End product repression of cobalamin synthesis in Salmonella typhimurium

The purpose of this research is to elucidate the mechanism that regulates the end-product repression of the genes for cobalamin biosynthesis (cob) in Salmonella typhimurium. Twenty-nine insertion mutants which exhibited altered repression phenotypes were isolated. The effect of these insertion mutations on the expression of the cob genes was quantified by using P-galactosidase assays to monitor the transcription of a coblr.lac operon fusion. The mutants were divided into two phenotypically distinct groups which were designated positive and negative. In the 11 negative mutants, the cobl operon was not expressed under any conditions. All of the insertion mutations in this group were mapped in or near the cobl operon itself. In the 18 positive mutants, the cobl operon was less sensitive to repression by cobalamin than in the parent strain. The positive mutants were subdivided into four groups by cotransduction and conjugational mapping. One group of mutations is located in the cobalamin transport genes (btuB and btuCED). A second group lies near the distal end of the ethanolamine utilization (eut) operon at 50 map units and prevents the mutants from utilizing ethanolamine as a sole carbon source. A third group lies between 47 and 62 map units on the S. typhimurium chromosome. The fourth group of positive mutations remains unmapped.

The insertion mutations that are located in the eut region appear to lie downstream from the genes that encode the enzymes and a known regulatory protein for ethanolamine utilization. We propose that these mutations affect the synthesis of a second regulatory protein that binds cobalamin. This protein plays a dual regulatory role by repressing transcription of the cobl operon and activating transcription of the eut operon.