The multisubunit chloroplast RNA polymerase A from mustard (Sinapis alba L.) - Integration of a prokaryotic core into a larger complex with organelle-specific functions

We previously identified two multisubunit plastid RNA polymerases termed A and B. The B enzyme has a bacterial-type polypeptide composition and is sen sitive to the prokaryotic transcription inhibitor rifampicin (Rif); the A e nzyme has a more complex subunit structure and is Rif-resistant. Here we re port results of N-terminal sequencing and MS carried out with the A enzyme, which establish that the latter contains rpo gene products and is structur ally related to the B enzyme. Furthermore, evidence is provided that the A enzyme can be converted into a Rif-sensitive enzyme form in a phosphorylati on-dependent manner in vitro by a treatment that results in depletion of a beta-like subunit. Database searches using sequence information derived fro m additional polypeptides that are present in purified A preparations revea led sequence similarity with chloroplast proteins involved in RNA processin g and redox control. This proteomics approach thus points to the complexity of the chloroplast transcription apparatus and its interconnections with p ost-transcriptional and signalling mechanisms.