Light-dependent, plastome-wide association of the plastid-encoded RNA polymerase with chloroplast DNA

Plastid genes are transcribed by two types of RNA polymerases: a plastid-encoded eubacterial-type RNA polymerase (PEP) and nuclear-encoded phage-type RNA polymerases (NEPs). To investigate the spatio-temporal expression of PEP, we tagged its -subunit with a hemagglutinin epitope (HA). Transplastomic tobacco plants were generated and analyzed for the distribution of the tagged polymerase in plastid sub-fractions, and associated genes were identified under various light conditions. RpoA:HA was detected as early as the 3rd day after imbibition, and was constitutively expressed in green tissue over 60days of plant development. We found that the tagged polymerase subunit preferentially associated with the plastid membranes, and was less abundant in the soluble stroma fraction. Attachment of RpoA:HA to the membrane fraction during early seedling development was independent of DNA, but at later stages of development, DNA appears to facilitate attachment of the polymerase to membranes. To survey PEP-dependent transcription units, we probed for nucleic acids enriched in RpoA:HA precipitates using a tobacco chloroplast whole-genome tiling array. The most strongly co-enriched DNA fragments represent photosynthesis genes (e.g. psbA, psbC, psbD and rbcL), whose expression is known to be driven by PEP promoters, while NEP-dependent genes were less abundant in RpoA:HA precipitates. Additionally, we demonstrate that the association of PEP with photosynthesis-related genes was reduced during the dark period, indicating that plastome-wide PEP-DNA association is a light-dependent process.