AC40 subunit of Eukaryotic RNA polymerase (RNAP) I and RNAP III
The eukaryotic AC40 subunit of RNA polymerase (RNAP) I and RNAP III 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. RNAP I is responsible for the synthesis of ribosomal RNA precursor, while RNAP III functions in the synthesis of 5S and tRNA. The AC40 subunit is the equivalent of the RPB3 subunit of RNAP II. The RPB3 subunit is similar to the bacterial RNAP alpha subunit in that 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 RPB3 subunit heterodimerizes with the RPB11 subunit, and together with RPB10 and RPB12, anchors the two largest subunits, RPB1 and RPB2, and stabilizes their association. The homology of AC40 to RPB3 suggests a similar function. The AC40 subunit is likely to associate with the RPB11 counterpart, AC19, to form a heterodimer, which stabilizes the association of the two largest subunits of RNAP I and RNAP III.