A HETEROLOGOUS MAIZE RPOB EDITING SITE IS RECOGNIZED BY TRANSGENIC TOBACCO CHLOROPLASTS

Single nucleotides in plant chloroplast transcripts are edited from th e genomically encoded C to U, often resulting in changes of the encode d protein sequence, Site-specific trans-acting factors are postulated to direct the selection of edited residues, In order to further define cis sequences required for RNA editing, we investigated whether two e diting sites present in maize rpoB mRNA would be recognized by the edi ting machinery of transformed tobacco chloroplasts. A 93-nucleotide (n t) segment surrounding site I is sufficient to direct editing of the m aize sequence in tobacco chloroplasts. However, an 86-nt segment surro unding maize site IV (which is genomically encoded as a T in tobacco) does not confer editing of this site, suggesting that trans-acting fac tors necessary for recognition of site IV are not present in tobacco, The maize sequences surrounding site I were found to compete with the endogenous rpoB for a depletable trans factor and to reduce editing of endogenous site I, The presence of exogenous maize site I was also fo und to decrease editing of endogenous tobacco site II, indicating that there is a shared aspect of editing for some closely spaced editing-s ites.