Louise Merson-Davies

Analysis of five tyiosin biosynthetic genes from the tyllBA region of the Streptomyces fradiae genome

The tyllBA region of the tylosin biosynthetic gene cluster of Streptomyces fradiae contains at least five open reading frames (ORFs). ORF1 {tyll) encodes a cytochrome P450 and mutations in this gene affect macrolide ring hydroxylation. The product of 0RF2 (tylB) belongs to a widespread family of proteins whose functions are speculative, although tylB mutants are defective in the biosynthesis or addition of mycaminose during tylosin production. ORFs 3 and 4 (tylA1 and tylA2) encode δTDP‐giucose synthase and δTDP‐glucose dehydratase, respectively, enzymes responsible for the first two steps common to the biosynthesis of all three deoxyhexose sugars of tylosin via the common intermediate, δTDP‐4‐keto, 6‐deoxygiucose. ORF5 encodes a thioesterase similar to one encoded in the erythromycin gene cluster of Saccharopolyspora erythraea.

The tylosin-biosynthetic genes of Streptomyces fradiae

The tylosin-biosynthetic (tyl) gene cluster occupies about 1% of the genome of Streptomyces fradiae and includes at least 43 open reading frames. In addition to structural genes required for tylosin production, the tylcluster contains three resistance determinants and several regulatory genes. Tylosin production is evidently controlled by pathway-specific and pleiotropic regulators with the likely involvement of γ-butyrolactone signalling factors. Accumulation of the polyketide aglycone is controlled by glycosylated macrolides and optimal performance of the complex polyketide synthase enzyme requires the activity of an editing thioesterase.

DNA alkylation sites of nitrogen mustards conjugated to polyamines and their implications for polyamine–DNA interactions

Polyamines conjugated to the nitrogen mustard chlorambucil increase the efficiency of DNA alkylation at N7 of guanine by factors in the range 103 to 104; the sequence selectivity of this alkylation (the alkylation ‘finger-print’) is largely unchanged, which is consistent with flexible, electrostatic binding and incompatible with tight, sequence-specific binding of the polyamine moiety.