CONTROL OF GENE-EXPRESSION BY BASE DEAMINATION - THE CASE OF RNA EDITING IN WHEAT MITOCHONDRIA

The term 'RNA editing' was used for the first time in 1986 to describe the process of uridylate insertion into trypanosomal mitochondrial tr anscripts. Since then, the term has been used more generally to descri be a large variety of processes involving base insertions, deletions a nd conversions that generate RNAs with a primary sequence different to those encoded by the gene. RNA editing has been observed in the mitoc hondrial fraction of trypanosomes, plants and other organisms, in the animal nuclear fraction in the case of the apolipoprotein B and glutam ate brain receptors mRNAs as well as in viruses like paramyxovirus, he patitis delta and probably HN. The role of cytidine and adenine deamin ation leading to C to U and A to I transitions has became pivotal to e xplain this process by base conversion. In this review we will focus m ainly on the work performed in our group on plant mitochondria and mor e specifically on the mechanism and the functional significance of RNA editing in wheat organelles. The original contributions of our labora tory in this field are: i) showing that RNA editing is reflected at th e protein level; ii) settling three in vitro systems to assay C to U c onversion using a wheat mitochondrial lysate as source of enzymes and factors, and unedited mRNA from the same source, as substrate; iii) de termination by double labelling of the unedited substrate that RNA edi ting in wheat mitochondria occurs via a deamination step; and iv) that introducing unedited proteins in the mitochondria of transgenic plant s leads to the emergence of cytoplasmic male sterility supporting the idea that the role of this process is to produce functional proteins. Using the antisense approach in transgenic plants we were able to obta in a significant male fertility restoration.