Brassica nap cytoplasmic male sterility is associated with expression of a mtDNA region containing a chimeric gene similar to the pol CMS-associated orf224 gene

Abstract

Two different cytoplasmic male-sterility (CMS) systems, nap and pol, are found in the oilseed rape (canola) species Brassica napus. Physical mapping studies have previously shown that organizational differences between the sterile pol and fertile cam mitochondrial genomes are restricted to a relatively small region immediately upstream of the atp6 gene. An approximately 4.5-kb pol mtDNA segment containing a chimeric open reading frame (orf224) co-transcribed with atp6 is missing from cam mtDNA and located at a different site on nap mtDNA; expression of the orf224/atp6 gene region is highly correlated with the pol CMS trait. Sequence analysis now shows that the transposed nap segment contains an open reading frame (ORF) related to, but distinct from, pol orf224. This open reading frame (orf222) potentially encodes a protein of 222 amino acids possessing 79% sequence similarity to the predicted product of the pol orf224 gene. nap orf222 is co-transcribed with the third exon of the trans-spliced nad5 gene and another ORF. orf222 transcripts are several times more abundant in nap CMS than in fertility restored nap-cytoplasm plants and qualitative transcript differences for the region between CMS and restored plants are found as well. Expression of the orf222/nad5c/orf139 region is specifically correlated with nap CMS: of 21 mitochondrial gene regions examined, including all the sites of rearrangement between the nap and fertile cam mitochondrial genomes and 22 known genes, only the orf222/nad5c/orf139 region detected transcript differences between maintainer cam cytoplasm, nap CMS- and fertility restored nap cytoplasm-plants. Our results suggest that expression of the orf222/nad5c/orf139 region may be associated with nap CMS, and, more generally, that different forms of CMS may be associated with genes encoding structurally similar proteins.