Enzyme lesions in galactose non-utilising mutants of Aspergillus nidulans

Abstract

1. 1. Accumulation of D-galactose from the medium by intact mycelium and the enzymes of the Leloir pathway for galactose metabolism have been investigated in wild type and galactose non-utilising mutants of Aspergillus nidulans. 2. 2. Accumulation of D-galactose is a reversible energy dependent process, antagonised by D-glucose, and is a constitutive property of the organism. No mutants were found to lack this function. 3. 3. The activity of galactokinase, galactose- I-phosphate uridyl transferase and UDP galactose-4-epimerase were measured in galactose grown organisms and in non-induced (glycerol grown) or induced (glycerol + galactose grown) organisms. All three enzymes are readily detectable in non-induced in non-induced organisms and icnrease 3–5 fold on induction when activities are comparable to those in galactose grown organisms. D-Fucose is not an inducer. 4. 4. Mutants at the gal9 locus lack galactokinase and those at the unlinked gal5 locus lack transferase. 5. 5. Recessive mutants at a third non-linked locus, gal1, contain normal constitutive enzyme activities, but there is no induction of galactokinase or transferase, though to epimerase increases normally, in cultures grown in the presence of galactose (up to 0.55 M). The mutants are permeable to galactose. 6. 6. Evidence is presented for two systems of galactose utilisation in A. nidulans regulated by pH of the medium. The wild type grows on media at pH 4.0–7.5 with galactose as carbon source. The following suggest that the Leloir pathway operates at pH 4.0–6.5 but some other route at Ph 7.5 (a) Wild type grown on galactose or induced at pH 4.0–6.5 contains all three enzymes, but when grown or induced at pH 7.5 no galactokinase activity can be detected though transferase and epimerase are present. (b) Mutants lacking galactokinase or transferase do not grow on galactose at pH 4.0–6.5, but grow normally at pH 7.5.