Evolution of the clusters of genes for lß-lactam antibiotics: a model for evolutive combinatorial assembly of new lß-lactams

The biosynthetic pathways of penicillins, cephalosporins and cephamycins are well known and most of the enzymes for the biosynthesis of these three classes of β-lactams have been now purified and characterized biochemically. However, the biosynthetic pathways for the more complex β-lactam (clavulanic acid, clavams and carbapenems) are still poorly known. Most of the genes for penicillins, cephalosporins and cephamycins have been cloned and sequenced. Comparison of the nucleotide and deduced amino acid sequences for the ACV tripeptide synthetase and the isopenicillin N synthase of prokaryotes (Streptomyces, Nocardia and Lysobacter species) and eukaryotes (Penicillium, Cephalosporium and Aspergillus species) has revealed a large conservation, suggesting that the genes encoding those enzymes were transferred horizontally from prokaryotic to eukaryotic β-lactam producers about 370 millions years ago, much later than the divergence of the prokaryotic and eukaryotic kingdoms. The horizontal transfer hypothesis is supported by the lack of introns in the pcbAB and pcbC genes in fungi, despite the large size (about 11 kb) of the pcbAB gene. The bacterial β-lactam clusters contain, in addition, genes for precursor (α-aminoadipic acid) formation and antibiotic exportation as well as resistance genes. Genes for α-aminoadipic biosynthesis were not transferred to the eukaryotic organisms since this precursor is synthesized by a different pathway in eukaryotic cells; furthermore, the β-lactam resistance genes are not needed in fungi. By contrast, the fungal β-lactam cluster has evolutively incorporated introns containing functional “late genes” that are not present in the bacterial βlactam clusters. Many other genes of the bacterial clusters which are not available in fungi provide interesting tools for combinatorial construction of β-lactams with improved pharmacological activities.