The tylLM region of the Streptomyces fradiae genome

Streptomyces fradiae produces the macrolide antibiotic tylosin which is composed of a polyketide lactone, tylactone, and three deoxyhexose sugars. The addition of the first sugar, mycaminose, confers antimicrobial activity onto the inert polyketide. The tyllBA and tylLM loci from the tylosin gene cluster are involved in the production and addition of mycaminose. tylL and tylA mutants are defective in the biosynthesis or addition of all three sugars, in contrast to the TylM and TylB lesions which are mycaminose specific. Little was known about the tylLM region, but the tylL and tylM mutants had both been complemented with an 8 kb fragment of tyl DNA (Fishman et. al. 1987).;Sequence has been generated for the tylLM region, identifying four ORFs (1*-4*), which could have roles in deoxyhexose metabolism (Gandecha et. al., 1997). The tylM mutant was complemented by integrating a wild-type copy of orf3* into the mutant genome; this gene is thought to be the methyltransferase which acts in the mycaminose pathway. The wild-type orf2* integrated into the tylL mutant chromosome restored tylosin biosynthesis, and an in-frame stop codon was located part way through orf2* amplified from the tylL genome. orf2* was a surprising candidate for the tylL gene because it is the mycaminosyl-glycosyltransferase. The tylM mutant could not convincingly convert OMT, DMT or desmycosin to tylosin. We therefore believe that the TylL phenotype is the result of multiple mutations or a physiological effect. The function of orf1* is still unknown, although matches with other genes involved in the biosynthesis of amino sugars suggest that it is involved in mycaminose production.;Analysis of the orfl* transcript suggest that, during antibiotic biosynthesis, orf1* may be co-transcribed with tylG, with two additional transcripts initiating independently of tylG. Promoter-probe vectors have been of limited use in identifying promoters.