The wheat mitochondrial gene for apocytochrome b: absence of a prokaryotic ribosome binding site.

The wheat mitochondrial gene for apocytochrome b (CYB) has been identified by its hybridization to a yeast CYB probe and its nucleotide sequence has been determined. The wheat CYB sequence predicts a cytochrome b apoprotein of 398 amino acids; it is almost identical to that of maize but has ten additional amino acids at the carboxy terminus. No introns are present in the wheat CYB gene, but an internal segment of the gene is repeated at another genomic location. Transcript analysis reveals a single wheat CYB mRNA of approximately 2.4 kb with a long untranslated leader. Sequences upstream of the CYB coding region are very similar in wheat and maize but the stretch proposed to be a ribosome binding site in maize is not conserved in wheat. The corresponding leader regions of the wheat mitochondrial mRNAs for cytochrome oxidase subunits I and II also lack complementarity to the 3'-end of the small subunit rRNA. We conclude that alternative signals are involved in the initiation of translation in plant mitochondria.