Flagellar phase variation in Salmonella enterica is mediated by a posttranscriptional control mechanism

Salmonella enterica has two antigenically distinct flagellin genes, fliC and fljB, that are alternatively expressed. The fljA gene is cotranscribed with fljB and encodes a protein that has been characterized as a transcriptional repressor of the unlinked fliC gene when FljB is expressed. In this study we report genetic evidence that FljA prevents the production of FliC protein through an interaction with the 5'-untranslated region of the fliC mRNA transcript. Studies with operon and gene fusions, Western analyses, and T-2 RNase protection assays were performed for strains with the fljBA promoter locked in either the on or the off orientation. beta-Galactosidase assays of fliC transcriptional and translational fusions to the lac operon demonstrated that while FljA inhibits fliC transcription fivefold in the fljBA(ON) orientation, it has a 200-fold effect on both fliC transcription and translation, indicating that the FljA inhibitor might act at both the transcriptional and translational level. T-2 RNase protection assays also demonstrated a fivefold decrease in fliC transcript levels for cells locked in the fljBA(ON) orientation compared to those in the fljBA(OFF) orientation, and an eightfold decrease in FliC protein levels was observed by Western analysis. This reduction in FliC protein levels is greater than the decrease observed for the transcript. These results are consistent with a new model whereby FljA inhibits FliC expression by an attenuation or translational control mechanism.