Unusual antibiotic biosynthesis enzymes: The observation that formation of trans-carboxymethylproline (t-CMP) and 6-methyl-t-CMP is catalyzed by ThnE from Streptomyces cattleya using glutamate semi-aldehyde and malonyl- or methylmalonyl-CoA implies the need for a C-5 epimerization step in the biosynthesis of thienamycin and reveals the possibility that a methyl group at C-6 might be introduced at an early stage.