Isolation and Characterization of a Mutant of Chlamydomonas reinhardtii Deficient in the CO₂ Concentrating Mechanism

Abstract

A Chiamydomonas reinhardtii mutant has been isolated that cannot grow photoautotrophically on low CO2 concentrations but can grow on elevated CO2. In a test cross, the high C02-requirement for growth showed a 2:2 segregation. This mutant, designated CIA-5, had a phenotype similar to previously identified mutants that were defective in some aspect of CO2 accumulation. Unlike previously isolated mutants, CIA-5 did not have detectable levels of the periplasmic carbonic anhydrase, an inducible protein that participates in the acquisition of CO2 by C. reinhardtii. CIA-5 also did not accumulate inorganic carbon to levels higher than could be accounted for by diffusion. This mutant strain did not synthesize any of the four polypeptides preferentially made by wild type C. reinhardtii when switched from an environment containing elevated CO2 levels to an environment low in CO2. It is concluded that this mutant fails to induce the CO2 concentrating system and is incapable of adapting to low CO2 conditions. Chlamydomonas reinhardtii, like other unicellular green algae, has the capacity to adapt to varying CO2 concentrations in the environment (1-3, 5). When grown on elevated levels of CO2 (5% (v/v) in air), C. reinhardtii has a relatively low affinity for C12 and exhibits high rates of photorespiration when placed in a low CO2/high O2 environment (2,16,19,26). However, if C. reinhardtii remains exposed to low levels of CO2 (ambient CO2 levels), it adapts to these conditions by inducing a CO2 concentrating mechanism (2). This CO2 concentrating mechanism is thought to increase the CO2 at the

Concerted Up-regulation of Aldehyde/Alcohol Dehydrogenase (ADHE) and Starch in Chlamydomonas reinhardtii Increases Survival under Dark Anoxia

“…In these conditions, the cells rely on CCM to concentrate CO 2 at the active site of the Rubisco (44). The ADHE abundance was first assessed in two CCM mutant strains: strain CIA5, which lacks Ccm1, a master gene regulator that controls the induction of CCM by sensing CO 2 availability in C. reinhardtii (45,46), and strain CIA3, which lacks CAH3, the thylakoid lumen carbonic anhydrase, which provides CO 2 to the Rubisco (47). The ADHE abundance in these two strains grown under atmospheric conditions was similar, being 1.5-1.8-fold higher than that in wild-type strain CC-124 (Table 4).…”

The Cia5 gene controls formation of the carbon concentrating mechanism in Chlamydomonas reinhardtii

“…A controversy has existed from soon after the time that the cia5 mutant was first isolated and characterized (6) as to whether this potential regulator of CCM formation and function served to control induction of CO 2 -responsive genes or if it was responsible for strong repression of these genes under conditions of high CO 2 . Although more complex mechanisms can be envisioned, the simplest hypothesis that is consistent with present data is one in which CIA5 is an inducer of gene transcription in cells grown in low CO 2 environments, but is inactivated by posttranslational modification in cells switched to high levels of CO 2 .…”

The Low CO2-Inducible 36-Kilodalton Protein Is Localized to the Chloroplast Envelope of Chlamydomonas reinhardtii

“…Moreover, at the same time that the cell induces the CCM and increases its affinity for externa1 Ci, at least five polypeptides that are either absent or present in low amounts in cells grown on high COz concentrations, are induced (Coleman and Grossman, 1984;Manuel and Moroney, 1988;Spalding and Jeffrey, 1989). At the present time, there is little information about location and functional role of these proteins in the CCM, but a mutant strain of C. reinhardtii, cia-5, which cannot make these proteins, requires elevated COz for growth, implying that these proteins are involved in Ci acquisition (Moroney et al, 1989).…”