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Genes Genet Syst. 1999 Aug;74(4):179-83.

Promoter analysis of the class 2 flagellar operons of Salmonella.

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Faculty of Applied Biological Science, Hiroshima University, Japan.


The Salmonella flagellar operons are divided into three classes with reference to their relative positions in the transcriptional hierarchy. Expression of the class 2 operons requires the class 1 gene products, FlhD and FlhC, and is enhanced by an unknown mechanism in the presence of the class 3-specific sigma factor, FliA, and in the absence of its cognate anti-sigma factor, FlgM. In this study, the transcriptional start site mapping was performed by primer extension analysis for five class 2 operons, flgA, flgB, flhB, fliE and fliL. In all cases, one or a few major transcriptional start sites were identified. These start signals disappeared in the flhDC-mutant background, and their intensity decreased and increased in the fliA-mutant and flgM-mutant backgrounds, respectively. Therefore, we conclude that the FlhD/FlhC-dependent transcription is responsible for the FliA-dependent enhancement. Sequence comparison revealed that an imperfect inverted repetitious sequence is conserved upstream of the class 2 operons. Truncation of this sequence from the flgB promoter reduced its transcriptional activity to the background level, indicating that this is an essential cis-acting element for transcription of the class 2 operons.

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