Cell-free extracts of antibiotic-negative mutants of Cephalosporium acremonium converted delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (LLD-tripeptide) into an antibiotic that was destroyed by penicillinase. The enzymic activity of the extracts was destroyed by boiling, but was not inhibited by cycloheximide. LLL-Tripeptide was totally inactive as substrate. The product resembled isopenicillin N, but not penicillin N, in its antibacterial spectrum. We propose that isopenicillin N is the first product of cyclization of LLD-tripeptide.

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