Involved in the biosynthesis of virginiae butanolide (VB), which regulates the production of antibiotic virginiamycin. Catalyzes the reduction of 6-dehydro-VB-A to VB-A, the last catalytic step in VB biosynthesis. In vitro, can use various synthetic A-factor-type analogs. a (3R,4R)-3-[(1S)-1-hydroxyalkyl]-4-(hydroxymethyl)oxolan-2-one + NADP(+) = a (3R,4R)-3-alkanoyl-4-(hydroxymethyl)oxolan-2-one + H(+) + NADPH Optimum pH is 7.5. Optimum temperature is 25 degrees Celsius. Homodimer. Constitutively expressed. Deletion mutant cannot produce VB or virginiamycin. Belongs to the short-chain dehydrogenases/reductases (SDR) family.