Homology in the region containing a tRNA(Trp) gene and a (complete or partial) tRNA(Pro) gene in wheat mitochondrial and chloroplast genomes.

We have used bean mitochondrial (mt) and chloroplast (cp) tRNA(Trp) as probes to locate the corresponding genes on the mt and cp genomes of wheat and we have determined the nucleotide sequences of the wheat mt and cp tRNA(Trp) genes and of the flanking regions. Sequence comparisons show that the wheat mt and cp tRNA(Trp) genes are 97% homologous. On the wheat cp DNA, a tRNA(UGGPro) gene was found 139 bp upstream of the cp tRNA(Trp) gene. On the wheat mt DNA, a sequence of 23 nucleotides completely homologous with the 3' end of this cp tRNA(Pro) gene was found 136 bp upstream of the mt tRNA(Trp) gene, but there is only 38% homology between cp and mt wheat genomes in the intergenic regions. The overall organization of this region in the chloroplast genome (a tRNA(Trp) gene separated by about 140 bp from a tRNA(Pro) gene) is also found in the mitochondrial genome, suggesting that this mitochondrial fragment might have originated from a chloroplast DNA insertion. A comparison of the genes and of the intergenic regions located between the tRNA(Trp) gene and the tRNA(Pro) (or partial tRNA(Pro)) gene shows that there is an almost complete conservation of these sequences in the mitochondrial DNA of wheat and maize, whereas wheat mt and cp intergenic regions show more sequence divergence. Wheat mt tRNA(Trp) gene is encoded by the main mt genome (accounted for by the master chromosome) but, in the case of maize mitochondria, this gene was found to be encoded by the 2.3 kb linear plasmid, indicating that this plasmid is not dispensable in maize mitochondria.