Heteroplasmic suppression of an amber mutation in the Chlamydomonas chloroplast gene that encodes the large subunit of ribulosebisphosphate carboxylase/oxygenase

Summary

The 18-5B and 18-7G mutants of Chlamydomonas reinhardtii lack ribulosebisphosphate carboxylase/oxygenase holoenzyme and contain nonsense mutations in the chloroplast gene that encodes the protein's large subunit. Spontaneous revertants of the 18-5B opal (UGA) mutant were found to be heteroplasmic in a previous study (Spreitzer et al. 1984). They appeared to contain both mutant and wild-type alleles of a suppressor gene within the chloroplast. However, revertants of the 18-7G amber (UAG) mutant could not be recovered spontaneously. In the present investigation, revertants of the opal and amber mutants were recovered after a mutagen treatment. Heteroplasmic suppression of the 18-7G amber mutant was observed, suggesting that heteroplasmic suppression may be a common genetic mechanism of polyploid genomes. Although a diversity of other revertant types was also observed, no significant alteration occurred in the oxygen sensitivity of ribulose-bisphosphate carboxylase acitivity.