An operon of Bacillus subtilis motility genes transcribed by the sigma D form of RNA polymerase.

Two genes controlling motility functions in Bacillus subtilis were identified by DNA sequence analysis of a chromosomal fragment containing a strong promoter for sigma D RNA polymerase. Previous studies had shown that this sigma D-dependent promoter controls synthesis of a 1.6-kb transcript in vivo and in vitro. Sequence analysis revealed that the 1.6-kb transcript contains two open reading frames coding for protein sequences homologous to the Escherichia coli motA and motB gene products, respectively, and ends in a rho-independent termination site. Direct evidence linking these genes to motility functions in B. subtilis was obtained by precise localization by polymerase chain reaction of Tn917 transposon insertion mutations of Mot- strains, isolated by Zuberi et al. (A. R. Zuberi, C. Ying, H. M. Parker, and G. W. Ordal, J. Bacteriol. 172:6841-6848, 1990), to within this mot. operon. Replacement of each wild-type gene by in-frame deletion mutations yielded strains possessing paralyzed flagella and confirmed that both motA and motB are required for the motility of B. subtilis. These current findings support our earlier suggestions that sigma D in B. subtilis plays a central role in the control of gene expression for flagellar assembly, chemotaxis, and motility functions. Sigma F, the enteric homolog of sigma D, controls similar functions in E. coli and Salmonella typhimurium, and these factors appear to be representative of a family of factors implicated in flagellar synthesis in many bacterial species, which we propose to designate the sigma 28 family.