BRASSICA NAP CYTOPLASMIC MALE-STERILITY IS ASSOCIATED WITH EXPRESSIONOF A MTDNA REGION CONTAINING A CHIMERIC GENE SIMILAR TO THE POL CMS-ASSOCIATED ORF224 GENE

Two different cytoplasmic male-sterility (CMS) systems, nap and pol, a re 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 rest ricted to a relatively small region immediately upstream of the atp6 g ene. An approximately 4.5-kb pol mtDNA segment containing a chimeric o pen reading frame (orf224) co-transcribed with atp6 is missing from ca m mtDNA and located at a different site on nap mtDNA; expression of th e orf224/atp6 gene region is highly correlated with the pal 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 produ ct of the pol orf224 gene. nap orf222 is co-transcribed with the third exon of the trans-spliced nad5 gene and another ORF. orf222 transcrip ts are several times more abundant in nap CMS than in fertility restor ed 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 site s of rearrangement between the nap and fertile cam mitochondrial genom es and 22 known genes, only the orf222/nad5c/orf139 region detected tr anscript differences between maintainer cam cytoplasm, nap CMS- and fe rtility restored nap cytoplasm-plants. Our results suggest that expres sion of the orf222/nad5c/orf139 region may be associated with nap CMS, and, more generally, that different forms of CMS may be associated wi th genes encoding structurally similar proteins.