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The Transcript from the σ28-Dependent Promoter Is Translationally Inert in the Expression of the σ28-Encoding Gene fliA in the fliAZ Operon of Salmonella enterica Serovar Typhimurium

Tanabe, Yasushi, Wada, Takeo, Ono, Katsuhiko, Abo, Tatsuhiko, Kutsukake, Kazuhiro
Journal of bacteriology 2011 v.193 no.22 pp. 6132-6141
DNA-directed RNA polymerase, Salmonella enterica subsp. enterica serovar Typhimurium, bacteriology, operon, ribosomes, transcription (genetics), translation (genetics)
There are three classes of promoters for flagellar operons in SALMONELLA: Class 2 promoters are transcribed by σ70 RNA polymerase in the presence of an essential activator, FlhD4C2, and activated by an auxiliary regulator, FliZ. Class 3 promoters are transcribed by σ28 RNA polymerase and repressed by an anti-σ28 factor, FlgM. σ28 (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, σ28, even in the absence of FlhD4C2. However, σ28-dependent transcription occurs in vivo only in the presence of FlhD4C2, 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.