EDITING OF THE CHLOROPLAST RPOB TRANSCRIPT IS INDEPENDENT OF CHLOROPLAST TRANSLATION AND SHOWS DIFFERENT PATTERNS IN BARLEY AND MAIZE

Sequence analysis of amplified cDNAs derived from the maize chloroplas t rpoB transcript which encodes the beta subunit of a chloroplast spec ific, DNA dependent RNA polymerase reveals four C-to-U editing sites c lustered within 150 nucleotides of the 5' terminal region of the rpoB message. These newly identified editing sites confirm the bias of chlo roplast editing for certain codon transitions and for second codon pos itions which both appear suggestive for an involvement of the translat ional apparatus in the editing process. This supposition prompted us t o investigate editing of the rpoB transcript from ribosome deficient, and hence protein synthesis deficient, plastids of the barley mutant a lbostrians. In this mutant editing is, however, not impaired at any of the editing sites functional in the barley wild type rpoB transcript. This demonstrates that chloroplast editing is neither linked to nor d ependent on the chloroplast translational apparatus. As a further cons equence any peptide components required for chloroplast editing must b e encoded in the nuclear genome. In spite of strong sequence conservat ion only three of the four editing sites identified in the maize rpoB transcript are functional in barley. This indicates that sequences sur rounding an editing site alone are not sufficient as determinants for the editing process in chloroplasts, but that trans-acting templates c arrying the editing information for each individual site may also be r equired.