Characterization of alternative respiratory pathways in the yeast Schwanniomyces castellii by the study of mutants deficient in cytochromes a+a3 and/or b.

: After a general review of the proposed mechanisms and physiological roles of the alternative respiratory pathways found in various organisms, the studies are focussed on the amylolytic yeast Schwaniomyces castellii. In addition to the cytochrome chain, the wild type presents two alternative pathways insensitive to antimycin A. One is salicylhydroxamic acid (SHAM)-sensitive and azide-insensitive; the other is SHAM-insensitive and sensitive to high azide concentration. Conditions for mutagenesis and screening are described, which allow isolation of mutants deficient in cytochromes a+a3 and/or b in this yeast previously classified as petite negative. The relative proportions of the alternative respiratory pathways are compared in the wild type and mutant strains following inhibition by SHAM and azide at optimal concentration as determined by iso-inhibition curves. The growth of the cytochrome deficient mutants on citrate, a non-fermentable carbon source, and the ability of the wild type to grow on citrate+antimycin A, after a lag of about 10 h, indicate an involvement of the alternative pathway(s) in energy production. Rotenone sensitivity of respiration and ATP level confirm the presence of a functional phosphorylation site 1. The role of each alternative respiratory pathway in energy production is discussed.