Small things considered: the small accessory subunits of RNA polymerase in Gram-positive bacteria

The DNA-dependent RNA polymerase core enzyme in Gram-positive bacteria consists of seven subunits. Whilst four of them (alpha(2)beta beta) are essential, three smaller subunits, delta, epsilon and omega (similar to 9-21.5 kDa), are considered accessory. Both d and. have been viewed as integral components of RNAP for several decades; however, epsilon has only recently been described. Functionally these three small subunits carry out a variety of tasks, imparting important, supportive effects on the transcriptional process of Gram-positive bacteria. While omega is thought to have a wide range of roles, reaching from maintaining structural integrity of RNAP to sigma factor recruitment, the only suggested function for epsilon thus far is in protecting cells from phage infection. The third subunit, delta, has been shown to have distinct influences in maintaining transcriptional specificity, and thus has a key role in cellular fitness. Collectively, all three accessory subunits, although dispensable under laboratory conditions, are often thought to be crucial for proper RNAP function. Herein we provide an overview of the available literature on each subunit, summarizing landmark findings that have deepened our understanding of these proteins and their function, and outline future challenges in understanding the role of these small subunits in the transcriptional process.