Evidence for informational suppression within the chloroplast of Chlamydomonas reinhardtii

Summary A balanced polymorphism within the single chloroplast of the green alga Chlamydomonas reinhardtii has been postulated to account for stable heteroplasmicity. Heteroplasmicity of mutant and wild-type alleles of a suppressor gene appears to be required for the suppression of a nonsense mutation in the chloroplast large-subunit gene (rbcL) of ribulosebisphosphate carboxylase/oxygenase. Whereas nonsense suppression has been demonstrated, its molecular basis remains to be determined. In the present study, the suppressed strain was found to synthesize both truncated and full-length large-subunit proteins. Since the rbcL nonsense mutation created a new restriction enzyme site, restriction-enzyme analysis and DNA hybridization were employed to show that only the mutant gene was present in the suppressed strain. It is thus likely that informational suppression is involved in the heteroplasmic suppression mechanism: the original nonsense mutation is present, but normal enzyme is produced comprised of full-length large subunits.