The induction of the CO2 concentrating mechanism in a starch-less mutant of Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii the formation of a starch sheath surrounding the pyrenoid is observed when cells grown under high CO 2 (5% CO 2 in air) are transferred to low CO 2 (0.03%) conditions. Formation of the starch sheath occurs coincidentally with induction of the CO 2 concentrating mechanism and with de novo synthesis of 5 polypeptides with molecular masses of 21, 36, 37, 42-44 kDa. We studied the effect of CO 2 concentrations on photosynthesis, ultrastructure and protein synthesis in Chlamydomonas reinhardtii cw-15 (wild phenotype for photosynthesis) and in the starch-less mutant BAFJ-6, with the aim to clarify the role of the pyrenoid starch sheath in the operation of the CO 2 concentrating mechanism and whether these low CO 2 -inducible polypeptides are involved in the formation of starch sheath. When wild type and starch-less mutant cells were transferred from high to low CO 2 , the CO 2 requirement for half-maximal rates of photosynthesis decreased from 40 μM to 2 μM CO 2 . 35 SO 2- 4 labeling analyses showed that the starch-less mutant induced the 5 low CO 2 -inducible polypeptides. These observations suggest that the starch-less mutant was able to induce a fully active CO 2 concentrating mechanism. Since the starch-less mutant did not form a pyrenoid starch sheath, we suggest that the starch sheath is not involved in the operation of the CO 2 concentrating mechanism and that none of these 5 low CO 2 -inducible proteins is involved in the formation of the starch sheath in Chlamydomonas.