DNA sequences obtained from S. clavuligerus ATCC 27064, recombinant vectors incorporating such sequences and hosts transformed with such vectors are disclosed. The DNA comprises one or more genes coding for one or more enzymes involved in the biosynthesis of penicillin and cephalosporin -lactams and such enzymes are expressed by hosts into which the recombinant vectors are transformed. The DNA and the enzymes encoded thereby have utility in the preparation of penicillins and cephalosporins, both known and novel, possessing pharmacological, especially antimicrobial, activity.