The plastid-encoded RNA polymerase of plant chloroplasts

Plant chloroplasts possess a dedicated genome (plastome) and a prokaryotic-type plastid-encoded RNA polymerase (PEP) that mediates its expression. PEP is composed of five bacteria-like core proteins and 16 nucleus-encoded PEP-associated proteins (PAPs). These are essential for PEP-driven transcription and chloroplast biogenesis, but their functions and structural arrangement in the PEP complex remained largely enigmatic. Recently, four independently determined cryogenic-electron microscopy (cryo-EM) structures of purified plant PEP complexes reported features of the prokaryotic core and the arrangement of PAPs around it, identified potential functional domains and cofactors, and described the interactions of PEP with DNA. We explore these data and critically discuss the proposed regulatory impact of PAPs on the transcription process. We further address the evolutionary implications and describe fields for future investigation.