The calcium-dependent antibiotic (CDA), from Streptomyces coelicolor, is an acidic lipopeptide comprising an N-terminal 2,3-epoxyhexanoyl fatty acid side chain and several nonproteinogenic amino acid residues. S. coelicolor grown on solid media was shown to produce several previously uncharacterized peptides with C-terminal Z-dehydrotryptophan residues. The CDA biosynthetic gene cluster contains open reading frames encoding nonribosomal peptide synthetases, fatty acid synthases, and enzymes involved in precursor supply and tailoring of the nascent peptide. On the basis of protein sequence similarity and chemical reasoning, the biosynthesis of CDA is rationalized. Deletion of SCO3229 (hmaS), a putative 4-hydroxymandelic acid synthase-encoding gene, abolishes CDA production. The exogenous supply of 4-hydroxymandelate, 4-hydroxyphenylglyoxylate, or 4-hydroxyphenylglycine re-establishes CDA production by the DeltahmaS mutant. Feeding analogs of these precursors to the mutant resulted in the directed biosynthesis of novel lipopeptides with modified arylglycine residues.

Nonribosomal peptides contain a wide range of unusual non-proteinogenic amino acid residues. As a result, these complex natural products are amongst the most structurally diverse secondary metabolites in nature, and possess a broad spectrum of biological activities. b-Hydroxylation of amino acid precursors or peptidyl residues and their subsequent processing by downstream tailoring enzymes are some of the most common themes in the biosynthetic diversification of these therapeutically important peptides. Identification and characterization of the biosynthetic intermediates and enzymes involved in these processes are thus pivotal in understanding nonribosomal peptide assembly and modification. To this end, the putative asparaginyl oxygenase-and 3-hydroxyasparaginyl phosphotransferase-encoding genes hasP and asnO were separately deleted from the calcium-dependent antibiotic (CDA) biosynthetic gene cluster of Streptomyces coelicolor. Whilst the parent strains produce a number of 3-hydroxyasparagine-and 3-phosphohydroxyasparagine-containing CDAs, the DhasP mutants produce exclusively non-phosphorylated CDAs. On the other hand, DasnO mutants produce several new Asn-containing CDAs not present in the wild-type, which retain calcium-dependent antimicrobial activity. This confirms that AsnO and HasP are required for the b-hydroxylation and phosphorylation of the Asn residue within CDA. INTRODUCTIONThe calcium-dependent antibiotics (CDAs) from Streptomyces coelicolor (Hojati et al., 2002;Kempter et al., 1997) belong to the group of structurally related acidic lipopetide antibiotics, which include A54145 (Fukuda et al., 1990; Miao et al., 2006), daptomycin (Baltz et al., 2005;Miao et al., 2005;Raja et al., 2003), friulimicins and amphomycins (Vértesy et al., 2000). All of these nonribosomally biosynthesized lipopeptides contain N-terminal fatty acid side chains, which is a trans-2,3-epoxyhexanoyl moiety in the case of CDA (Fig. 1), along with decapeptide lactone or lactam cores. Within the decapeptide cores are a number of conserved amino acids, including acidic residues responsible for co-ordination of calcium ions, which are essential for antimicrobial activity. Notably, daptomycin was recently approved for clinical use, becoming the first new structural class of natural antibiotic to reach the clinic in over 30 years (Baltz et al., 2005;Raja et al., 2003). As a result of this, there has been considerable interest in establishing the biosynthetic origins of the acidic lipopeptide group of antibiotics, with the specific aim of engineering new lipopeptide variants with improved therapeutic properties. To this end we have focused on engineering the biosynthesis of CDAs, using adenylation domain active site modifications to replace Asp In addition to D-HPG, CDA contains a number of other non-proteinogenic amino acids, including L-3-methylglutamic acid (3-MeGlu; at position 10), which is also present at the same relative position in the decapeptide cores of daptomycin and A54145 (Milne et al., 2006), and a Cterminal Z-dehydro...