The Transcript from the σ 28 -Dependent Promoter Is Translationally Inert in the Expression of the σ 28 -Encoding Gene fliA in the fliAZ Operon of Salmonella enterica Serovar Typhimurium

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.