An inorganic carbon transport system responsible for acclimation specific to air levels Of CO2 in Chiamlydomonas reinhardtii

Many photosynthetic microorganisms acclimate to CO2 limited environments by induction and operation of CO2-concentrating mechanisms (CCMs). Despite their central role in CCM function, inorganic carbon (G) transport systems never have been identified in eukaryotic photosynthetic organisms. In the green alga Chlamy-domonas reinhardtii, a mutant, pmp1, was described in 1983 with deficiencies in Ci transport, and a Pmp1 protein-associated G uptake system has been proposed to be responsible for Ci uptake in low CO2. (air level)-acclimated cells. However, even though pmp1 represents the only clear genetic link to Ci transport in microalgae and is one of only a very few mutants directly affecting the CCM itself, the identity of Pmp1 has remained unknown. Physiological analyses indicate that C reinhardtii possesses multiple Ci transport systems responsible for acclimation to different levels of limiting CO2 and that the Pmp1-associated transport system is required specifically for low (air level) CO2 acclimation. In the current study, we identified and characterized a pmp1 allelic mutant, air dier 1 (adi) that, like pmp1, cannot grow in low CO2 (350 ppm) but can grow either in high CO2 (5% CO2) or in very low CO2 (< 200 ppm). Molecular analyses revealed that the Ad1/Pmp1 protein is encoded by LciB, a gene previously identified as a CO2-responsive gene. LciB and three related genes in C reinhardtii compose a unique gene family that encode four closely related, apparently soluble plastid proteins with no clearly identifiable conserved motifs.