An Overview of RNA Polymerase Sigma Factors in Phototrophs

Abstract

Sigma factors are dissociable subunits that confer promoter specificity on eubacte-rial core RNA polymerase and are required for transcription initiation. Two major families of sigma factors occur in eubacteria: the a70 (RpoD) family [1,2]) and the a54 (RpoN) family [3]; which are both named after the originally identified Escherichia coli proteins. Based upon sequence comparisons and functional considerations, the σ70 family has been divided into three groups [1]. The primary sigma factors, which are essential for cell viability, comprise group 1. Group 2 includes alternative sigma factors that are highly similar in sequence to the respective group 1 members, whereas group 3 sigma factors are alternative sigma factors that vary more significantly in sequence from the other two groups and include functional groupings such as heat shock and sporulation sigma factors. Sigma factors of groups 2 and 3 are not required for cell viability. Group 1 and group 2 members are related sufficiently closely that cross-hybridization is easily detected in genomic Southern analyses.