Study on the transcriptional control of flagellar regulon in Salmonella typhimurium.

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1. An overall structure of flagellar regulon of Salmonella has been established. Based on the transcriptional hierarchy. flagellar operons can be divided into three classes (Class I. Class II, and Class III). The transcription of the class III operons is controlled positively by the gene product of fliA which belongs to class II and negatively by the product of a newly identified gene, rflB, which belongs to class III.2. The fliA gene was cloned and sequenced. The deduced animo acid sequence of the FliA protein was revealed to share a great deal of homology with those of the known sigma factors of RNA polymerase. The purified FliA protein had an activity to promote specifically the transcription from the promoter of flagellar operons when added to the in vitro transcription system containing RNA polymerase core enzyme. This indicates that the FliA protein is an alternative sigma factor specific for flagellar regulon.3. The rflB gene was cloned and sequenced. The purified RflB protein had an activity to repress specifically the transcription from the promoter of flagellar operons when added to the in vitro transcription system containing FliA-RNA polymerase.4. A number of derepressed mutants of the class III operons were isolated. Some of them were found to have mutations in the fliA gene, which suggests that there may be a molecular interaction between the FliA and the RflB proteins. This was proved in vitro using the purified FliA and RflB proteins.5. Expression of the class II operons was shown to be enhanced in the rflB mutants. This enhancement disappeared when the fliA defect was introduced into the same strains. Therefore, it has been concluded that the activity of the whole flagellar regulon is controlled positively and negatively by the FliA and the RflB proteins. respectively.