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 decarboxylation on the malonyl group attached to PigH to yield a C2 carbanion of acetyl-S-PigH. Then, the heterocyclic pyrrole group of PigG moves to the PigJ active site Cys-525 to generate a transient pyrrolyl-S-cysteinyl PigJ intermediate (acyl donor) whose pyrrolyl group is captured by the C2 carbanion of acetyl-S-PigH to yield the pyrrolyl-beta-ketoacyl-S-PigH. Antibiotic biosynthesis; prodigiosin biosynthesis. It seems that PigJ is composed by an active ketosynthase (KS) domain and by a chain length factor (CLF) partner domain that potentially decarboxylates the malonyl group of PigH. Cells lacking this gene show a white phenotype and produce 2-methyl-3-n-amyl-pyrrole (MAP). Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.