TEMPERATURE-CONDITIONAL NUCLEAR MUTATION OF CHLAMYDOMONAS-REINHARDTIIDECREASES THE CO2 O2 SPECIFICITY OF CHLOROPLAST RIBULOSEBISPHOSPHATE CARBOXYLASE OXYGENASE/

The Chlamydomonas reinhardtii (Dangeard) temperature-conditional mutan t 68-11AR is phenotypically indistinguishable from the wild type at th e permissive temperature (25-degrees-C), but has greatly reduced photo synthetic ability and requires acetate for growth at the restrictive t emperature (35-degrees-C). The mutant strain is deficient in ribulose- 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) holoenzy me when grown at 35-degrees-C. This decrease in the level of enzyme ap pears to be due to degradation of assembled holoenzyme rather than to a reduction in the synthesis of enzyme subunits. When grown at 25-degr ees-C, the mutant has a substantial amount of Rubisco. Enzyme purified from 25-degrees-C-grown mutant cells was found to have a 16% decrease in the CO2/O2 specificity factor when compared to the wild-type enzym e. This alteration was accompanied by changes in the kinetic constants for both carboxylation and oxygenation. Although the Rubisco active s ite is located on the chloroplast-encoded large subunit, genetic analy sis showed that the 68-11AR strain arose from a nuclear-gene mutation. The two nuclear genes that encode the Rubisco small subunits (rbcS1 a nd rbcS2) were cloned from mutant 68-11AR and completely sequenced, bu t no mutation was found. Analysis of restriction-fragment length polym orphisms also failed to detect linkage between mutant and rbcS gene lo ci. These results indicate that nuclear genes can influence Rubisco ca talysis without necessarily encoding polypeptides that reside within t he holoenzyme.