TRANSCRIPT PROCESSING INTERNAL TO A MITOCHONDRIAL OPEN READING FRAME IS CORRELATED WITH FERTILITY RESTORATION IN MALE-STERILE SORGHUM

A chimeric mitochondrial DNA (mtDNA) configuration of the cytoplasmic male-sterile (cms) sorghum line IS1112C includes a 321 bp open reading frame designated orf107, encoding a predicted polypeptide product of 11.85 kDa. The open reading frame, similar to several other genes asso ciated with cms, consists of amino-terminal sequences derived from an obligate gene. Unlike other examples to date, however, the carboxy-ter minal sequences are highly similar to the carboxy terminus of an open reading frame implicated in cms of rice, orf79. The amino-terminal 31 residues of orf107 are 84% similar to atp9, and the carboxy-terminal 4 9 residues are 57% identical and 80% similar to the carboxy terminus o f orf79. Transcripts of orf107 are edited, with four C-to-U changes th at alter amino acids. Sorghum lines partially or fully restored to fer tility exhibit a high-efficiency internal-orf107 transcript processing activity, precluding abundant whole-length transcripts, while male-st erile lines exhibit only a trace of the activity. Previous data on the abundance of a 12 kDa in organello-synthesized polypeptide in male-st erile versus male-fertile lines are correlated with differential orf10 7 transcript processing activity of these lines. Examinations of backc ross and F-2 lines suggest a gametophytic mode of restoration, and ind icate that enhanced transcript processing activity is necessary, but n ot sufficient, to restore full fertility. These novel observations ind icate that mitochondrial open reading frames associated with cms in di fferent species can include highly similar motifs, and that fertility restoration could involve a mechanism by which synthesis of a ems-asso ciated gene product may be precluded through internal transcript cleav age.