Fused and overlapping rpoB and rpoC genes in helicobacters, campylobacters, and related bacteria

The genes coding for the beta (rpoB) and beta' (rpoC) subunits of RNA polym erase are fused in the gastric pathogen Helicobacter pylori but separate in other taxonomic groups. To better understand how the unique fused structur e evolved, we determined DNA sequences at and around the rpoB-rpoC junction in 10 gastric and nongastric species of Helicobacter acid in members of th e related genera Wolinella, Arcobacter, Sulfurospirillum, and Campylobacter . We found the fusion to be specific to Helicobacter and Wolinella genera; rpoB and rpoC overlap in the other genera. The fusion may have arisen by a frameshift mutation at the site of rpoB and rpoC overlap. Loss of good Shin e-Dalgarno sequences might then have fixed the fusion in the Helicobacterac eae, even if fusion itself did not confer a selective advantage.