RNA editing is a process that results in the production of a messenger RNA with nucleotide sequences that differ from those of the template DNA, and provides another mechanism for modulating gene expression. The phenomenon was initially described in the mitochondria of protozoa. Here we report that RNA editing is also required for the correct expression of plant mitochondrial genes. It has previously been proposed that in plant mitochondria there is a departure from the universal genetic code, with CGG specifying tryptophan instead of arginine. This was because CGG codons are often found in plant mitochondrial genes at positions corresponding to those encoding conserved tryptophans in other organisms. We have now found, however, wheat mitochondrial gene sequences containing C residues that are edited to U residues in the corresponding mRNA sequences. In this way, CGG codons can be changed to UGG codons in the mRNA so that tryptophan may be encoded according to the universal genetic code. Furthermore, for each codon modification resulting from a C----U conversion that we studied, we found a corresponding change in the amino acid that was encoded. RNA editing in wheat mitochondria can thus maintain genetic information at the RNA level and as a result contribute to the conservation of mitochondrial protein sequences among plants.

Very closely related short sequences are present at the 5′ end of cytoplasmic mRNAs in Euglena as evidenced by comparison of cDNA sequences and hybrid‐arrested translation experiments. By cloning Euglena gracilis nuclear DNA and isolating the rbcS gene (encoding the small subunit of ribulose‐1,5‐bisphosphate carboxylase/oxygenase), we have shown that the short leader sequence does not flank the nuclear gene sequence. The leader sequences were found to constitute the 5′ extremities of a family of small RNAs. Sequencing six members of this family revealed a striking similarity to vertebrate U snRNAs. We propose that a trans‐splicing mechanism transfers the spliced leader (SL) sequence from these small RNAs (SL RNAs) to pre‐mature mRNAs. Transfer of leader sequences to mRNAs by trans‐splicing has been shown only in trypanosomes where cis‐splicing is unknown, and in nematodes where not more than 10% of the mRNAs have leader sequences. Our results strongly suggest that Euglena is a unique organism in which both a widespread trans‐splicing and a cis‐splicing mechanism co‐exist.

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