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. Belongs to the RNA polymerase beta' chain family. Truncated N-terminus.