Mycaminose metabolism and control of tylosin synthesis in Streptomyces fradiae

Production of the macrolide antibiotic, tylosin, occurs by a combination of polyketide synthesis and deoxyhexose sugar metabolism in S. fradiae. Polyketide synthesis was stimulated by glycosylated tylosin-precursors, and by tylosin itself, in an S. fradiae strain disrupted in the glycosyltransferase responsible for the addition of mycaminose. The addition of mycaminose is the essential first step in the conversation of the polyketide ring, tylactone, to tylosin. In the absence of glycosylated material, the strain disrupted in mycaminose addition produced no significant amounts of any tylosin precursor. The possibility that this effect might have been the result of downstream effects on a gene, ccr, thought to be involved in polyketide synthesis was refuted by analysis of a strain disrupted in ccr. The behaviour of strains disrupted in orf1*, putatively encoding an isomerase and in tylMI, encoding a methyltransferase involved in mycaminose synthesis were also analysed. orf1* was revealed to contain an authentic mycaminose-specific gene, designated tylMIII. Both these disrupted strains and the tylB mutant, altered in an aminotransferase gene also required for mycaminose synthesis but located over 45 kb from its functional counterparts, behaved similarly to the mycaminosyltransferase disrupted strain with respect to stimulation of polyketide synthesis. The stimulation of polyketide synthesis was shown to be a catalytic effect which could also be provoked by other glycosylated macrolides, rosaramicin and spiramycin but not by erythromycin, carbomycin or chalcomycin. The possible involvement of recently discovered regulatory genes associated with the tylosin biosynthetic cluster was investigated and interesting preliminary data were obtained.