, a leucine or an arginine codon at rtTA amino acid position 37 that differs in at least two nucleotides from an alanine, a lysine or a serine codon.  
     
     
       60. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a codon at rtTA amino acid position 9 encoding isoleucine, and/or a codon at rtTA amino acid position 19 encoding alanine, cysteine, aspartate, phenylalanine, histidine, isoleucine, lysine, leucine, methionine, asparagine, glutamine, arginine, serine, threonine, valine, tryptophan or tyrosine, and/or a codon at rtTA amino acid position 37 encoding cysteine, methionine, glutamine, threonine, histidine, leucine or arginine, and/or a codon at rtTA amino acid position 56 encoding lysine or glutamine, and/or a codon at rtTA amino acid position 67 encoding serine, and/or a codon at rtTA amino acid position 68 encoding arginine, and/or a codon at rtTA amino acid position 86 encoding tyrosine, and/or a codon at rtTA amino acid position 138 encoding aspartate or serine, and/or a codon at rtTA amino acid position 157 encoding lysine, and/or a codon at rtTA amino acid position 171 encoding lysine, and/or a codon at rtTA amino acid position 177 encoding leucine, and/or a codon at rtTA amino acid position 195 encoding serine, and/or a codon at rtTA amino acid position 209 encoding threonine.  
     
     
       61. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises at least one mutation as depicted in FIG. 14B or FIG. 14C.  
     
     
       62. The synthetic or recombinant nucleic acid sequence according to claim 55, wherein said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises at least one mutation as compared to an rtTA encoding nucleic acid sequence depicted in FIG. 19.  
     
     
       63. A synthetic or recombinant amino acid sequence encoded by the nucleic acid sequence according to claim 55.   
     
     
       64. In a method of inducing expression of a nucleic acid sequence of interest, the improvement comprising: utilizing the synthetic or recombinant nucleic acid sequence of claim 55 for inducible expression of a nucleic acid sequence of interest.  
     
     
       65. A vector comprising the nucleic acid sequence of claim 55.   
     
     
       66. An inducible viral replicon, comprising: the nucleic acid sequence of claim 55, and at least one viral sequence that is essential for replication under direct or indirect control of said nucleic acid sequence.  
     
     
       67. The inducible viral replicon according to claim 66, comprising all viral sequences essential for replication under direct or indirect control of said nucleic acid sequence.  
     
     
       68. The inducible viral replicon according to claim 66, which is derived from a human immunodeficiency virus.  
     
     
       69. The inducible viral replicon of claim 66, wherein the nucleic acid sequence is inserted into the nef gene.  
     
     
       70. The inducible viral replicon of claim 66, further comprising at least one tetO motif in at least one functional LTR.  
     
     
       71. The inducible viral replicon of claim 70, further comprising at least 2, 4, 6, or 8 such elements in at least one functional LTR.  
     
     
       72. The inducible viral replicon of claim 66, wherein at least one LTR is modified to avoid reversion to wild type virus.  
     
     
       73. A method for producing a virus dependent upon an inducing agent for replication, the method comprising: providing a permissive cell with the inducible viral replicon of claim 66, culturing said cell in the presence of said inducing agent, and harvesting said dependent virus from said culture.  
     
     
       74. The method according to claim 73, in which said dependent virus is a human immunodeficiency virus.  
     
     
       75. The method according to claim 73, in which said virus is an attenuated virus.  
     
     
       76. A virus dependent on an inducing agent for replication obtainable by the method according to claim 73.   
     
     
       77. The virus according to claim 76, which is a human immunodeficiency virus.  
     
     
       78. A method for the controlled replication of a virus or a viral replicon, the method comprising: providing a permissive cell with the inducible viral replicon of claim 66; culturing said cell in the presence of said inducing agent and manipulating the amount of inducing agent present.  
     
     
       79. An isolated cell comprising the nucleic acid sequence of claim 55.   
     
     
       80. A transactivator, having the DNA sequence (5′- 3′ orientation): 
       
         
           
                 
               
                   (SEQ ID NO: 28) 
                 
                   atgtctagactggacaagagcaaagtcataaactctgctctggaattact 
                 
                     
                 
                   caatggagtcggtatcgaaggcctgacgacaaggaaactcgctcaaaagc 
                 
                     
                 
                   tgggagttgagcagcctaccctgtactggcacgtgaagaacaagcgggcc 
                 
                     
                 
                   ctgctcgatgccctgccaatcgagatgctggacaggcatcatacccactc 
                 
                     
                 
                   ctgccccctggaaggcgagtcatggcaagactttctgcggaacaacgcca 
                 
                     
                 
                   agtcataccgctgtgctctcctctcacatcgcgacggggctaaagtgcat 
                 
                     
                 
                   ctcggcacccgcccaacagagaaacagtacgaaaccctggaaaatcagct 
                 
                     
                 
                   cgcgttcctgtgtcagcaaggcttctccctggagaacgcactgtacgctc 
                 
                     
                 
                   tgtccgccgtgggccactttacactgggctgcgtattggaggaacaggag 
                 
                     
                 
                   catcaagtagcaaaagaggaaagagagacacctaccaccgattctatgcc 
                 
                     
                 
                   cccacttctgaaacaagcaattgagctgttcgaccggcagggagccgaac 
                 
                     
                 
                   ctgccttccttttcggcctggaactaatcatatgtggcctggagaaacag 
                 
                     
                 
                   ctaaagtgcgaaagcggcgggccgaccgacgeccttgacgattttgactt 
                 
                     
                 
                   agacatgctcccagccgatgcccttgacgactttgaccttgatatgctgc 
                 
                     
                 
                   ctgctgacgctcttgacgattttgaccttgacatgctccccgggtaa. 
                 
             
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
       
        
       
     
     
       81. A transactivator, having the amino acid sequence:
 MSRLDKSKVINSALELLNGVGIEGLTTRKLAQKLGVEQPTLYWHVKNKRALLDALPIEML DRHHTHSCPLEGESWQDFLRNNAKSYRCALLSHRDGAKVHLGTRPTEKQYETLENQLAFL CQQGFSLENALYALSAVGHFTLGCVLEEQEHQVAKEERETPTTDSMPPLLKQAIELFDRQ GAEPAFLFGLELIICGLEKQLKCESGGPTDALDDFDLDMLPADALDDFDLDMLPADALDDFDLDMLPG*(SEQ ID NO: 29).    
     
     
       82. A method for inducibly expressing a nucleic acid sequence of interest, the method comprising the steps of:
 providing a nucleic acid construct comprising said nucleic acid sequence of interest operably linked to an inducible gene expression system that comprises a reverse tetracycline-controlled transactivator (rtTA) encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, said rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprising a mutation in a codon at rtTA amino acid position 67;   introducing said nucleic acid construct to a suitable expression system; and   allowing for inducible expression of said nucleic acid sequence of interest.    
     
     
       83. A synthetic or recombinant nucleic acid sequence comprising a rtTA encoding nucleic acid sequence and/or a single chain rtTA encoding nucleic acid sequence, which rtTA encoding nucleic acid sequence and/or single chain rtTA encoding nucleic acid sequence comprises a mutated codon at rtTA amino acid position 67.