Polyadenylation occurs at multiple sites in maize mitochondrial cox2 mRNA and is independent of editing status

Polyadenylation of nucleus-encoded transcripts has a well-defined role in g ene expression. The extent and function of polyadenylation in organelles an d prokaryotic systems, however, are less well documented. Recent reports of polyadenylation-mediated RNA destabilization in Escherichia coli and in va scular plant chloroplasts prompted us to look for polyadenylation in plant mitochondria. Here, we report the use of reverse transcription-polymerase c hain reaction to map multiple polyadenylate addition sites in maize mitocho ndrial cox2 transcripts. The lack of sequence conservation surrounding thes e sites suggests that polyadenylation may occur at many 3' termini created by endoribonucleolytic and/or exoribonucleolytic activities, including thos e activities involved in 3' end maturation. Endogenous transcripts could be efficiently polyadenylated in vitro by using maize mitochondrial lysates w ith an activity that added AMP more efficiently than GMP. Polyadenylated su bstrates were tested for stability in maize mitochondrial S100 extracts, an d we found that, compared with nonpolyadenylated RNAs, the polyadenylated s ubstrates were less stable. Taken together with the low abundance of polyad enylated RNAs in maize mitochondria, our results are consistent with a degr adation-related process. The fact that polyadenylation does not dramaticall y destabilize plant mitochondrial transcripts, at least in vitro, is in agr eement with results obtained for animal mitochondria but differs from those obtained for chloroplasts and E. coli. Because fully edited, partially edi ted, and unedited transcripts were found among the cloned polyadenylated co x2 cDNAs, we conclude that RNA editing and polyadenylation are independent processes in maize mitochondria.