STIMULATION OF POLYKETIDE METABOLISM IN STREPTOMYCES-FRADIAE BY TYLOSIN AND ITS GLYCOSYLATED PRECURSORS

Three glycosyltransferases are involved in tylosin biosynthesis in Str eptomyces fradiae. The first sugar to be added to the polyketide aglyc one (tylactone) is mycaminose and the gene encoding mycaminosyltransfe rase is orf2 (tylM2). However, targeted disruption of orf2* did not l ead to the accumulation of tylactone under conditions that normally fa vour tylosin production; instead, the synthesis of tylactone was virtu ally abolished. This may, in part, have resulted from a polar effect o n the expression of genes downstream of orf2, particularly orf4* (ccr ) which encodes crotonyl-CoA reductase, an enzyme that supplies 4-carb on extender units for polyketide metabolism However, that cannot be th e entire explanation, since tylosin production was restored at about 1 0% of the wild-type level when orf2 was re-introduced into the disrup ted strain. When glycosylated precursors of tylosin were fed to the di srupted strain, they were converted to tylosin, confirming that two of the three glycosyltransferase activities associated with tylosin bios ynthesis were still intact. Interestingly, however, tyladone also accu mulated under such conditions and, to a much lesser extent, when tylos in was added to similar fermentations. It is concluded that glycosylat ed macrolides exert a pronounced positive effect on polyketide metabol ism in S. fradiae.