Residue threonine-149 of the Salmonella typhimurium CysB transcription activator: mutations causing constitutive expression of positively regulated genes of the cysteine regulon.
In both Salmonella typhimurium and Escherichia coli, CysB is a LysR family transcriptional activator, which regulates genes of the cysteine regulon. Transcription activation of cys genes also requires an inducer, N-acetyl-L-serine, and cysB mutants that do not require inducer are termed constitutive, i.e. cysBc. After finding that two independently isolated cysBc mutants are substituted at amino acid residue threonine-149 (T149), we isolated the other 17 single-amino-acid substitutions by site-directed mutagenesis. Of the 19 mutant alleles, 11 supported normal growth on sulphate, and nine of these were cysBc. Four other mutants were 'leaky' cysB+, and four were cysB-. Insertions of up to 14 amino acids were also tolerated at T149, and two of three such mutants were cysBc. An allele containing a TAG translation terminator at codon 149 had no detectable function in a delta cysB strain, but gave a constitutive phenotype when introduced into either wild-type S. typhimurium or the E. coli strain NK1, which contains a cysB- mutation in a predicted helix-turn-helix region that interferes with specific binding of CysB to DNA and with autoregulation of cysB. The peptide encoded by the T149ter allele is proposed to interact with the wild-type CysB peptide or with the NK1 mutant peptide to form hetero-oligomers that do not require N-acetyl-L-serine for cys gene activation.
Colyer
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Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710.
Kredich
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D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000502:physiology; D003545:Cysteine / Q000096:biosynthesis*; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000502:physiology; D015964:Gene Expression Regulation, Bacterial; D019811:Hydroxylamine; D006898:Hydroxylamines; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D018085:Regulon; D012486:Salmonella typhimurium / Q000235:genetics*; D013912:Threonine; D014158:Transcription, Genetic; D015533:Transcriptional Activation
D001426:Bacterial Proteins; C062886:CysB protein, Bacteria; C489265:CysB protein, Salmonella typhimurium; D004268:DNA-Binding Proteins; D006898:Hydroxylamines; D019811:Hydroxylamine; D013912:Threonine; D003545:Cysteine