Molecular systematic studies of eubacteria, using σ⁷⁰-type sigma factors of group 1 and group 2

Sigma factors of the σ⁷⁰ family were used as a phylogenetic tool to compare evolutionary relationships among eubacteria. Several new sigma factor genes were cloned and sequenced to increase the variety of available sequences. Forty-two group 1 sigma factor sequences of various species were analyzed with the help of a distance matrix method to establish a phylogenetic tree. The tree derived by using sigma factors yielded subdivisions, including low-G+C and high-G+C gram-positive bacteria, cyanobacteria, and the α, β, γ, and δ subdivisions of proteobacteria, consistent with major bacterial groups found in trees derived from analyses with other molecules. However, some groupings (e.g., the chlamydiae, mycoplasmas, and green sulfur bacteria) are found in different positions than for trees obtained by using other molecular markers. A direct comparison to the most extensively used molecule in systematic studies, small-subunit rRNA, was made by deriving trees from essentially the same species set and using similar phylogenetic methods. Differences and similarities based on the two markers are discussed. Additionally, 31 group 2 sigma factors were analyzed in combination with the group 1 proteins in order to detect functional groupings of these alternative sigma factors. The data suggest that promoters recognized by the major vegetative sigma factors of eubacteria will contain sequence motifs and spacing very similar to those for the σ⁷⁰ sigma factors of Escherichia coli.