A 345 bp open reading frame of T cytoplasm mtDNA codes for a predicted polypeptide of 13 kd (Dewey et al., Cell 44:439, 1986). A polypeptide of this size is produced by amino acid incorporation by isolated T mitochondria (Forde and Leaver, Proc. Natl. Acad. Sci. 77:418, 1980). Plants regenerated from callus tissue culture of T cytoplasm, exhibiting male fertility and resistance to the toxins of race T of Cochliobolus heterostrophus (Bipolaris maydis) (Gengenbach et al., Proc. Natl. Acad. Sci. 74:5113, 1977), have this gene deleted, or in the case of a mutant designated T-4, the gene has a G to A transition adjacent to a 5 bp insertion (Wise et al., MNL 60:63, 1986; Proc. Natl. Acad. Sci., submitted). This insertion event places a TGA stop codon in frame 4 bp from the insertion, truncating the predicted polypeptide at 8.3 kd. Transcription of this region is unaltered in the T-4 mutant compared to T cytoplasm, suggesting that T-4 may synthesize an 8.3 kd polypeptide. 35 S-methionine incorporation by isolated mitochondria showed that T synthesized a prominent 13 kd polypeptide, which was absent in N, T-4, or T-7 (a deletion mutant). A unique polypeptide migrating at approximately 8 kd was synthesized by T-4. A highly immunogenic region of URF13-T was selected for the synthesis of a 17 amino acid polypeptide, designated PEP17. Polyspecific antibody to PEP17 was raised, and immunoprecipitation of native polypeptides from mitochondrial amino acid incorporation revealed precipitation of the 13 kd polypeptide, indicating that it is a gene product of URF13-T. A polypeptide of approximately 7.2 kd, similar to the size of a polypeptide identified as subunit 9 of ATPase, was also precipitated by the antibody from mitochondria of T but not T-7 or N, suggesting that the 13 kd protein may be part of a complex with the atp9 product.

Roger P. Wise, Albert E. Fliss, Jr., Daryl R. Pring and Burle G. Gengenbach
 
 

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