Involved in cellular response to chemical stress and may contribute to resistance toward antimicrobial natural compounds as well as drugs (Probable). Catalyzes the methylation and detoxification of the P.aeruginosa toxin 2-heptyl-1-hydroxy-4(1H)-quinolinone (HQNO) to 2-heptyl-1-methoxy-4(1H)-quinolinone (HMOQ) (PubMed:33064871). Can also methylate 3-bromo-2-heptyl-1-hydroxy-4(1H)-quinolinone, and shows much lower activity with 1-hydroxyquinolin-4(1H)-one, quercetin, 4-hydroxyquinolin-2(1H)-one (DHQ) and 4-hydroxyisoquinolin-1(2H)-one (PubMed:33064871). 2-heptyl-1-hydroxy-4(1H)-quinolinone + S-adenosyl-L-methionine = 2-heptyl-1-methoxy-4(1H)-quinolinone + H(+) + S-adenosyl-L-homocysteine 3-bromo-2-heptyl-1-hydroxy-4(1H)-quinolinone + S-adenosyl-L-methionine = 3-bromo-2-heptyl-1-methoxy-4(1H)-quinolinone + H(+) + S-adenosyl-L-homocysteine kcat is 2.0 sec(-1) with HQNO as substrate. Monomer. Belongs to the methyltransferase superfamily.