SpoIIIAB represents the stage III sporulation protein AB, which is encoded in a spore formation operon: spoIIIAABCDEFGH that is under sigma G regulation [1]. A comparative genome analysis of all sequenced genomes of Firmicutes shows that the proteins are strictly conserved among the sub-set of endospore-forming species.

1: The sigmaE regulon and the identification of additional sporulation genes in Bacillus subtilis. Eichenberger P, Jensen ST, Conlon EM, van Ooij C, Silvaggi J, Gonzalez-Pastor JE, Fujita M, Ben-Yehuda S, Stragier P, Liu JS, Losick R; J Mol Biol 2003;327:945-972. PMID:12662922