RF8 AND RF-ASTERISK MEDIATE UNIQUE T-URF13-TRANSCRIPT ACCUMULATION, REVEALING A CONSERVED MOTIF ASSOCIATED WITH RNA PROCESSING AND RESTORATION OF POLLEN FERTILITY IN T-CYTOPLASM MAIZE

Rf8 is a newly described nuclear gene that can substitute for Rf1 to p artially restore pollen fertility to male-sterile, T-cytoplasm maize. Families segregating for Rf8 were used to investigate the mechanism of this fertility restoration and to compare it to the restoration condi tioned by Rf1. Although Rf8 is unlinked to the rf1 locus, it also alte rs T-urf13 mitochondrial transcript accumulation and reduces the accum ulation of the URF13 protein. Like the 1.6- and 0.6-kilobase (kb) T-ur f13 transcripts that accumulate in T-cytoplasm plants carrying Rf1, 1. 42- and 0.42-kb transcripts accumulate in plants that are partially re stored by Rf8. A survey of T-cytoplasm maize lines, inbreds, and F-1 h ybrids by mitochondrial RNA gel blot analyses revealed that Rf8 is rar e in maize germplasm. These surveys revealed the presence of another r are, weak restorer factor, Rf, which is uniquely associated with the accumulation of 1.4- and 0.4-kb T-urf13 transcripts. Primer extension analyses position the 5' termini of the 1.42/0.42-kb and 1.4/0.4-kb tr anscripts at +137 and +159 nucleotides, respectively, 3' of the AUG in itiation codon of the T-urf13 reading frame. The conserved motif, 5'-C NACNNU-3', overlaps the 5' termini of the Rf1-, Rf8-, and Rf-associat ed transcripts and the 380 nucleotide, Rf3-associated orf107 transcrip t from cytoplasmic male sterility sorghum. These results demonstrate t hat multiple unlinked, nuclear genes can have similar but distinct eff ects on the expression of the unique T-urf13 mitochondrial coding sequ ence to restore pollen fertility to T-cytoplasm maize.