The group 1 and group 2 sigma70-type sigma factors of the green sulfur bacterium Chlorobium tepidum and of the green nonsulfur bacterium Chloroflexus aurantiacus were cloned and characterized. Cb. tepidum was found to contain one sigma70-type sigma factor; the expression of the gene was analyzed by Northern blot hybridization and primer-extension mapping. Cf. aurantiacus has genes encoding four sigma factors of groups 1 and 2. The expression of these genes was examined in cells grown aerobically and anaerobically. The sigC gene was expressed at approximately equal levels under both conditions, resulting in its designation as the group 1 sigma factor of this organism. The only other detectable transcripts arose from the sigB gene, which was expressed at higher levels during aerobic growth. A phylogenetic tree was obtained using the group 1 sigma factors of Cb. tepidum, Cf. aurantiacus, and diverse eubacteria as the molecular marker. The resulting phylogenetic tree shows that Cb. tepidum and Cf. aurantiacus are related to each other and to the cyanobacteria. The relationship of the group 2 sigma factors of Cf. aurantiacus and the cyanobacteria was more specifically examined phylogenetically. The group 2 sigma factors of Cf. aurantiacus probably arose by gene duplication events after the split of the green nonsulfur bacteria from other photosynthetic eubacteria.