Truncated and dispersed rpl2 and rps19 pseudogenes are co-transcribed with neighbouring downstream genes in wheat mitochondria

Abstract.

The wheat mitochondrial genome contains only partial coding sequences for the L2 and S19 ribosomal proteins, unlike in rice or liverwort mitochondria, where these genes are functional and have a bacterial-type linkage. A single-copy stretch corresponding to the extreme 3′ terminus of the wheat rpl2 gene is co-transcribed with the trans-splicing nad1 exon 4; and, at another unique location, the rps19 segment lacking the 5′ coding region is co-transcribed with the downstream nad4L gene. In both cases, the 5′ termini of these transcripts map to promoter consensus motifs acquired through genomic reorganization, enabling continued expression of essential downstream genes. In both wheat and rice, the rpl2 and rps19 genomic regions differ in their RNA profiles between germinating embryos and seedlings. The absence of intact rpl2 and rps19 genes in wheat mitochondria is consistent with their inactivation through DNA rearrangement/deletion after the successful transfer of functional copies to the nucleus.