Identification and characterization of FliY, a novel component of the Bacillus subtilis flagellar switch complex

Abstract

The Bacillus subtilis gene encoding FMY has been cloned and sequenced. The gene encodes a 379‐amino‐acid protein with a predicted molecular mass of 41 054 daltons. FliY is partly homologous to the Escherichia coli and Salmonella typhimurium switch proteins FliM and FliN. The N‐terminus of FliY has 33% identity with the first 122 amino acids of FliM, whereas the C‐terminus of FliY has 52% identity with the last 30 amino acids of FMN. The middle 60% of FliY is not significantly homologous to either of the proteins. A fliY::cat null mutant has no flagella. Motility can be restored to the mutant by expression of fliY from a plasmid, although chemotaxis is still defective since the strain exhibits smooth swimming behaviour. fliY::cat is in the cheD complementation group. One of the cheD point mutants does not switch although the population grown from a single cell has both smooth swimming and tumbling bacteria, implying that the switch is locked. Expression of fliY in wild‐type B. subtilis makes the cells more smooth‐swimming but does not appear to affect chemotaxis. Expression of fliY in wild‐type S. typhimurium severely inhibits chemotaxis and also makes the cells smooth swimming. Expression in a non‐motile S. typhimurium fliN mutant restores motility but not chemotaxis, although expression in a non‐motile E. coli fliM mutant does not restore motility. The homology, multiple phenotypes, and inter‐species complementation suggest that FliY forms part of the B. subtilis switch complex. The function of the unique part of FliY and the evolutionary relationship between FliY and FliN are discussed in terms of the novel properties of B. subtilis chemotaxis.