PTK, the chloroplast RNA polymerase-associated protein kinase from mustard(Sinapis alba), mediates redox control of plastid in vitro transcription

The major RNA polymerase from mustard chloroplasts is a multi-subunit enzym e consisting of core components and associated factors. Among the latter is a heterotrimeric factor named PTK (plastid transcription kinase) because o f its serine/threonine-type protein kinase activity. PTK activity itself de pends on its phosphorylation state. In addition, we show that it responds t o glutathione but not to other redox-reactive reagents that were tested, an d both glutathione and phosphorylation act antagonistically. Using a homolo gous in vitro system, we find that PTK selectively phosphorylates subunit(s ) of plastid RNA polymerase and is involved in determining the level of fai thful transcription from the chloroplast psbA promoter. Together, these res ults establish a role for phosphorylation and redox state in the regulation of plastid gene expression.