The Transcription Apparatus and the Regulation of Transcription Initiation

Abstract

SummaryOur knowledge of transcription in cyanobacteria has increased dramatically during the last decade. The aspects of transcription for which most is known include the properties of the transcription apparatus and the regulation of transcription initiation. The structure of the RNA polymerase of Escherichia coli was believed to be universal among eubacteria, and the finding that cyanobacteria possess a transcriptional apparatus of unique structure was unexpected. Cyanobacterial RNA polymerases contain equivalents of the α, β and σ subunits of the E. coli enzyme, but differ in having two subunits that correspond to the β′ subunit of E. coli. The genes encoding the sigma factors for the major form of RNA polymerase, as well as additional sigma factor genes have been identified in several cyanobacteria. All of the genes encode sigma factors with features of the principal sigma factors of other eubacteria. The roles of these multiple sigma factors remain to be elucidated, and factors of the alternative sigma class have not yet been identified. Comparisons of promoter sequences, the analysis of promoter activities in vivo and in vitro, and other studies indicate that the promoters of E. coli and cyanobacteria are found at similar positions relative to transcription initiation sites. Most cyanobacterial promoters display a conserved element at -10 from the transcription initiation site that conforms to the E. coli -10 promoter consensus. For the majority of genes, however, an element that conforms to the E. coli -35 promoter element is lacking. These data and other evidence suggest that many of the cyanobacterial genes thus far characterized are subject to regulation by activator proteins. A number of DNA binding proteins and cis regulatory regions have been identified that are implicated in the control of transcription initiation.