Nuclear-gene mutations suppress a defect in the expression of the chloroplast-encoded large subunit of ribulose-1,5-bisphosphate Carboxylase/Oxygenase.

The green alga Chlamydomonas reinhardtii mutant 76-5EN lacks photosynthesis because of a nuclear-gene mutation that specifically inhibits expression of the chloroplast gene encoding the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39). Photosynthesis-competent revertants were selected from mutant 76-5EN to explore the possibility of increasing Rubisco expression. Genetic analysis of 10 revertants revealed that most arose from suppressor mutations in nuclear genes distinct from the original 76-5EN mutant gene. The revertant strains have regained various levels of Rubisco holoenzyme, but none of the suppressor mutations increased Rubisco expression above the wild-type level in either the presence or absence of the 76-5EN mutation. One suppressor mutation, S107-4B, caused a temperature-conditional, photosynthesis-deficient phenotype in the absence of the original 76-5EN mutation. The S107-4B strain was unable to grow photosynthetically at 35 degreesC, but it expressed a substantial level of Rubisco holoenzyme. Whereas the 76-5EN gene encodes a nuclear factor that appears to be required for the transcription of the Rubisco large-subunit gene, the S107-4B nuclear gene may be required for the expression of other chloroplast genes.