Structure and function of lineage-specific sequence insertions in the bacterial RNA polymerase β′ subunit

The large β and β′ subunits of the bacterial core RNA polymerase (RNAP) are highly conserved throughout evolution. Nevertheless, large sequence insertions in β and β′ characterize specific evolutionary lineages of bacteria. The Thermus aquaticus RNAP β′ subunit contains a 283 residue insert between conserved regions A and B that is found in only four bacterial species. The Escherichia coli RNAP β′ subunit contains a 188 residue insert in the middle of conserved region G that is found in a wide range of bacterial species. Here, we present structural studies of these two β′ insertions. We show that the inserts comprise repeats of a previously characterized fold, the sandwich-barrel hybrid motif (as predicted from previous sequence analysis) and that the inserts serve significant roles in facilitating protein/protein and/or protein/nucleic acid interactions.