Cyanobacterial sigma factors: Current and future applications for biotechnological advances

A sigma (sigma) factor is a constituent of bacterial RNA polymerase that guides the holoenzyme to promoter sequences and initiates transcription. In addition to a primary housekeeping sigma factor, bacteria contain a number of alternative sigma factors which recognize a specific set of promoters. By replacing the primary sigma factor with alternative variants, the cell controls transcription of the whole sets of genes, typically to acclimate to changes in the environment. As key regulatory elements, sigma factors are frequent targets of genetic manipulation aimed at the improvement of bacterial stress tolerance and capacity for bioproduction. Cyanobacteria are a phylum of bacteria capable of oxygenic photosynthesis and there is a great interest to employ them as biochemical and biofuel production hosts. Engineering of sigma factor genes has become an important strategy to improve robustness and suitability of cyanobacteria for the production of high-value metabolites such as polyhydroxybutyrate, succinate, sucrose or hydrogen. Here, we summarize the current knowledge about the regulatory role of different sigma factor classes in cyanobacteria, highlighting their biotechnological potential.