Using gene shuffling to increase genetic diversity in genes involved in beta-lactam biosynthesis

摘要

The actinomycetes are gram-positive bacteria that produce more than two-thirds of the known biologically active microbial natural products, including many commercially important antibiotics, anti-cancer agents, other pharmacologically useful agents, animal health products andagrochemicals.The prevailing utilization of antibiotics continues to be the mainstay against microbial infections and a majority ofthe over six thousand antibiotics discovered thus far are from Streptomyces spp.One of the most well-characterized antibiotic biosynthetic pathway is the one involving the biosynthesis of the penicillins, cephalosporins and cephamycins. This pathway involves two initial steps which are common in filamentous fungi, lower eukaryotes and prokaryotes. The penam nucleus of penicillins and the cephem nucleus of both cephamycins andcephalosporins are formed by the condensation of the three precursor amino acids L-a-aminoadipic acid, Lcysteineand L-valine by a mechanism designated as 'non-ribosomal peptide synthesis', which involves activation and condensation of the three component amino acids and epimerization of the L- to D-valine to form a linear acyclic tripeptide called o-(L-a-aminoadipyl)-L-cysteinyl-Dvaline (ACV) by the action of a peptide synthetase. ACV is then cyclized to form isopenicillin N, an intermediate that contains an L-a-aminoadipyl side-chain attached to the penem nucleus (Fig. 1.2) by isopenicilin N synthase (IPNS or Cyclase) and this encompasses the creation of theBeta-lactam and thiazolidine rings.A broad range of ~-lactam producing Streptomyces spp were grown, the DNA extraction procedure optimised and total chromosomal DNA isolated. A bioinformatics analysis of knownIPNS gene sequences allowed the synthesis of PCR primers for the iso-penicillin N synthase gene. IPNS genes and lPNS-like genes were successfully amplified from the total DNA of ten strains including two novel thermophilic strains, A. and B. Sequencing was carried out on thegenes from S. hygroscopicus, S. tanashiensis and the two thermophiles A and B. This allowed development of the conditions for gene shuffiing of the IPNS gene which was carried out pairwise and resulted in the reconstitution of shuffied genes of the correct size. The resultingmixed gene sequences were cloned into the pTrcHis2-TOPO expression vector and the plasmid DNA screened and assayed for IPNS activity using HPLC which showed ten fold increase in IPNS activity as a result of the shuffiing.