Incidence and function of sigma factors in Ralstonia metallidurans and other bacteria.

Bacterial sigma factors are essential for directing the bacterial RNA polymerase to promoter regions during transcription initiation. Genomic sequencing of the highly heavy-metal-resistant beta-proteobacterium Ralstonia metallidurans strain CH34 revealed 17 candidate genes for sigma factors. This review compares the sigma factor machinery of R. metallidurans to that of other bacteria. The sigma factors of 105 bacterial genomes were assigned to sigma factor clusters and families formed around the factors from Escherichia coli, Bacillus subtilis, and R. metallidurans. Genes for between 1 and 65 sigma-factor-related proteins were found in these genomes. Although prediction of sigma factor function from sequence comparisons can be misleading, organization of the R. metallidurans sigma factors into clusters and protein families, together with a discussion of the physiological function of members of these clusters, might yield insight into the cellular roles of bacterial sigma factors and the genes that depend on them for their expression.