Expression of a Low CO2–Inducible Protein, LCI1, Increases Inorganic Carbon Uptake in the Green Alga Chlamydomonas reinhardtii    

Abstract

Aquatic photosynthetic organisms can modulate their photosynthesis to acclimate to CO 2 -limiting stress by inducing a carbon-concentrating mechanism (CCM) that includes carbonic anhydrases and inorganic carbon (Ci) transporters. However, to date, Ci-specific transporters have not been well characterized in eukaryotic algae. Previously, a Chlamydomonas reinhardtii mutant (lcr1) was identified that was missing a Myb transcription factor. This mutant had reduced lightdependent CO 2 gas exchange (LCE) activity when grown under CO 2 -limiting conditions and did not induce the CAH1 gene encoding a periplasmic carbonic anhydrase, as well as two as yet uncharacterized genes, LCI1 and LCI6. In this study, LCI1 was placed under the control of the nitrate reductase promoter, allowing for the induction of LCI1 expression by nitrate in the absence of other CCM components. When the expression of LCI1 was induced in the lcr1 mutant under CO 2 -enriched conditions, the cells showed an increase in LCE activity, internal Ci accumulation, and photosynthetic affinity for Ci. From experiments using indirect immunofluorescence, LCI1-green fluorescent protein fusions, and cell fractionation procedures, it appears that LCI1 is mainly localized to the plasma membrane. These results provide strong evidence that LCI1 may contribute to the CCM as a component of the Ci transport machinery in the plasma membrane.

Transcriptome-Wide Changes in Chlamydomonas reinhardtii Gene Expression Regulated by Carbon Dioxide and the CO2-Concentrating Mechanism Regulator CIA5/CCM1

“…Some of these LCI genes, such as CAH3, LCI1, LCIA, LCIB, and HLA3, reportedly play important roles in the C. reinhardtii CCM (Galván et al, 2002;Wang and Spalding, 2006;Duanmu et al, 2009aDuanmu et al, , 2009bOhnishi et al, 2010), and the function of others, such as CCP1, CCP2, and LCIC, in the CCM also has been implicated (Pollock et al, 2004;Wang and Spalding, 2006;Yamano et al, 2010). However, only 53 of 106 previously reported LCI genes were supported as L-CO 2 or VL-CO 2 upregulated DE genes in this study.…”

“…Two CIA5-regulated genes encoding candidate transporters have been implicated in C i transport across the plasma membrane: high light-induced gene3 (HLA3) encodes a putative ATP binding cassette type transporter and is induced under low CO 2 conditions, and knockdown of its expression impairs photosynthesis, C i uptake, and growth in alkaline conditions (Duanmu et al, 2009a). LCI1 encodes a plasma membrane protein reported to increase C i uptake in LCR1 mutants when expressed transgenically (Ohnishi et al, 2010). Two Rhesus-like proteins, RHP1 and RHP2, also are predicted to be plasma membrane located (Yoshihara et al, 2008).…”

Acclimation to Very Low CO2: Contribution of Limiting CO2 Inducible Proteins, LCIB and LCIA, to Inorganic Carbon Uptake in Chlamydomonas reinhardtii

“…However, if LCIA does function as a channel to facilitate passive entry of HCO 3 2 into the chloroplast, it would have to depend on the activity of plasma membrane Ci transporters (e.g. HLA3 and LCI1; Duanmu et al, 2009a;Ohnishi et al, 2010) to generate a sufficiently high HCO 3 2 concentration in the relatively small volume between the plasma membrane and chloroplast envelope to establish a HCO 3 2 concentration gradient sufficient to overcome the electrical gradient and push HCO 3 2 across the inner envelope. An LCIA channel would also have to be highly regulated to prevent reversed flow when the HCO 3 2 gradient was not favorable for uptake.…”

“…Some areas of uncertainty regarding key features of the microalgal CCM remain to be addressed. First, Ci, including the charged species, bicarbonate (HCO 3 reach Rubisco, yet the roles, if any, of proposed and confirmed Ci transporters, including the plasma membrane proteins HLA3 and Limiting CO 2 Inducible1 (LCI1) and the chloroplast envelope proteins LCIA (NAR1.2), CCP1, and CCP2 (Im and Grossman, 2002;Miura et al, 2004;Pollock et al, 2004;Mariscal et al, 2006;Duanmu et al, 2009a;Ohnishi et al, 2010), in acclimation and Ci uptake under different limiting CO 2 conditions are not yet clearly defined. Second, previous physiological studies have demonstrated that active uptake of both CO 2 and HCO 3 2 occurs in C. reinhardtii (Moroney and Tolbert, 1985;Sültemeyer et al, 1989), but the molecular components and the underlying mechanism responsible for active CO 2 uptake are still largely unknown.…”

Inorganic carbon utilization by aquatic photoautotrophs and potential usages of algal primary production

“…Key components, which have been found recently, for a pyrenoid-based CCM, LCIB/C, were described and a CO 2 recapturing hypothesis by LCIB/C complex surrounding the pyrenoid was proposed in the review. Also, candidate HCO 3 -transporter proteins, required for transport across the plasma membrane, chloroplast envelope, and the thylakoid membranes, and a possible CO 2 channel, the Rhesus factor, HLA, were described (one of those, LCI1, has now been shown to be HCO 3 -transporter; Ohnishi et al 2010). The pyrenoid forming factor LCIB/C was found by the analysis of pmp1 and ad1 mutants of C. reinhardtii, which are unable to grow at air-level CO 2 but able to grow under very low CO 2 conditions.…”