Sigma factors and promoters in Corynebacterium glutamicum

The Corynebacterium glutamicum genome codes for 7 sigma subunits (factors) of RNA polymerase (RNAP): primary sigma factor SigA (σA), primary-like SigB and 5 other alternative sigma factors (SigC, SigD, SigE, SigH and SigM). Each sigma factor is responsible for recognizing promoters of genes belonging to a regulon (sigmulon) involved in specific functions of the cell. Most promoters of C. glutamicum housekeeping genes are recognized by RNAP + σA, whereas σB is involved in transcription of a large group of genes active during the transition phase between the exponential and stationary growth phases when various stress factors threaten to damage the cell. The σH regulon consists of the genes involved in heat shock response including those coding for regulators and other sigma factors. It seems therefore that σH occupies a central position in the cross-regulated network of sigma factors and controls their concerted response to various stress conditions in C. glutamicum. The σM factor was found to regulate genes responding to oxidative stress. The main role of σE is to activate genes involved in response to a cell surface stress. Promoters of individual classes recognized by different sigma factors are compiled and the respective consensus sequences of their key recognition motifs (−35 and −10 regions) are derived. In a number of genes, two or more promoters controlled by the same or different sigma factors were discovered. These multiple, overlapping or dual promoters contribute to a complex gene transcription control mechanisms that integrate internal and external signals and tune gene expression in cells as required by environmental and physiological conditions.