Although Escherichia coli and Salmonella enterica serovar Typhimurium have a similar flagellar regulatory system, the response of flagellar synthesis to nutrient conditions is quite different between the two: that is, in low-nutrient conditions, flagellar synthesis is inhibited in Salmonella and enhanced in E. coli. In Salmonella, this inhibition is mediated by an anti-FlhD4C2 factor, YdiV, which is expressed in low-nutrient conditions and binds to FlhD4C2 to inhibit the expression of the class 2 flagellar genes. The fliZ gene encodes a repressor of the ydiV gene, and thus is required for efficient flagellar gene expression in low-nutrient conditions in Salmonella. In this study, we showed that the E. coli ydiV gene encodes a protein which inhibits motility and flagellar production when expressed from a multicopy plasmid. We showed further that E. coli YdiV binds to FlhD4C2 and inhibits its binding to the class 2 flagellar promoter. These results indicate that E. coli YdiV can also act as an anti-FlhD4C2 factor. However, although the ydiV gene was transcribed efficiently in E. coli cells, the intracellular level of the YdiV protein was extremely low due to its inefficient translation. Consistent with this, E. coli cells did not require FliZ for efficient motility development. This indicates that, unlike in Salmonella, the FliZ-YdiV regulatory system does not work in the nutritional control of flagellar gene expression in E. coli.
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