• Aspergillus nidulans ;
  • d-galactose;
  • Leloir pathway;
  • oxido-reductive pathway;
  • beta-galactosidase;
  • lactose permease


In this study, we analyze the expression of the Aspergillus nidulans bgaD-lacpA gene couple (encoding an intracellular beta-galactosidase and a lactose permease) in the presence of d-galactose. This monosaccharide can be catabolized via alternative, independent pathways in this model organism. The inductive capabilities of intermediates of the two alternative routes of d-galactose utilization were addressed in loss-of-function mutants defective in a defined step in one of the two pathways. In a galactokinase (galE9) mutant, the cluster is strongly induced by d-galactose, suggesting that formation of Leloir pathway intermediates is not required. The expression profiles of bgaD and lacpA were similar in wild type, l-arabinitol dehydrogenase (araA1), and hexokinase (hxkA1) negative backgrounds, indicating that intermediates of the oxido-reductive pathway downstream of galactitol are not necessary either. Furthermore, bgaD-lacpA transcription was not induced in any of the tested strains when galactitol was provided as the growth substrate. An hxkA1/galE9 double mutant cannot grow on d-galactose at all, but still produced bgaD and lacpA transcripts upon transfer to d-galactose. We therefore concluded that the physiological inducer of the bgaD-lacpA gene cluster upon growth on d-galactose is the nonmetabolized sugar itself.