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The GenBank accession numbers for the sequences determined in this work are AF225461–AF225466.
Conservation of the sporulation processes between Bacillus spp. and Clostridium spp. was investigated through evolutionary and complementation analyses of σE. Alignment of partial predicted σE 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 various sigma factor sequences from endospore-forming bacteria revealed that homologues of σE, σK and σG clustered together regardless of genus, suggesting a common origin of sporulation sigma factors. The functional equivalence between Clostridium acetobutylicum σE and Bacillus subtilis σE was investigated by complementing a non-polar B. subtilis σE 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 σE 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 σE in Bacillus spp. and Clostridium spp., although regulation of σE expression may differ in these two genera.
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