Analysis of bacterial carbapenem antibiotic production genes reveals a novel beta-lactam biosynthesis pathway

Carbapenems are beta-lactam antibiotics which have an increasing utility in chemotherapy, particularly for nosocomial, multidrug-resistant infections. Strain GS101 of the bacterial phytopathogen, Erwinia carotovora, makes the simple beta-lactam antibiotic, 1-carbapen-2-em-3-carboxylic acid. We have mapped and sequenced the Erwinia genes encoding carbapenem production and have cloned these genes into Escherichia coli where we have reconstituted, for the first time, functional expression of the beta-lactam in a heterologous host. The carbapenem synthesis gene products are unrelated to enzymes involved in the synthesis of the so-called sulphur-containing beta-lactams, namely penicillins, cephamycins and cephalosporins. However, two of the carbapenem biosynthesis genes, carA and carC, encode proteins which show significant homology with proteins encoded by the Streptomyces clavuligerus gene cluster responsible for the production of the beta-lactamase inhibitor, clavulanic acid. These homologies, and some similarities in genetic organization between the clusters, suggest an evolutionary relatedness between some of the genes encoding production of the antibiotic and the beta-lactamase inhibitor. Our observation are consistent with the evolution of a second major biosynthetic route to the production of beta-lactam-ring-containing antibiotics.