SEQUENCING AND MUTAGENESIS OF GENES FROM THE ERYTHROMYCIN BIOSYNTHETIC GENE-CLUSTER OF SACCHAROPOLYSPORA-ERYTHRAEA THAT ARE INVOLVED IN L-MYCAROSE AND D-DESOSAMINE PRODUCTION

The nucleotide sequence on both sides of the eryA polyketide synthase genes of the erythromycin-producing bacterium Saccharopolyspora erythr aea reveals the presence of ten genes that are involved in L-mycarose (eryB) and D-desosamine (eryC) biosynthesis or attachment. Mutant stra ins carrying targeted lesions in eight of these genes indicate that th ree (eryBIV, eryBV and eryBVI) act in L-mycarose biosynthesis or attac hment, while the other five (eryCII, eryCIII, eryCIV, eryCV and eryCVI ) are devoted to D-desosamine biosynthesis or attachment. The remainin g two genes (eryBII and eryBVII) appear to function in L-mycarose bios ynthesis based on computer analysis and earlier genetic data. Three of these genes, eryBII, eryCIII and eryCII, lie between the eryAIII and eryG genes on one side of the polyketide synthase genes, while the rem aining seven, eryBIV, eryBV, eryCVI, eryBVI, eryCIV, eryCV and eryBVII lie upstream of the eryAI gene on the other side of the gene cluster. The deduced products of these genes show similarities to: aldohexose 4-ketoreductases (eryBIV), aldoketo reductases (eryBII), aldohexose 5- epimerases (eryBVII), the dnmT gene of the daunomycin biosynthetic pat hway of Streptomyces peucetius (eryBVI), glycosyltransferases (eryBV a nd eryCIII), the AscC 3,4-dehydratase from the ascarylose biosynthetic pathway of Yersinia pseudotuberculosis (eryCIV), and mammalian N-meth yltransferases (eryCVI). The eryCII gene resembles a cytochrome P450, but lacks the conserved cysteine residue responsible for coordination of the haem iron, while the eryCV gene displays no meaningful similari ty to other known sequences. From the predicted function of these and other known eryB and eryC genes, pathways for the biosynthesis of L-my carose and D-desosamine have been deduced.