The mRNAs of maternally and paternally inherited mtDNAs of the mussel Mytilus galloprovincialis: start/end points and polycistronic transcripts.

Transcription of the mitochondrial genomes of the Mediterranean mussel Mytilus galloprovincialis has been studied by RT-PCR and RNA circularization. This species has an egg-transmitted (F) and a sperm-transmitted (M) mitochondrial genome, in accordance with the doubly uniparental inheritance (DUI) pattern of mtDNA transmission. The primary transcript is cleaved into ten transcripts, eight of which are monocistronic, one is tricistronic and one is most likely, but not certainly, bicistronic. The start/end points of these transcripts have been determined. In the majority of cases cleavage is mediated according to the "tRNA punctuation" model. However, we have identified four cases of cleavage that do not coincide with the presence of a tRNA. In these cases transcription starts immediately or only a few bases from the end point of the preceding gene and cleavage is, most likely, mediated by a stem-loop structure formed at the start point of the gene. The identification of a tricistronic transcript is a novel finding for metazoan mtDNA. We propose that its evolution has been facilitated by the fact that all coding genes are transcribed from the same DNA strand and that co-transcription is sustained by selection emanating from the fact that proteins derived from all three co-transcribed genes participate in the formation of the same oxidative phosphorylation complex.