TRANSCRIPTION FACTOR PHOSPHORYLATION BY A PROTEIN-KINASE ASSOCIATED WITH CHLOROPLAST RNA-POLYMERASE FROM MUSTARD (SINAPIS-ALBA)

The chloroplast transcription machinery involves multiple components w ith both catalytic and regulatory functions. Here we describe a serine -specific protein kinase activity that is associated with the major ch loroplast RNA polymerase and phosphorylates sigma-like transcription f actors in vitro. The kinase activity can be assigned to a 54 kDa polyp eptide of partially purified RNA polymerase (KPC, kinase polymerase co mplex). This polypeptide is also present in a smaller complex that con tains several putative polymerase subunits and reveals kinase activity but lacks transcription activity (KC, kinase complex). Although the 5 4 kDa component could not be chromatographically separated from the re st of this complex without loss of activity, it retained residual kina se activity in an electrophoretic blot assay. The polymerase-associate d kinase is itself affected by in vitro phosphorylation and dephosphor ylation, which raises the possibility that it is part of a signalling cascade that controls chloroplast transcription in vivo by factor phos phorylation.