Characterization of a Chlamydomonas reinhardtii mutant defective in a maltose transporter

Microalgae are potential sources of energy and high-value materials. To decipher the process of energy metabolism in green algae, we created a mutant pool of strain CC-503 of the model green microalga Chlamydomonas reinhardtii, by random insertion of an antibiotic resistance gene, and screened the pool for lines with altered carbon metabolism. We identified a mutant that harbored the antibiotic resistance gene in CrMEX1, a putative Maltose Exporter-Like protein 1 (Cre12.g486600.t1.2). The mutant had reduced levels of CrMEX1 expression and, similarly to the Arabidopsis mex1 knockout mutant, which cannot export maltose from the chloroplast, it over-accumulated starch granules in the chloroplast. The mutant’s lipid levels were slightly higher than those of the wild type, and its initial growth kinetics were not significantly different from those of the wild type, but the mutant culture did not reach the same high cell density as the wild type in acetate-containing culture medium under continuous light. These results suggest that CrMEX1 encodes a maltose transporter protein, and that export of photoassimilates from chloroplasts is necessary for normal Chlamydomonas growth, even under continuous light with an ample supply of carbon in the form of acetate.