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. 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. Young tissue from this organism is photosynthetic and contains some thylakoids, although the photosynthetic activity does not exceed the light compensation point.