DNA encoding the gene for the synthetase enzyme capable of generating &dgr; (L-a-aminoadipyl)-L-crysteinyl-D-valine (ACV) from its constituent amino acids was obtained from several penicillin and cephalosporin producing organisms, e.g. Penicillium chrysogenum, cephalosporium and a Flavobacterium species. The DNA was used to prepare recombinant vectors comprising the ACV synthetase gene and hosts transformed with such vectors. The ACV synthetase gene can form part of a gene cluster comprising other genes involved in -&bgr;-lactam biosynthesis and the production of penicillin by expression of the entire biosynthetic gene cluster for the synthesis of penicillin from primary amino acids is described. Suitable hosts in which expression can take place include heterologous hosts which are naturally non-producers of penicillin.