Structural basis of chloroplast transcription

Chloroplasts contain a dedicated genome, which encodes subunits of the photosynthesis machinery. Transcription of photosynthesis genes is predominantly carried out by a plastid-encoded RNA polymerase (PEP), a nearly 1 MDa complex composed of core subunits with homology to eubacterial RNA polymerases (RNAPs) and at least 12 additional chloroplast-specific PEP-associated proteins (PAPs). However, the architecture of this complex and the functions of the PAPs remain unknown. In this work, a 19-subunit PEP complex from Sinapis alba was purified by glycerol gradient centrifugation. All known subunits of the complex could be identified by LC-MS and the structure of the complex was determined by cryo-electron microscopy.