SEQUENCE-ANALYSIS OF THE FLGA GENE AND ITS ADJACENT REGION IN SALMONELLA-TYPHIMURIUM, AND IDENTIFICATION OF ANOTHER FLAGELLAR GENE, FLGN

The flagellar genes flgA and flgM are located at the terminus of the r egion-I flagellar gene cluster on the chromosome of Salmonella typhimu rium. The flgA gene is involved in P-ring formation of the flagellar b asal body, whereas flgM encodes the anti-sigma factor which acts as a negative regulator of the flagellar regulon. The nucleotide sequence o f the DNA fragment containing these flagellar genes and the adjacent r egion was determined. The flgA gene was found to encode a 219-amino-ac id (aa) protein of 23 556 Da. The N-terminal region of FlgA has the ch aracteristics of a typical signal sequence, suggesting that FlgA may f unction in the periplasmic space where P-ring assembly takes place. Th e flgM gene was found to constitute an operon together with an ORF whi ch encodes a 140-aa protein of 15 899 Da. A gene disruption mutant was constructed by inserting a cat gene cartridge into the ORF on the chr omosome. This mutant showed only weak motility, indicating that the pr oduct of the ORF is involved in flagellar formation. Therefore, this O RF was designated as flgN. Electron microscopic observation revealed t hat most of the flagellar structures produced by the flgN mutant are h ook-basal body complexes lacking the filament portions. Based on these results, we concluded that the flgN product is required for the effic ient initiation of filament assembly.