Promotes RNA polymerase assembly. Latches the N- and C-terminal regions of the beta' subunit thereby facilitating its interaction with the beta and alpha subunits. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Consists of a sigma factor and the RNAP core enzyme which is composed of 2 alpha chains, 1 beta chain, 1 beta' chain and 1 subunit omega. Belongs to the RNA polymerase subunit omega family.