Abstract
The core RNA polymerases from bacterial and eukaryotic cells, which are homologous in structure and function (Allison et al. 1985; Biggs et al. 1985; Ahearn et al. 1987; Sweetser et al. 1987; Darst et al. 1989, 1991; Schultz et al. 1993; Polyakov et al. 1995), are catalytically active in RNA chain elongation but are incapable of promoter recognition and specific initiation. Promoter-specific transcription initiation requires additional protein factors. In bacteria, specific initiation by RNA polymerase (RNAP) requires a single polypeptide known as a σ factor, which binds to core RNAP to form the holoenzyme (Burgess et al. 1969; Travers and Burgess 1969). One primary σ factor directs the bulk of transcription during exponential growth. Specialized, alternative σ factors direct transcription of specific regulons during unusual physiological or developmental conditions (reviewed in Helmann and Chamberlin 1988; Gross et al. 1992). The primary and most of the alternative σ factors comprise a highly homologous family of proteins (Stragier et al. 1985; Gribskov and Burgess 1986) with four regions of highly conserved amino acid sequence (Fig. 1; reviewed in Lonetto et al. 1992). Based on the results of genetic and biochemical experiments, specific functions have been assigned to some of the conserved regions(summarized in Fig. 1).