Isolation and Characterization of a Mutant of Chlamydomonas reinhardtii Deficient in the CO(2) Concentrating Mechanism.

A Chlamydomonas reinhardtii mutant has been isolated that cannot grow photoautotrophically on low CO(2) concentrations but can grow on elevated CO(2). In a test cross, the high CO(2)-requirement for growth showed a 2:2 segregation. This mutant, designated CIA-5, had a phenotype similar to previously identified mutants that were defective in some aspect of CO(2) accumulation. Unlike previously isolated mutants, CIA-5 did not have detectable levels of the periplasmic carbonic anhydrase, an inducible protein that participates in the acquisition of CO(2) by C. reinhardtii. CIA-5 also did not accumulate inorganic carbon to levels higher than could be accounted for by diffusion. This mutant strain did not synthesize any of the four polypeptides preferentially made by wild type C. reinhardtii when switched from an environment containing elevated CO(2) levels to an environment low in CO(2). It is concluded that this mutant fails to induce the CO(2) concentrating system and is incapable of adapting to low CO(2) conditions.