Structure of the Essential Diversity-Generating Retroelement Protein bAvd and Its Functionally Important Interaction with Reverse Transcriptase

SummaryDiversity-generating retroelements (DGRs) are the only known source of massive protein sequence variation in prokaryotes. These elements transfer coding information from a template region (TR) through an RNA intermediate to a protein-encoding variable region. This retrohoming process is accompanied by unique adenine-specific mutagenesis and, in the prototypical BPP-1 DGR, requires a reverse transcriptase (bRT) and an accessory variability determinant (bAvd) protein. To understand the role of bAvd, we determined its 2.69 Å resolution structure, which revealed a highly positively charged pentameric barrel. In accordance with its charge, bAvd bound both DNA and RNA, albeit without a discernable sequence preference. We found that the coding sequence of bAvd functioned as part of TR but identified means to mutate bAvd without affecting TR. This mutational analysis revealed a strict correspondence between retrohoming and interaction of bAvd with bRT, suggesting that the bRT-bAvd complex is important for DGR retrohoming.

Graphical AbstractFigure optionsDownload high-quality image (244 K)Download as PowerPoint slideHighlights
► bAvd forms a highly positively charged pentameric barrel
► bAvd binds both DNA and RNA, but without sequence preference
► The coding sequence for bAvd serves dual purposes
► The interaction of bAvd with bRT is likely to be important for retrohoming