Involved in the biosynthesis of 4-methoxy-2,2'-bipyrrole-5-carbaldehyde (MBC), one of the terminal products involved in the biosynthesis of the red antibiotic prodigiosin (Pig). Catalyzes the transfer of a methyl group from S-adenosyl-L-methionine (SAM) to the hydroxyl group of 4-hydroxy-2,2'-bipyrrole-5-carbaldehyde (HBC) to yield 4-methoxy-2,2'-bipyrrole-5-carbaldehyde (MBC). Antibiotic biosynthesis; prodigiosin biosynthesis. Cells lacking this gene show an orange phenotype and produce norprodigiosin. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.