Flagellar biosynthesis requires an alternative sigma (sigma) subunit of RNA polymerase to allow recognition of the promoters for flagellin and other late genes of the flagellar regulon. We have now overproduced and characterized Bacillus subtilis sigma D: the prototype of the sigma 28 family of flagellar sigma factors. Limited protease digestion studies indicate that sigma D contains an amino-terminal domain, comprising conserved regions 1.2 and 2, and a carboxyl-terminal domain containing conserved regions 3.2 and 4. The protease-sensitive region between these two domains correlates with a region of very low sequence conservation among bacterial sigma factors. Unlike the primary sigma factor, sigma D binds to DNA. In non-denaturing polyacrylamide gel electrophoresis the sigma D-DNA complex has an apparent equilibrium dissociation constant of 1 microM. Binding of sigma D to the promoter for flagellin, PD-6, appears to lead to an altered DNA structure near the -35 and -10 recognition elements as detected by DNase I footprinting and by the enhanced reactivity of several bases to dimethylsulfate.