The role of the putative P450 monooxygenase OxyD and the chlorination time point in the biosynthesis of the glycopeptide antibiotic balhimycin produced by Amycolatopsis balhimycina were analyzed. The oxyD gene is located directly downstream of the bhp (perhydrolase) and bpsD (nonribosomal peptide synthetase D) genes, which are involved in the synthesis of the balhimycin building block ␤-hydroxytyrosine (␤-HT). Reverse transcriptase experiments revealed that bhp, bpsD, and oxyD form an operon. oxyD was inactivated by an in-frame deletion, and the resulting mutant was unable to produce an active compound. Balhimycin production could be restored (i) by complementation with an oxyD gene, (ii) in cross-feeding studies using A. balhimycina JR1 (a null mutant with a block in the biosynthesis pathway of the building blocks hydroxy-and dihydroxyphenylglycine) as an excretor of the missing precursor, and (iii) by supplementation of ␤-HT in the growth medium. These data demonstrated an essential role of OxyD in the formation pathway of this amino acid. Liquid chromatography-electrospray ionization-mass spectrometry analysis indicated the biosynthesis of completely chlorinated balhimycin by the oxyD mutant when culture filtrates were supplemented with nonchlorinated ␤-HT. In contrast, supplementation with 3-chloro-␤-HT did not restore balhimycin production. These results indicated that the chlorination time point was later than the stage of free ␤-HT, most likely during heptapeptide synthesis.

In the mutasynthetic approach, the DeltadpgA mutant of the vancomycin-type glycopeptide antibiotic producer Amycolatopsis balhimycina, which is deficient in the synthesis of 3,5-dihydroxyphenylglycine (DPg), was supplemented with synthetic DPg analogues to obtain the corresponding modified glycopeptides. Sterically more demanding 3,5-disubstituted methoxy derivatives as well as monosubstituted DPg analogues were accepted as substrates. These facts indicate that steric and electronic requirements suffice in several cases for the oxidative closure of the AB ring, thus leading to the generation of novel antibiotically active glycopeptide derivatives. The results represent a further step in evaluating the potential of mutasynthesis for peptidic secondary metabolites.

Halogenase enzymes involved in glycopeptide antibiotic biosynthesis accept aminoacyl-carrier protein substrates.

The putative hydrolase gene bhp from the balhimycin biosynthetic gene cluster has been cloned and overexpressed in Escherichia coli. The corresponding enzyme Bhp was purified to homogeneity by nickel-chelating chromatography and characterized. Although Bhp has sequence similarities to hydrolases with "haloperoxidase"/perhydrolase activity, it did not show any enzymatic activity with standard "haloperoxidase"/perhydrolase substrates (e.g., monochlorodimedone and phenol red), nonspecific esterase substrates (such as p-nitrophenyl acetate, p-nitrophenyl phosphate and S-thiophenyl acetate) or the model lactonase substrate dihydrocoumarin. However, Bhp could be shown to catalyse the hydrolysis of S-beta-hydroxytyrosyl-N-acetyl cysteamine thioester (beta-OH-Tyr-SNAC) with 15 times the efficiency of S-L-tyrosyl-N-acetyl cysteamine thioester (L-Tyr-SNAC). This is in agreement with the suggestion that Bhp is involved in balhimycin biosynthesis, during which it was supposed to catalyse the hydrolysis of beta-OH-Tyr-S-PCP (PCP=peptidyl carrier protein) to free beta-hydroxytyrosine (beta-OH-Tyr) and strongly suggests that Bhp is a thioesterase with high substrate specificity for PCP-bound beta-OH-Tyr and not a "haloperoxidase"/perhydrolase or nonspecific esterase.