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.