https://phys.org/news/2018-06-fungi-renewable-energy.html

Alazi first found the activator GaaR, a protein that activates the expression of the genes that encode pectinases. Roughly said, when A. niger senses pectin, GaaR is activated. Furthermore, she found the repressor GaaX. When the fungus doesn't sense pectin in its environment, the repressor GaaX is activated. GaaX then disables the activator GaaR and the pectinase production is stopped. The third component she found was the inducer molecule that causes abundant pectinase production: 2-keto-3-deoxy-L-galactonate, a catabolic product of pectin. "My research provided proper clues about the exact mechanism, so our hypothesis is now as follows: The inducer binds to the repressor GaaX, so it can no longer inhibit the activator GaaR," Alazi explains. "In this way, GaaR can start to activate gene expression and pectinases are produced. Other researchers are now aiming to confirm this hypothesis in the lab."

After understanding the mechanism, Alazi mutagenized the fungus with UV-light in order to isolate mutants with unique pectinase production properties. One mutant was found to possess a GaaR allele which is not inhibited by the repressor GaaX. "So normally, the repressor GaaX inactivates GaaR, so that A. niger does not produce pectinases," she says. "But when the inducer is present, it binds to GaaX, the activator GaaR is free to go and the fungus starts producing pectinases." However, says Alazi, with this allele the presence of the inducer is not necessary: the GaaR allele activates the expression of the pectinase genes automatically. Moreover, she also made mutants that produce more of the activator GaaR than normal. These mutants produced even more pectinases without the need for the inducer to be present.