A NEW GENE, CBL, ENCODING A MEMBER OF THE LYSR FAMILY OF TRANSCRIPTIONAL REGULATORS BELONGS TO ESCHERICHIA-COLI CYS REGULON

The cbl (cysB-like) gene has been identified in Escherichia coli, The analysis of the cloned cbl sequence revealed strict homology to an ORF of unknown function found initially in Klebsiella aerogenes [Schwacha and Bender, J. Bacteriol. 175 (1993) 2107-2115]. The predicted Cbl pr otein has structural features of the LysR family of transcriptional ac tivators. It is also strongly similar to the CysB protein, the activat or of the cys regulon. The position of cbl on the Ec physical map has been established at a 2070-kb (43.5 min) region between asnU and asnV. The gene is expressed in vivo as a 1-kb monocistronic transcript star ting from one major transcription start point. Unexpectedly, the in vi vo expression of cbl has shown dependence on CysB, belonging to the sa me family of proteins. The promoter region of cbl binds purified CysB protein in a manner similar to other CysB-responsive promoters. A cbl disruption mutant was constructed by insertion of a Km(R) gene cartrid ge into the ORF on the chromosome. Phenotypes related to cbl expressio n suggest the involvement of the gene in an accessory regulatory circu it within the cys regulon engaging, in the last step, the function of the cysM gene encoding O-acetylserine (thiol)-lyase B.