The D subunit of archaeal RNA polymerase (RNAP) 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. A single distinct RNAP complex is found in archaea, which may be responsible for the synthesis of all RNAs. The archaeal RNAP harbors homologues of all eukaryotic RNAP II subunits with two exceptions (RPB8 and RPB9). The 12 archaeal subunits are designated by letters and can be divided into three functional groups that are engaged in: (I) catalysis (A'/A", B'/B" or B); (II) assembly (L, N, D and P); and (III) auxiliary functions (F, E, H and K). The D subunit is equivalent to the RPB3 subunit of eukaryotic RNAP II. It contains two subdomains: one subdomain is similar 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 archaeal RNAP subunits that provide most of the enzyme's catalytic functions depends on the presence of the archaeal D/L heterodimer.