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Sequence analysis of oxazolomycin (OZM) biosynthetic gene cluster from Streptomyces albus JA3453 revealed a gene,ozmH, encoding a hybrid polyketide and non-ribosomal peptide enzyme. Tandem ketosynthase (KS) domains (KS10-1 and KS10-2) were characterized and they show significant homology with known KSs. Using an alteative method that involves RecA-mediated homologous recombination, the negative selection marker sacB gene, and temperature-sensitive replications, site-directed mutagenesis of the catalytic triad amino acid cysteines were carried out in each of the tandem KS domains to test the function they play in OZM biosynthesis.HPLC-mass spectrometry analysis of the resulting mutant strains showed that KS10-2 is essential for OZM biosynthesis but KS10-1 is not indispensable and might serve as a "redundant" domain. These results confirmed the existence of an "extra domain" in complex polyketide synthase.