Phylogenetic evidence for the emergence of bacterial σ-factors by domain fusion

{sigma}-factors are dissociable subunits of RNA polymerase (RNAP), forming the basal regulatory machinery for bacterial transcription. The minimal {sigma}-factors contain two domains (called {sigma}2 or {sigma}4), both of which bind RNAP as well as the promoter DNA through their helix-turn-helix motif. Housekeeping {sigma}-factors contain four domains. We identify proteins containing at least one of the four {sigma}-factor domains in bacterial and bacteriophage genomes; we also show that proteins containing either {sigma}2 or {sigma}4, which are not defined as {sigma}-factors, are commonly found in both sets of organisms. Using phylogenetic analysis of these two domains, we suggest that the highly conserved major housekeeping {sigma}-factor, with its complex domain architecture, represents a late-evolving monophyletic clade, whereas single-domain proteins and minimal {sigma}-factor architectures comprising {sigma}2 and {sigma}4 might be ancestral. Thus, {sigma}-factors probably evolved from single-domain DNA-binding transcription factors by domain accretion.