The two eukaryotic subunits Rpb3 and Rpb11 dimerise to from a platform onto which the other subunits of the RNA polymerase assemble (D/L in archaea). The prokaryotic equivalent of the Rpb3/Rpb11 platform is the alpha-alpha dimer. The dimerisation domain of the alpha subunit/Rpb3 is interrupted by an insert domain (PF01000). Some of the alpha subunits also contain iron-sulphur binding domains (PF00037). Rpb11 is found as a continuous domain. Members of this family include: alpha subunit from eubacteria, alpha subunits from chloroplasts, Rpb3 subunits from eukaryotes, Rpb11 subunits from eukaryotes, RpoD subunits from archaeal spp, and RpoL subunits from archaeal spp.
The core of the bacterial RNA polymerase (RNAP) consists of four subunits, two alpha, a beta and a beta', which are conserved from bacteria to mammals. The alpha subunit (RpoA) initiates RNAP assembly by dimerising to form a platform on which the beta subunits can interact, and plays a direct role in promoter recognition [PUBMED:10972792]. In eukaryotes, RNA polymerase (RNAP) II is responsible for all mRNA synthesis. RNAP-II consists of 12 subunits, where subunits Rpb3 and Rpb11 form a heterodimer that is functionally analogous to the bacterial RpoA homodimer [PUBMED:12860379]. Archaeal RNAP closely resembles eukaryotic RNAP-II, and is composed of 12 subunits, of which D and L form a heterodimer resembling the Rpb3/Rpb11 and RpoA/RpoA dimers [PUBMED:12694606].
Murakami K, Kimura M, Owens JT, Meares CF, Ishihama A; , Proc Natl Acad Sci USA 1997;94:1709-1714.: The two alpha subunits of Escherichia coli RNA polymerase are asymmetrically arranged and contact different halves of the DNA upstream element. PUBMED:9050843 EPMC:9050843.