Role of the AFRD1-encoded fumarate reductase in hypoxia and osmotolerance in Arxula adeninivorans.

Fumarate reductase is an enzyme involved in maintaining redox balance through regeneration of reduced cofactors during oxygen deficiency conditions. This work reports the identification and characterization of the gene and its promoter and terminator elements that encodes cytosolic fumarate reductase enzyme in the nonconventional yeast, Arxula adeninivorans. The gene harbours an ORF of 1446 bp, encoding a 482-amino acid protein. The deduced amino acid sequence is similar to those of fumarate reductases from other yeast and fungi, such as the two fumarate reductases of Saccharomyces cerevisiae, Frd1p (44%) and Osm1p (41%). This enzyme is located in the cytosol and has a pH optimum of ca. 7.5 and a Michaelis constant (K(M)) of 2.9 mM with fumarate as the substrate. Expression of AFRD1 is regulated by the cultivation conditions. A shift from NaCl-free to NaCl-supplemented media and aerobic to hypoxic growth conditions leads to reduced AFRD1 transcription levels, but not to alteration in the concentration of Afrd1p. The functional analyses of Afrd1p were performed in A. adeninivorans and S. cerevisiae disruption mutants. The A. adeninivorans fumarate reductase is capable of functional complementation of the missing S. cerevisiae genes during anoxia; however, it is not involved in yeast growth under osmotic stress.