CHARACTERIZATION OF PROTEIN-SYNTHESIS BY ISOLATED RICE MITOCHONDRIA

Bacteria-free mitochondria were isolated from aseptically grown, etiol ated and green seedlings of both cytoplasmic male-sterile (WA-type) an d male-fertile rice (Oryza sativa L.). Protein synthesis in these isol ated mitochondria was characterized by gel electrophoresis/fluorograph y and by the incorporation of [S-35]-methionine into protein. In the p resence of cycloheximide, a set of some 25 discrete polypeptides and a n electrophoretically unresolved population were synthesized. This pat tern of protein synthesis in organello was essentially the same in mit ochondria isolated from both male-fertile and male-sterile cytoplasms. Our data does not preclude the possibility, however, that the WA-type CMS possesses a tissue-specific and/or a low abundance mitochondrial protein(s), whose synthesis eluded detection under our experimental co nditions. The synthesis of the mitochondria-encoded polypeptides by is olated rice mitochondria was inhibited by chloramphenicol and incomple tely inhibited by erythromycin. A minor chloramphenicol-insensitive, c ycloheximide-sensitive translation activity was found consistently to copurify with the mitochondria. This activity generated a reproducible electrophoretic profile of a poorly resolved, weakly labelled populat ion of polypeptides and of a few conspicuous polypeptides, including a 42 kDa species.