Periplasmic Carbonic Anhydrase Structural Gene (Cah1) Mutant in Chlamydomonas reinhardtii

To survive in various conditions of CO₂ availability, Chlamydomonas reinhardtii shows adaptive changes, such as induction of a CO₂-concentrating mechanism, changes in cell organization, and induction of several genes, including a periplasmic carbonic anhydrase (pCA1) encoded by Cah1. Among a collection of insertionally generated mutants, a mutant has been isolated that showed no pCA1 protein and no Cah1 mRNA. This mutant strain, designated cah1-1, has been confirmed to have a disruption in the Cah1 gene caused by a single Arg7 insert. The most interesting feature of cah1-1 is its lack of any significant growth phenotype. There is no major difference in growth or photosynthesis between the wild type and cah1-1 over a pH range from 5.0 to 9.0 even though this mutant apparently lacks Cah1 expression in air. Although the presence of pCA1 apparently gives some minor benefit at very low CO₂ concentrations, the characteristics of this Cah1 null mutant demonstrate that pCA1 is not essential for function of the CO₂-concentrating mechanism or for growth of C. reinhardtii at limiting CO₂ concentrations.