Bacillus subtilis σ factor σ²⁹ is the product of the sporulation-essential gene spoIIG

Evidence is presented that the sporulation-essential locus spoIIG codes for both σ²⁹ and a structurally related protein, P³¹. This demonstrates that at least one specific Bacillus subtilis RNA polymerase binding protein provides a critical function in endospore formation. spoIIG-specific RNA is present in B. subtilis cultures that are synthesizing P³¹ and σ²⁹ and is absent in those that are not. A monoclonal antibody specific for an antigenic determinant on P³¹/σ²⁹ detected crossreacting proteins (P²⁵/P²¹) but not P³¹ or σ²⁹ in a Spo^- B. subtilis strain with a mutation at the spoIIG locus (spoIIG41). The appearance of P²⁵ and P²¹ occurs in this mutant at a time when P³¹ and σ²⁹ would normally appear and suggests that they are homologous proteins. Transformation of the spoIIG41 strain with plasmid DNA carrying the structural gene for spoIIG complements the Spo^- phenotype and results in the synthesis of P³¹, σ²⁹, P²⁵, and P²¹ at the appropriate times during sporulation. In Escherichia coli, the cloned spoIIG sequence encoded a protein that reacted with the anti-P³¹/σ²⁹ monoclonal antibody and had the electrophoretic mobility of authentic P³¹.