The eukaryotic RPB3 subunit of RNA polymerase (RNAP), as well as its archaeal (D subunit) and bacterial (alpha subunit) counterparts, is involved in the assembly of RNAP subunits. RNAP is a large multi-subunit complex responsible for the synthesis of RNA. It is the principal enzyme of the transcription process, and is a final target in many regulatory pathways that control gene expression in all living cells. At least three distinct RNAP complexes are found in eukaryotic nuclei: RNAP I, RNAP II, and RNAP III, for the synthesis of ribosomal RNA precursor, mRNA precursor, and 5S and tRNA, respectively. A single distinct RNAP complex is found in prokaryotes and archaea, which may be responsible for the synthesis of all RNAs. The RPB3 subunit is similar to the bacterial RNAP alpha subunit in that it contains two subdomains: one subdomain is similar to the eukaryotic Rpb11/AC19/archaeal L subunit which is involved in dimerization; and the other is an inserted beta sheet subdomain. The assembly of the two largest eukaryotic RNAP subunits that provide most of the enzyme's catalytic functions depends on the presence of RPB3/RPB11 heterodimer subunits. This is also true for the archaeal (D/L subunits) and bacterial (alpha subunit) counterparts.