RNA EDITING IN MAIZE CHLOROPLASTS IS A PROCESSING STEP INDEPENDENT OFSPLICING AND CLEAVAGE TO MONOCISTRONIC MESSENGER-RNAS

The psbB operon contained in the plastomes of higher plants consists o f the genes psbB, psbH, petB and petD. The primary transcript of this operon is subject to a series of processing steps which include cleava ges resulting in four monocistronic mRNAs and splicing of the petB and petD transcripts. A search for editing sites within the two latter tr anscripts from maize led us to the detection of one editing site withi n the petB coding region which is conserved at the DNA level in other graminean species and in tobacco. This shows that editing must be cons idered as an additional processing step of the psbB operon encoded pri mary transcript. As is evident from cDNA sequences derived from the di cistronic and/or unspliced petB/D transcripts which are completely edi ted, editing is an early step of mRNA processing which precedes both s plicing and cleavage to the monocistronic mRNAs and which most, theref ore, be independent of the latter two steps. This conclusion is confir med by a similar observation with the editing site of the rpl2 transcr ipt which is contained in the polycistronic transcript of the rpoA ope ron, although here only partial editing is observed for the unspliced dicistronic rpl23/rpl2 transcript.