A pyrenoid-localized protein SAGA1 is necessary for Ca2+-binding protein CAS-dependent expression of nuclear genes encoding inorganic carbon transporters in Chlamydomonas reinhardtii.

Microalgae induce a CO2-concentrating mechanism (CCM) to maintain photosynthetic affinity for dissolved inorganic carbon (Ci) under CO2-limiting conditions. In the model alga Chlamydomonas reinhardtii, the pyrenoid-localized Ca2+-binding protein CAS is required to express genes encoding the Ci-transporters, high-light activated 3 (HLA3), and low-CO2-inducible protein A (LCIA). To identify new factors related to the regulation or components of the CCM, we isolated CO2-requiring mutants KO-60 and KO-62. These mutants had insertions of a hygromycin-resistant cartridge in the StArch Granules Abnormal 1 (SAGA1) gene, which is necessary to maintain the number of pyrenoids and the structure of pyrenoid tubules in the chloroplast. In both KO-60 and the previously identified saga1 mutant, expression levels of 532 genes were significantly reduced. Among them, 10 CAS-dependent genes, including HLA3 and LCIA, were not expressed in the saga1 mutants. While CAS was expressed normally at the protein levels, the localization of CAS was dispersed through the chloroplast rather than in the pyrenoid, even under CO2-limiting conditions. These results suggest that SAGA1 is necessary not only for maintenance of the pyrenoid structure but also for regulation of the nuclear genes encoding Ci-transporters through CAS-dependent retrograde signaling under CO2-limiting stress.