| Erythromycin was a kind of broad-spectrum macrolide antibiotics produced by Saccharopolyspora erythraea. Like other antibiotics, bacterial resistances to erythromycin also came out in clinic. It was urgent for us to develop new antibiotics, which could overcome some problems of bacterial resistance. The third generation erythromycin that had been gotten onto the market was one of ketolides that was synthesized by chemical modification of erythromycin. L-cladinose at position C3 of macrolactone ring had been replaced by carbonyl by chemical modification, this carbonyl enabled ketolides to bind to their targets without triggering the inducible resistance to macrolide-lincosamide-streptogramin B(MLSB)drugs, and so improve antimicrobial activity to a great extent. With the development of molecular biology and deep understanding of molecular mechanism of erythromycin biosynthesis process, combination biosynthesis became one way of exploring to synthesize antibiotics.Biosynthesis process of erythromycin had two steps: macrolactone ring synthesis and post modification. The macrolactone ring was synthesized by polyketide sythase(PKS). PKS were multiplex enzymatic system and composed of six modules whose function and structure was close. Whether C3 hydroxyl of macrolactone ring would be reduced depended on the ketoreductase (KR6) domain of the sixth module of polyketide synthase, so we could make KR6 domain completely or partly inactivated and turn C3 hydroxyl into carbonyl by genetic engineering for the ketolides biosynthesis. Tyr2699 was a key amino acid in the catalytic site of KR6 domain. Mutant Tyr2699 could completely inactivate KR6. Saccharopolyspora erythraea A226-YA had been constructed with homologous recombination, and it could synthesize the compound 3-deoxy-3-oxo-erythronolide B without detecting erythromycin.In order to mutate the codon TAC of Tyr2699 to the codon GCC of Ala of catalytic triplet in KR6 and ensure effective chromosome integration and the second...
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