Knock-down of the COX3 and COX17 gene expression of cytochrome c oxidase in the unicellular green alga Chlamydomonas reinhardtii

Abstract

The COX3 gene encodes a core subunit of mitochondrial cytochrome c oxidase (complex IV) whereas the COX17 gene encodes a chaperone delivering copper to the enzyme. Mutants of these two genes were isolated by RNA interference in the microalga Chlamydomonas. The COX3 mRNA was completely lacking in the cox3-RNAi mutant and no activity and assembly of complex IV were detected. The cox17-RNAi mutant presented a reduced level of COX17 mRNA, a reduced activity of the cytochrome c oxidase but no modification of its amount. The cox3-RNAi mutant had only 40% of the wild-type rate of dark respiration which was cyanide-insensitive. The mutant presented a 60% decrease of H(2)O(2) production in the dark compared to wild type, which probably accounts for a reduced electron leakage by respiratory complexes III and IV. In contrast, the cox17-RNAi mutant showed no modification of respiration and of H(2)O(2) production in the dark but a two to threefold increase of H(2)O(2) in the light compared to wild type and the cox3-RNAi mutant. The cox17-RNAi mutant was more sensitive to cadmium than the wild-type and cox3-RNAi strains. This suggested that besides its role in complex IV assembly, Cox17 could have additional functions in the cell such as metal detoxification or Reactive Oxygen Species protection or signaling. Concerning Cox3, its role in Chlamydomonas complex IV is similar to that of other eukaryotes although this subunit is encoded in the nuclear genome in the alga contrary to the situation found in all other organisms.

Divergent functions of the Arabidopsis mitochondrial SCO proteins: HCC1 is essential for COX activity while HCC2 is involved in the UV-B stress response

“…Plants with only one intact HCC1 copy looked phenotypically indistinguishable from the WT (Figure 2D). In agreement with this, COX-deprived mutants of the green alga Chlamydomonas reinhardtii continued to grow like the WT when kept in the light (Colin et al, 1995; Remacle et al, 2010). Apparently, the reduced production of ATP is compensated by photosynthesis, or other metabolic pathways such as glycolysis may contribute some ATP.…”

Biogenesis and assembly of eukaryotic cytochrome c oxidase catalytic core

“…In both cases, Cox1 and Cox2 are rapidly degraded in the absence of Cox3. Also in Chlamydomonas , where Cox3 is encoded by a nuclear gene and presents a lower hydrophobicity than other Cox3 mitochondria-encoded proteins [20], the lack of this subunit leads to the absence of COX [206]. These results show that, independently of its genetic origin, the essential role of Cox3 in the assembly or stability of the catalytic core of COX is conserved though evolution.…”

Elemental Economy

“…Since then, C. reinhardtii has become a premier reference organism for understanding mechanisms of Cu sensing and sparing in photosynthetic systems (Merchant, 1998). C. reinhardtii has 3 abundant Cu proteins, plastocyanin, Cyt oxidase and a multicopper oxidase involved in Fe assimilation, plus a number of other less abundant ones (Li et al, 1996, Chen et al, 2008, Remacle et al, 2010, Castruita et al, 2011). In an Fe-replete situation (when ferroxidase expression is repressed), the Cu quota is determined largely by plastocyanin and Cyt oxidase.…”