Salmonella typhimurium PtsJ is a novel MocR‐like transcriptional repressor involved in regulating the vitamin B 6 salvage pathway

The vitamin B 6 salvage pathway, involving pyridoxine 5′‐phosphate oxidase ( PNPO x) and pyridoxal kinase ( PLK ), recycles B 6 vitamers from nutrients and protein turnover to produce pyridoxal 5′‐phosphate ( PLP ), the catalytically active form of the vitamin. Regulation of this pathway, widespread in living organisms including humans and many bacteria, is very important to vitamin B 6 homeostasis but poorly understood. Although some information is available on the enzymatic regulation of PNPO x and PLK , little is known on their regulation at the transcriptional level. In the present work, we identified a new MocR‐like regulator, PtsJ from Salmonella typhimurium , which controls the expression of the pdxK gene encoding one of the two PLK s expressed in this organism ( PLK 1). Analysis of pdxK expression in a ptsJ knockout strain demonstrated that PtsJ acts as a transcriptional repressor. This is the first case of a MocR‐like regulator acting as repressor of its target gene. Expression and purification of PtsJ allowed a detailed characterisation of its effector and DNA ‐binding properties. PLP is the only B 6 vitamer acting as effector molecule for PtsJ. A DNA ‐binding region composed of four repeated nucleotide sequences is responsible for binding of PtsJ to its target promoter. Analysis of binding stoichiometry revealed that protein subunits/ DNA molar ratio varies from 4 : 1 to 2 : 1, depending on the presence or absence of PLP . Structural characteristics of DNA transcriptional factor‐binding sites suggest that PtsJ binds DNA according to a different model with respect to other characterised members of the MocR subgroup.