Inactivates the type B streptogramin antibiotics by linearizing the lactone ring at the ester linkage, generating a free phenylglycine carboxylate and converting the threonyl moiety into 2-amino-butenoic acid. Monomer. Has unequivocally been shown (PubMed:11467949) to cleave the ester bond of streptogramin B antibiotics by an elimination reaction and not by hydrolysis, therefore is a lyase and not a hydrolase. The olefinic dehydrobutyrine thus generated in the N-terminal position has a Z stereochemistry. Belongs to the Vgb family.