Summary: The genetic organization of the left edge ( region) of the tylosin biosynthetic gene cluster from has been determined. Sequence analysis of a 12·9 kb region has revealed the presence of 11 ORFs, 10 of them belonging to the biosynthetic cluster. The putative functions of the proteins encoded by these genes are as follows: peptidase (ORF1, ), tylosin resistance determinant (ORF2, ), glycosyltransferase (ORF3, ), methyltransferase (ORF4, ), ketoreductase (ORF5, ), ferredoxin (ORF6, ), cytochrome P450 (ORF7, ), methyltransferase (ORF8, ), epimerase (ORF9, ), acyl-CoA oxidase (ORF10, ) and receptor of regulatory factors (ORF11, ). The functional identification of the genes in the proposed tylosin biosynthetic pathway has been deduced by database searches and previous genetic complementation studies performed with tylosin idiotrophic mutants blocked at various stages in tylosin biosynthesis. The gene has been shown to be useful as a tylosin resistance marker in and and the effect of on macrocin depletion has been confirmed. A pathway for the biosynthesis of 6-deoxy-D-allose, the unmethylated mycinose precursor, involving the genes and is proposed.


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