The nuclear gene Rf3 affects the expression of the mitochondrial chimeric sequence R implicated in S-type male...

The mitochondrial genomes of maize plants exhibiting S-type cytoplasmic male sterility (cms-S) contain a repeated DNA region designated R. This region was found to be rearranged in the mitochondria of all cms-S cytoplasmically revertant fertile plants in all nuclear backgrounds analyzed. A 1.6-kb mRNA transcribed from the R region in mitochondria of sterile plants was absent from all cytoplasmic revertants examined. The nuclear gene Rf3, which suppresses the cms-S phenotype, was found to have a specific effect on the expression of the R sequence; the abundance of the major R transcripts, including the cms-S-specific 1.6-kb mRNA, is decreased in mitochondria of restored plants. Nucleotide sequence analysis of R has revealed similarities to the R1 plasmid found in some South American maize races with RU cytoplasm, to the M1 plasmid found in one source of Zea luxurians teosinte, to the atp9 mitochondrial gene and its 3' flanking sequence, and also to a region 3' to the orf221 gene. The derived amino acid sequence of the R region predicts two open reading frames (ORFs). These ORFs contain the similarities to R1, M1, atp9 and orf221. The present report reveals the chimeric nature of the R region, describes the complex effect of Rf3 on the expression of the R sequence and implicates R in the sterile phenotype of cms-S maize.