Transcriptional and translational control of the Salmonella fliC gene

The flagellin gene fliC encodes the major component of the flagellum in Salmonella enterica serovar Typhimurium. This study reports the identification of a signal within the 5' untranslated region (5'UTR) of the fliC transcript required for the efficient expression and assembly of FliC into the growing flagellar structure. Primer extension mapping determined the transcription start site of the fliC flagellin gene to be 62 bases upstream of the AUG start codon. Using telA-fliC operon fusions, we show that the entire 62-base 5'UTR region of fliC was required for sufficient fliC mRNA translation to allow normal FliC flagellin assembly, suggesting that translation might be coupled to assembly. To identify sequence that might couple fliC mRNA translation to FliC secretion, the 5' end of the chromosomal fliC gene was mutagenized by PCR-directed mutagenesis. Single base sequences important for fliC-dependent transcription, translation, and motility were identified by using fliC-lacZ transcriptional and translational reporter constructs. Transcription-specific mutants identified the -10 and -35 regions of the consensus flagellar class 3 gene promoter. Single base changes defective in translation were located in three regions: the AUG start codon, the presumed ribosomal binding site region, and a region near the very 5' end of the fliC mRNA that corresponded to a potential stem-loop structure in the 5'UTR. Motility-specific mutants resulted from base substitutions only in the fliC-coding region. The results suggest that fliC mRNA translation is not coupled to FliC secretion by the flagellar type III secretion system.