A Mutant in the ADH1 Gene of Chlamydomonas reinhardtii Elicits Metabolic Restructuring during Anaerobiosis1[W]

• Published in: Plant physiology (Bethesda) Vol. 158; no. 3; pp. 1293 - 1305
• Main Authors: Magneschi, Leonardo, Catalanotti, Claudia, Subramanian, Venkataramanan, Dubini, Alexandra, Yang, Wenqiang, Mus, Florence, Posewitz, Matthew C, Seibert, Michael, Perata, Pierdomenico, Grossman, Arthur R
• Format: Journal Article
• Language: English
• Published: Rockville American Society of Plant Biologists 01.03.2012
• Subjects: Anoxia; Anoxic conditions; Aquatic plants; E coli; Environmental Stress and Adaptation to Stress; Ethanol; Metabolites; Mutants
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.111.191569

The green alga Chlamydomonas reinhardtii has numerous genes encoding enzymes that function in fermentative pathways. Among these, the bifunctional alcohol/acetaldehyde dehydrogenase (ADH1), highly homologous to the Escherichia coli AdhE enzyme, is proposed to be a key component of fermentative metabolism. To investigate the physiological role of ADH1 in dark anoxic metabolism, a Chlamydomonas adh1 mutant was generated. We detected no ethanol synthesis in this mutant when it was placed under anoxia; the two other ADH homologs encoded on the Chlamydomonas genome do not appear to participate in ethanol production under our experimental conditions. Pyruvate formate lyase, acetate kinase, and hydrogenase protein levels were similar in wild-type cells and the adh1 mutant, while the mutant had significantly more pyruvate:ferredoxin oxidoreductase. Furthermore, a marked change in metabolite levels (in addition to ethanol) synthesized by the mutant under anoxic conditions was observed; formate levels were reduced, acetate levels were elevated, and the production of CO(2) was significantly reduced, but fermentative H(2) production was unchanged relative to wild-type cells. Of particular interest is the finding that the mutant accumulates high levels of extracellular glycerol, which requires NADH as a substrate for its synthesis. Lactate production is also increased slightly in the mutant relative to the control strain. These findings demonstrate a restructuring of fermentative metabolism in the adh1 mutant in a way that sustains the recycling (oxidation) of NADH and the survival of the mutant (similar to wild-type cell survival) during dark anoxic growth.