Involved in the incorporation of salicylate into the virulence-conferring salicylate-based siderophore mycobactin. Catalyzes the initial conversion of chorismate to yield the intermediate isochorismate (isochorismate synthase activity), and the subsequent elimination of the enolpyruvyl side chain in a lyase reaction to give salicylate (isochorismate pyruvate-lyase activity). In the absence of magnesium, MbtI displays a chorismate mutase activity and converts chorismate to prephenate. chorismate = isochorismate isochorismate = pyruvate + salicylate chorismate = prephenate Siderophore biosynthesis; mycobactin biosynthesis. Monomer. Belongs to the anthranilate synthase component I family. Salicylate synthase subfamily.