Revealing secrets of the enigmatic omega subunit of bacterial RNA polymerase

The conserved omega (omega) subunit of RNA polymerase (RNAP) is the only nonessential subunit of bacterial RNAP core. The small omega subunit (7 kDa-11.5 kDa) contains three conserved alpha helices, and helices alpha 2 and alpha 3 contain five fully conserved amino acids of omega. Four conserved amino acids stabilize the correct folding of the omega subunit and one is located in the vicinity of the beta ' subunit of RNAP. Otherwise omega shows high variation between bacterial taxa, and although the main interaction partner of omega is always beta ', many interactions are taxon-specific. omega-less strains show pleiotropic phenotypes, and based on in vivo and in vitro results, a few roles for the omega subunits have been described. Interactions of the omega subunit with the beta ' subunit are important for the RNAP core assembly and integrity. In addition, the omega subunit plays a role in promoter selection, as omega-less RNAP cores recruit fewer primary sigma factors and more alternative sigma factors than intact RNAP cores in many species. Furthermore, the promoter selection of an omega-less RNAP holoenzyme bearing the primary sigma factor seems to differ from that of an intact RNAP holoenzyme.

Automated summary

The omega subunit of RNA polymerase is the only nonessential subunit in the core of bacterial RNAP. It stabilizes correct folding, interacts with the beta ' subunit, and plays a crucial role in core assembly and promoter selection. Omega-less strains exhibit pleiotropic phenotypes and interactions with other subunits differ between bacterial taxa.