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Mitochondrial DNA in the T male-sterile cytoplasm (cms-T) of maize contains an open reading frame (ORF 13) associated with the T type of sterility. Antibodies raised to a chemically synthesized oligopeptide corresponding to ORF 13 were used to establish the expression of a 13-kDa protein from this reading frame. The 13-kDa polypeptide is synthesized uniquely in cms-T maize and purifies with the membrane fraction of T mitochondria. We assign the symbol urpf3-T to designate this mitochondrial gene. Presence of the nuclear restorer gene Rfl in cms-T plants results in a decrease in abundance of 13-kDa protein and alteration in the transcripts of urfl3-T.Cytoplasmic male sterility (cms) in higher plants represents one of the few well-characterized examples of heritable variability transmitted through the cytoplasm. Pollen production is aborted in cms plants, yet female fertility is unaffected. In maize (Zea mays L.), the trait has been used extensively in the commercial production of hybrid seed as a means of preventing self-fertilization. The three major malesterile cytoplasms of maize-S, C, and T-are distinguished according to the pattern of fertility restoration by nuclear restorer genes. The S and C cytoplasms require a single dominant restorer gene for fertility restoration-RJ3 and Rf4, respectively-whereas two restorer genes, Rfl and Rf2, are necessary to restore T cytoplasm cms (cms-T) plants to male fertility (1-4).In maize, abundant evidence indicates that changes in organization and expression of the mitochondrial genome are responsible for the cms trait (for review, see ref. 5). Maize plants carrying the T cytoplasm are also distinguished by an apparently inseparable association between male sterility and susceptibility to the fungal pathogen Bipolaris maydis, race T (6-8). This pathogen produces a host-specific toxin that affects the permeability of cms-T mitochondria and promotes the uncoupling of oxidative phosphorylation and the leakage of NAD+ and Ca2l (9-11). In contrast, mitochondria from male fertile (N), cms-C, and cms-S cytoplasms are insensitive to the toxin.Protein synthesis studies of isolated mitochondria have revealed differences between the protein products of T versus N mitochondria (12). In particular, a 13-kDa polypeptide is observed in T mitochondria that is absent in N, and a 21-kDa protein is synthesized in N mitochondria that is not detected in T. The 13-kDa protein is further characterized by a dramatically reduced abundance in cms-T plants that are restored to fertility by nuclear restorer genes Rfl and Rf2 (13).We have previously reported the identification of a highly rearranged DNA sequence, designated TURF 2H3, unique to the T cytoplasm of maize and associated with this type of sterility (14). TURF 2H3 contains two major open reading frames, ORF 13 and ORF 25, which may encode polypeptides of 13 and 25 kDa, respectively. The organization and transcription of ORF 13, so far as we know, are unique to the T cytoplasm of maize. Significantly, ORF 13 transcripts are modified...

The Texas cytoplasm of maize carries two cytoplasmically inherited traits, male sterility and disease susceptibility, which have been of great interest both for basic research and plant breeding. The two traits are inseparable and are associated with an unusual mitochondrial gene, T- urf13 , which encodes a 13-kilodalton polypeptide (URF13). An interaction between fungal toxins and URF13, which results in permeabilization of the inner mitochondrial membrane, accounts for the specific susceptibility to the fungal pathogens.

The Texas male-sterile cytoplasm (cms-T) of maize carries the cytoplasmically inherited trait of male sterility. Mitochondria isolated from cms-T maize are specifically sensitive to a toxin (BmT-toxin) produced by the fungal pathogen Bipolaris maydis, race T, and the carbamate insecticide methomyl. A mitochondrial gene unique to cms-T maize, which produces a 13-kilodalton polypeptide associated with cytoplasmic male sterility, was expressed in Escherichia coli. After addition of BmT-toxin or methomyl, inhibition of whole cell respiration and swelling of spheroplasts were observed in Escherichia coli cultures producing the novel mitochondrial protein; these effects are similar to those observed with isolated cms-T mitochondria. The amino-terminal region of the 13-kilodalton polypeptide appears to be essential for proper interaction with the BmT-toxin and methomyl. These results implicate the 13-kilodalton polypeptide in conferring toxin sensitivity to mitochondria of cms-T maize.

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