Promoter analysis of the class 2 flagellar operons of Salmonella

The Salmonella flagellar operons are divided into three classes with refere nce to their relative positions in the transcriptional hierarchy. Expressio n of the class 2 operons requires the class 1 gene products, F1hD and F1hC, and is enhanced by an unknown mechanism in the presence of the class 3-spe cific sigma factor, FliA, and in the absence of its cognate anti-sigma fact or, FlgM. In this study, the transcriptional start site mapping was perform ed 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 site s were identified. These start signals disappeared in the flhDC-mutant back ground, 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 enhance ment. Sequence comparison revealed that an imperfect inverted repetitious s equence is conserved upstream of the class 2 operons. Truncation of this se quence from the flgB promoter reduced its transcriptional activity to the b ackground level, indicating that this is an essential cis-acting element fo r transcription of the class 2 operons.