title: Flexibility in anaerobic metabolism as revealed in a mutant of Chlamydomonas reinhardtii lacking hydrogenase activity.
abstract: The green alga Chlamydomonas reinhardtii has a network of fermentation pathways that become active when cells acclimate to anoxia. Hydrogenase activity is an important component of this metabolism, and we have compared metabolic and regulatory responses that accompany anaerobiosis in wild-type C. reinhardtii cells and a null mutant strain for the HYDEF gene (hydEF-1 mutant), which encodes an [FeFe] hydrogenase maturation protein. This mutant has no hydrogenase activity and exhibits elevated accumulation of succinate and diminished production of CO2 relative to the parental strain during dark, anaerobic metabolism. In the absence of hydrogenase activity, increased succinate accumulation suggests that the cells activate alternative pathways for pyruvate metabolism, which contribute to NAD(P)H reoxidation, and continued glycolysis and fermentation in the absence of O2. Fermentative succinate production potentially proceeds via the formation of malate, and increases in the abundance of mRNAs encoding two malate-forming enzymes, pyruvate carboxylase and malic enzyme, are observed in the mutant relative to the parental strain following transfer of cells from oxic to anoxic conditions. Although C. reinhardtii has a single gene encoding pyruvate carboxylase, it has six genes encoding putative malic enzymes. Only one of the malic enzyme genes, MME4, shows a dramatic increase in expression (mRNA abundance) in the hydEF-1 mutant during anaerobiosis. Furthermore, there are marked increases in transcripts encoding fumarase and fumarate reductase, enzymes putatively required to convert malate to succinate. These results illustrate the marked metabolic flexibility of C. reinhardtii and contribute to the development of an informed model of anaerobic metabolism in this and potentially other algae.
authors.affiliation: Environmental Science and Engineering Division, Colorado School of Mines, Golden, Colorado 80401, USA.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Dubini
authors.forename: Florence
authors.initials: F
authors.lastname: Mus
authors.forename: Michael
authors.initials: M
authors.lastname: Seibert
authors.forename: Arthur R
authors.initials: AR
authors.lastname: Grossman
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Posewitz
mesh_terms: D020418:Algal Proteins / Q000235:genetics / Q000378:metabolism*; D000693:Anaerobiosis / Q000502:physiology; D000818:Animals; D002245:Carbon Dioxide / Q000378:metabolism; D016825:Chlamydomonas reinhardtii / Q000201:enzymology* / Q000235:genetics; D010450:Endopeptidases / Q000235:genetics / Q000378:metabolism*; D006864:Hydrogenase / Q000235:genetics / Q000378:metabolism*; D008291:Malate Dehydrogenase / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D009249:NADP / Q000235:genetics / Q000378:metabolism; D010084:Oxidation-Reduction; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism*; D019289:Pyruvic Acid / Q000378:metabolism*; D013385:Succinate Dehydrogenase / Q000235:genetics / Q000378:metabolism; D019802:Succinic Acid / Q000378:metabolism
chemical_list: D020418:Algal Proteins; D015800:Protozoan Proteins; D002245:Carbon Dioxide; D009249:NADP; D019289:Pyruvic Acid; D019802:Succinic Acid; D008291:Malate Dehydrogenase; D006864:Hydrogenase; D013385:Succinate Dehydrogenase; D010450:Endopeptidases; C119558:hydrogenase maturating endopeptidase HYBD