THE A-FORMS AND B-FORMS OF PLASTID DNA-DEPENDENT RNA-POLYMERASE FROM MUSTARD (SINAPIS-ALBA L.) TRANSCRIBE THE SAME GENES IN A DIFFERENT DEVELOPMENTAL CONTEXT

Two RNA polymerases, termed A (cp-pol A) and B (cp-pol B), are known t o be present in mustard plastids. In vitro, the two enzymes have diffe rent requirements for DNA binding, but both bind to, and transcribe fr om, the same set of chloroplast promoters. The B enzyme is sensitive t o rifampicin (Rif), whereas the A enzyme is not. When seedlings were g rown in the presence of Rif, RNA pool sizes of the photosynthesis-rela ted plastid genes rbcL and psbA were smaller than in untreated control s, whereas transcripts of the non-photosynthetic genes rps16, trnG, rr n and rpoB remained virtually unaffected by the drug. The Rif inhibiti on patterns of rbcL and psbA transcripts reflect the relative abundanc e of the A and B enzymes at different stages and light/dark conditions . These genes can thus be transcribed by either of the two enzymes in vivo, whereas the non-photosynthetic genes are transcribed mostly or e xclusively by the A enzyme, or by another Rif-resistant plastid polyme rase. Among several nuclear gene transcripts that were tested for Rif inhibition, only those of the RbcS gene family for the plastid-bound s mall subunit of Rubisco revealed a decrease in pool size, which may im ply that mechanisms exist that serve to coordinate patterns of gene ex pression in the different cellular compartments.