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) 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). The N-terminal domain is essential for RNAP assembly and basal transcription, whereas the C-terminal domain is involved in interaction with transcriptional regulators and with upstream promoter elements. Unlike most plants, the gene for this protein is encoded in the nucleus, thus the protein is synthesized as a precursor and imported into the chloroplast. Belongs to the RNA polymerase alpha chain family.