DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Binds 1 Mg(2+) ion per subunit. Binds 2 Zn(2+) ions per subunit. In plastids the minimal PEP RNA polymerase catalytic core is composed of four subunits: alpha, beta, beta', and beta''. When a (nuclear-encoded) sigma factor is associated with the core the holoenzyme is formed, which can initiate transcription. Beta' and beta'' are fused in this algae. In this algae the rpoC1 (beta') and rpoC2 (beta'') genes are fused into one ORF. This seems to be the result of evolution acting to reduce protein size rather than horizontal transfer from a bacteria. Thus this is considered an atypical member of the bacterial RpoC family. In the N-terminal section; belongs to the RNA polymerase beta' chain family. RpoC1 subfamily. In the C-terminal section; belongs to the RNA polymerase beta' chain family. RpoC2 subfamily.