Howard Hughes Medical Institute

Recombination event in cms-T regenerated plants during reversion to fertility --Christiane M.-R. Fauron and Marie Havlik The entire sequence complexity of the mitochondrial genome of the maize cytoplasmic male sterile cms-T is contained in a circular molecule also called master chromosome of 540 kb (Fauron et al., MGG 216:395, 1989). The master circle containing the entire sequence complexity is part of the multipartite structure arising via recombination at repeated sequences. In 1986 Dewey et al. (Cell 44:439) identified a rearranged sequence unique to the cms-T mitochondrial genome containing a 345 bp open reading frame (T-urf13) encoding a 13 kd polypeptide associated with the male sterile phenotype (Dewey et al., PNAS 84:5374, 1987). This sequence is located in cms-T on a 6.6 kb XhoI fragment that is absent in the revertant progenies obtained through tissue culture with the exception of two mutants, T4 (Umbeck and Gengenbach, Crop Sci. 23:584, 1983) and V18 (Brettell et al., MNL 56:13, 1982). This locus is located near a 4.6 kb repeat not found in the normal genome. It is either deleted (Fauron et al., Curr. Genet. 11:339, 1987; Rottman et al., EMBO J. 6:1541, 1987), or truncated (Wise et al., PNAS 84:2858, 1987) in the male fertile regenerated plants. We have shown that the deletion of the urf13 locus in the revertant V3 is the result of a recombination event involving two sets of repeats (Fauron et al., Genetics 123, 1990). As shown on Figure 1, associated with the 0.423 kb deletion encompassing the T-urf13 gene is a duplication of 165 kb. The study of various independently isolated revertants has shown that different sets of repeated sequences might be involved in the recombinational events responsible for the mt genome reorganization (Fauron et al., TAG in press, 1990).

Figure 1. Comparison of the diagrams of the master chromosomes of cms-T and V3 mitochondrial genomes with the location of the repeated sequences and the rRNA and protein coding genes. Located on the cms-T circle are the 165 kb region found duplicated in the V3 genome and the 0.423 kb region deleted in the V3 genome. The duplicated region is also marked on the V3 circle.

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