DNA-dependent RNA polymerase (RNAP) catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. This subunit plays an important role in subunit assembly since its dimerization is the first step in the sequential assembly of subunits to form the holoenzyme. Part of the processive rRNA transcription and antitermination complex (rrnTAC). The complex forms an RNA-chaperone ring around the RNA exit tunnel of RNAP. It supports rapid transcription and antitermination of rRNA operons, cotranscriptional rRNA folding, and annealing of distal rRNA regions to allow correct ribosome biogenesis. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Homodimer. The RNAP catalytic core consists of 2 alpha, 1 beta, 1 beta' and 1 omega subunit. When a sigma factor is associated with the core the holoenzyme is formed, which can initiate transcription. Both the N- and C-terminus interact with different regions of transcriptional regulator CRP. The rRNA transcription and antitermination complex (rrnTAC) consists of RNAP, NusA, NusB, NusE (rpsJ), NusG, SubB, ribosomal protein S4, DNA and precursor rRNA; S4 is more flexible than other subunits (PubMed:32871103). The N-terminal domain is essential for RNAP assembly and basal transcription, whereas the C-terminal domain is involved in interaction with transcriptional regulators (such as CRP) and with upstream promoter elements. Acetylated on Lys-297 and Lys-298 in the presence of glucose. PatZ controls acetylation of Lys-298 but not of Lys-297. (Microbial infection) ADP-ribosylated on both alpha subunits by the phage T4 protein ModA (PubMed:10634320, PubMed:15489438). ADP-ribosylated on only one of the alpha subunits by the phage T4 protein Alt (PubMed:15489438). Belongs to the RNA polymerase alpha chain family.