Zmp TAC 12 binds single‐stranded nucleic acids and is essential for accumulation of the plastid‐encoded polymerase complex in maize

• Published in: The New phytologist 2015-05, Vol.206 (3), p.1024-1037
• Main Authors: Pfalz, Jeannette, Holtzegel, Ute, Barkan, Alice, Weisheit, Wolfram, Mittag, Maria, Pfannschmidt, Thomas
• Format: Article
• Language: English
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.13248

The plastid‐encoded plastid RNA polymerase ( PEP ) represents the major transcription machinery in mature chloroplasts. Proteomic studies identified four plastome‐ and at least ten nuclear‐encoded proteins making up this multimeric enzyme. Depletion of single subunits is known to result in strongly diminished PEP activity causing severe defects in chloroplast biogenesis. Here, we characterized one PEP subunit in maize, Zmp TAC 12, and investigated the molecular basis underlying PEP ‐deficiency in Zmptac12 mutants. We show that the ZmpTAC12 gene encodes two different protein isoforms, both of which localize dually in chloroplasts and nuclei. Moreover, both variants assemble into the PEP ‐complex. Analysis of PEP ‐complex assembly in various maize mutants lacking different PEP ‐complex components demonstrates that Zmp TAC 12, Zmp TAC 2, Zmp TAC 10 and ZmMurE are each required to accumulate a fully assembled PEP ‐complex. Antibodies to Zmp TAC 12 coimmunoprecipitate a subset of plastid RNA s that are synthesized by PEP ‐dependent transcription. Gel mobility shift analyses with recombinant Zmp TAC 12 revealed binding capabilities with ss RNA and ss DNA , but not ds DNA . Collectively these data demonstrate that Zmp TAC 12 is required for the proper build‐up of the PEP ‐complex and that it interacts with single‐stranded nucleic acids. See also the Commentary by Peter Kindgren and Åsa Strand