Transcription factor that regulates the expression of the gene clusters that mediate the biosynthesis of the host-selective toxins (HSTs) AK-toxins responsible for Japanese pear black spot disease by the Japanese pear pathotype (Probable). AK-toxins are esters of 9,10-epoxy 8-hydroxy 9-methyldecatrienoic acid (EDA) (PubMed:22846083). On cellular level, AK-toxins affect plasma membrane of susceptible cells and cause a sudden increase in loss of K(+) after a few minutes of toxin treatment (PubMed:22846083). Gene clusters encoding host-selective toxins (HSTs) are localized on conditionally dispensable chromosomes (CDCs), also called supernumerary chromosomes, where they are present in multiple copies (PubMed:10975654). The CDCs are not essential for saprophytic growth but controls host-selective pathogenicity (PubMed:10975654).