Search

There are three classes of promoters for flagellar operons in Salmonella . Class 2 promoters are transcribed by σ 70 RNA polymerase in the presence of an essential activator, FlhD 4 C 2 , and activated by an auxiliary regulator, FliZ. Class 3 promoters are transcribed by σ 28 RNA polymerase and repressed by an anti-σ 28 factor, FlgM. σ 28 (FliA) and FliZ are encoded by the fliA and fliZ genes, respectively, which together constitute an operon transcribed in this order. This operon is transcribed from both class 2 and class 3 promoters, suggesting that it should be activated by its own product, σ 28 , even in the absence of FlhD 4 C 2 . However, σ 28 -dependent transcription occurs in vivo only in the presence of FlhD 4 C 2 , indicating that transcription from the class 2 promoter is a prerequisite to that from the class 3 promoter. In this study, we examined the effects of variously modified versions of the fliA regulatory region on transcription and translation of the fliA gene. We showed that FliA is not significantly translated from the class 3 transcript. In contrast, the 5′-terminal AU-rich sequence found in the class 2 transcript confers efficient fliA translation. Replacement of the Shine-Dalgarno sequence of the fliA gene with a better one improved fliA translation from the class 3 transcript. These results suggest that the 5′-terminal AU-rich sequence of the class 2 transcript may assist ribosome binding. FliZ was shown to be expressed from both the class 2 and class 3 transcripts.

Flagellar operons are divided into three classes with respect to their transcriptional hierarchy in Salmonella enterica serovar Typhimurium. The class 1 gene products FlhD and FlhC act together in an FlhD 4 C 2 heterohexamer, which binds upstream of the class 2 promoters to facilitate binding of RNA polymerase. In this study, we showed that flagellar expression was much reduced in the cells grown in poor medium compared to those grown in rich medium. This nutritional control was shown to be executed at a step after class 1 transcription. We isolated five Tn 5 insertion mutants in which the class 2 expression was derepressed in poor medium. These insertions were located in the ydiV ( cdgR ) gene or a gene just upstream of ydiV . The ydiV gene is known to encode an EAL domain protein and to act as a negative regulator of flagellar expression. Gene disruption and complementation analyses revealed that the ydiV gene is responsible for nutritional control. Expression analysis of the ydiV gene showed that its translation, but not transcription, was enhanced by growth in poor medium. The ydiV mutation did not have a significant effect on either the steady-state level of flhDC mRNA or that of FlhC protein. Purified YdiV protein was shown in vitro to bind to FlhD 4 C 2 through interaction with FlhD subunit and to inhibit its binding to the class 2 promoter, resulting in inhibition of FlhD 4 C 2 -dependent transcription. Taking these data together, we conclude that YdiV is a novel anti-FlhD 4 C 2 factor responsible for nutritional control of the flagellar regulon.