Conservation of the sporulation processes between Bacillus spp. and Clostridium spp. was investigated through evolutionary and complementation analyses of σ E . Alignment of partial predicted σ E amino acid sequences from three Bacillus spp., Paenibacillus polymyxa and five Clostridium spp. revealed that amino acid residues previously reported to be involved in promoter utilization (M124, E119 and N120) and strand opening (C117) are conserved among all these species. Phylogenetic analyses of

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... ious sigma factor sequences from endospore-forming bacteria revealed that homologues of σ E , σ K and σ G clustered together regardless of genus, suggesting a common origin of sporulation sigma factors. The functional equivalence between Clostridium acetobutylicum σ E and Bacillus subtilis σ E was investigated by complementing a non-polar B. subtilis σ E null mutant with the spoIIG operon from either B. subtilis (spoIIG Bs ) or C. acetobutylicum (spoIIG Ca ). Single-copy integration of spoIIG Bs into the amyE locus of the σ E null mutant completely restored the wild-type sporulation phenotype, while spoIIG Ca only partially restored sporulation. Maximal expression of spoIIG Ca -lacZ occurred approximately 12 h later than maximal expression of spoIIG Bs -lacZ. Differences in temporal expression patterns for spoIIG Ca and spoIIG Bs in the B. subtilis background may at least partially explain the observed sporulation complementation phenotypes. This study suggests a common phylogenetic ancestor for σ E in Bacillus spp. and Clostridium spp., although regulation of σ E expression may differ in these two genera.