The Chlamydomonas reinhardtii temperature-sensitive mutant 68-4PP results from a mutation within the chloroplast gene that encodes the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. When grown at the permissive temperature (25 degrees C), the mutant has a reduced level of holoenzyme protein, and the purified enzyme has a lower CO2/O2 specificity than the wild-type enzyme. At the nonpermissive temperature (35 degrees C), the holoenzyme level is greatly reduced, and the mutant is unable to grow photosynthetically. When photosynthesis-competent revertants of 68-4PP were selected at 35 degrees C, a nuclear mutation was identified that suppresses the temperature-sensitive phenotype by enhancing both the activity and amount of the mutant enzyme. More significantly, the reduced CO2/O2 specificity of the 68-4PP enzyme is restored to the wild-type value. However, the nuclear suppressor mutation alone does not produce a phenotype different from wild type, and the CO2/O2 specificity of the suppressor strain's enzyme is normal. We have cloned and completely sequenced the two small-subunit genes from the suppressor strain, but no mutation has been found. These results suggest that some other nuclear-encoded protein is able to influence the structure of the holoenzyme, which in turn influences the CO2/O2 specificity factor.