A CYTOPLASMIC MALE STERILITY-ASSOCIATED MITOCHONDRIAL PEPTIDE IN COMMON BEAN IS POSTTRANSLATIONALLY REGULATED

Cytoplasmic male sterility in the common bean plant is associated with a dominant mitochondrial mutation designated pvs-orf239 (for Phaseolu s vulgaris sterility sequence open reading frame 239). The sequence is transcribed in both vegetative and reproductive tissues, but the tran slation product, ORF239, is present only in reproductive tissues. We p resent evidence to support a model of post-translational regulation of ORF239 expression based on the following observations. In organello t ranslation experiments using purified mitochondria from young seedling s demonstrated accumulation of ORF239 only when a protease inhibitor w as included. Proteolytic activity against ORF239 was observed in mitoc hondrial extracts fractionating with the mitochondrial inner membrane. The DNA sequence encoding a serine-type protease, similar to the ion protease gene of Escherichia coli, was cloned from the Arabidopsis gen ome. The expression product of this sequence demonstrated proteolytic activity against ORF239 in vitro, with features resembling the activit y detected in mitochondrial inner membrane preparations. Antibodies ge nerated against the overexpressed Lon homolog reduced proteolytic acti vity against ORF239 when added to mitochondrial extracts. Our data sug gest that ORF239 was undetected in vegetative tissue due to rapid turn over by at least one mitochondrial protease that acts against ORF239 p ost-translationally.