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. Belongs to the RNA polymerase alpha chain family.