The unicellular green alga Chlamydomonasreinhardtii acclimates to low-CO2 (LC) conditions by actively transporting inorganic carbon (Ci) into the cell, resulting in an increase in photosynthetic efficiency. This mechanism is called the carbon-concentrating mechanism (CCM), and soluble protein LCIB is essential for the CCM. LCIB is localized in the vicinity of pyrenoid, a prominent structure in the chloroplast, under LC conditions in the light. In contrast, in the dark or in high-CO2 conditions, where the CCM is inactive, LCIB diffuses away from the pyrenoid. Although the functional importance of LCIB for the CCM has been shown, the significance and mechanism of the change in suborganellar localization of LCIB remain to be elucidated. In this study, we screened 13,000 DNA-tagged mutants and isolated twelve aberrant LCIB localization (abl) mutants under LC conditions. abl-1 and abl-3 with dispersed and speckled localization of LCIB in the chloroplast showed significant decreases in Ci affinity, Ci accumulation, and CO2 fixation. Ten abl mutants (abl-1, abl-3, abl-4, abl-5, abl-6, abl-7, abl-8, abl-9, abl-11, and abl-12) showed not only aberrant LCIB localization but also reduced pyrenoid sizes. Moreover, three abl mutants (abl-10, abl-11, and abl-12) showed the increased numbers of pyrenoids per cell. These results suggested that the specific LCIB localization could be related to pyrenoid development.
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We thank Ms. Hiro Iguchi for technical assistance. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Nos 23120514, 22380059, and 25120714 to H. F., and 25840109 to T. Y.), and the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA).
Fig. S1Subcellular localization of endogenous LCIB and LCIC in parental strain cells and abl mutants. Parental strain cells and abl mutants were grown in low-CO2 (LC) conditions, and subcellular localization of LCIB and LCIC was analyzed using an indirect immunofluorescence assay, as described previously (Yamano et al. 2010). DIC, differential image contrast. Bar 5 μm. Supplementary material 1 (PDF 1407 kb)
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