Aspergillus galactose metabolism is more complex than that of Saccharomyces: the story of GalDGAL7 and GalEGAL1

Abstract

Saccharomyces cerevisiae Hansen GAL1 (galactokinase) generates galactose-1-phosphate; GAL7 (galactose-1-phosphate uridylyltransferase) transfers UDP between galactose or glucose and their respective sugar-1-phosphate conjugates, and both are essential on galactose. Aspergillus nidulans ANID_04957 has 41% amino acid sequence identity with GAL1; ANID_06182 has 50% sequence identity with GAL7. The names Aspergillus nidulans GalE (galactokinase) and GalD (galactose-1-phosphate uridylyltransferase) are consistent with prior studies. Complemented galDΔ:ScGAL7 and galEΔ:ScGAL1 strains had wild-type phenotype, demonstrating functional homology. The galD5 and galE9 alleles were truncated. Strains galDΔ and galD5 were impaired on minimal medium containing 1% galactose (MM-Gal) at pH 7.5 and did not grow on MM-Gal pH 4.5. Strains galEΔ and galE9 grew on MM-Gal at both pH levels. Strains galDΔ and galEΔ produced wild-type conidiophores on minimal medium containing 1% glucose (MM-Glu) but few spores; for both, sporulation was lower on MM-Gal pH 7.5. GalD-GFP (green fluorescent protein) and GalE-GFP were cytosolic and upregulated on MM-Gal, consistent with quantitative real-time polymerase chain reaction. Galactofuranose immunolocalization in galDΔ resembled wild type on MM-Glu but was reduced on MM-Gal. The galEΔ strains had immunolocalizable Galf on all these media. Strains galDΔ and galEΔ were more sensitive to calcofluor, caspofungin, and itraconazole on MM-Gal. Neither galD nor galE is essential on galactose at high pH, implying additional routes for galactose metabolism in Aspergillus. Aspergillus galactose metabolism is more complex than that of S. cerevisiae.