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. 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 (By similarity). In C.reinhardtii the gene for this protein is split in two. Belongs to the RNA polymerase beta' chain family. RpoC1 subfamily.