Characterization of csh203::Tn917lac, a mutation in Bacillus subtilis that makes the sporulation sigma factor sigma-H essential for normal vegetative growth.

spo0H encodes a sigma factor, sigma-H, of RNA polymerase that is required for sporulation in Bacillus subtilis. Null mutations in spo0H block the initiation of sporulation but have no obvious effect on vegetative growth. We have characterized an insertion mutation, csh203::Tn917lac, that makes spo0H essential for normal growth. In otherwise wild-type cells, the csh203::Tn917lac insertion mutation has no obvious effect on cell growth, viability, or sporulation. However, in combination with a mutation in spo0H, the csh203 mutation causes a defect in vegetative growth. The csh203::Tn917lac insertion mutation was found to be located within orf23, the first gene of the rpoD (sigma-A) operon. The transposon insertion separates the major vegetative promoters P1 and P2 from the coding regions of two essential genes, dnaG (encoding DNA primase) and rpoD (encoding the major sigma factor, sigma-A) and leaves these genes under the control of minor promoters, including P4, a promoter controlled by sigma-H. The chs203 insertion mutation caused a 2- to 10-fold increase in expression of promoters recognized by RNA polymerase containing sigma-H. The increased expression of genes controlled by sigma-H in the csh203 single mutant, as well as the growth defect of the csh203 spo0H double mutant, was due to effects on rpoD and not to a defect in orf23 or dnaG.