DNA-dependent RNA polymerase (RNAP) catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Forms the clamp head domain. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Binds at least 2 Zn(2+) per subunit. Part of the RNA polymerase complex. This protein undergoes a protein self splicing that involves a post-translational excision of the intervening region (intein) followed by peptide ligation. Belongs to the RNA polymerase beta' chain family.