DNA-dependent RNA polymerase (RNAP) catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Forms part of the jaw domain. Reconstitution experiments show this subunit is required for basic activity. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Part of the 13-subunit RNA polymerase complex. Belongs to the RNA polymerase beta' chain family.