Transcription factor phosphorylation by a protein kinase associated with chloroplast RNA polymerase from mustard (Sinapis alba).

The chloroplast transcription machinery involves multiple components with both catalytic and regulatory functions. Here we describe a serine-specific protein kinase activity that is associated with the major chloroplast RNA polymerase and phosphorylates sigma-like transcription factors in vitro. The kinase activity can be assigned to a 54 kDa polypeptide of partially purified RNA polymerase (KPC, kinase polymerase complex). This polypeptide is also present in a smaller complex that contains several putative polymerase subunits and reveals kinase activity but lacks transcription activity (KC, kinase complex). Although the 54 kDa component could not be chromatographically separated from the rest of this complex without loss of activity, it retained residual kinase activity in an electrophoretic blot assay. The polymerase-associated kinase is itself affected by in vitro phosphorylation and dephosphorylation, which raises the possibility that it is part of a signalling cascade that controls chloroplast transcription in vivo by factor phosphorylation.