RNA polymerase is the enzyme responsible for recognizing appropriate genes under specific environmental conditions, and for creating the mRNA transcripts that can be translated into new proteins. Sigma factors are dissociable subunits of prokaryotic RNA polymerase. When a sigma factor associates with a core RNA polymerase to form RNA polymerase holoenzyme, it directs the holoenzyme to recognize conserved DNA motifs called promoter sites (or regions) that precede gene sequences.