Studies on the beta-galactosidase system in Aspergillus nidulans.

Conditions affecting the formation of beta-galactosidase activity in A.nidulans have been investigated. It is shown that mycelium grown on lactose or galactose as carbon sourse contains beta-galactosidase with a specific activity more than thirty times that in mycelium grown on many other carbon sources. The enzyme occurs in two forms with molecular weights of 120,000 and 450,000. A histochemical staining technique has been developed to detect beta-galactosidase activity in colonies grown on solid medium and used to isolate mutants unable to form beta-galactosidase activity. These mutants are defective at single loci, and the two forms of the enzyme probably represent different molecular states of a single polypeptide. Mutants unable to form beta-galactosidase grow on lactose on solid medium: however in liquid culture conidia of these strains do not germinate, indicating that the physiological role of the enzyme is connected with conidial germination when lactose is carbon source. The mutants have been tentatively assigned to three loci designated bgaA, bgaB and bgaC. As beta-galactosidase is not essential for the growth of mycelium on lactose, another pathway for lactose utilisation must exist. The nature of this pathway is not known. Evidence has accumulated that galactose is metabolised in two ways in the organism: via the Leloir pathway and by another unknown system. This alternate pathway is implicated in induction of beta-galactosidase, and mutants have been isolated which are unable to form the activity in response to galactose, though able to do so when grown on lactose. Mutants/contd. Mutants have been obtained which show an induction pattern in liquid culture which is different from that in solid culture. These mutants appear to synthesise beta-galactosidase constitutively on solid medium, but not in liquid culture. The difference is thought to be due to induction of the enzyme by metabolites derived from agar.