Hitachimycin is a macrolactam antibiotic with (S)-β-phenylalanine (β-Phe) at the starter position of its polyketide skeleton. To understand the incorporation mechanism of β-Phe and the modification mechanism of the unique polyketide skeleton, the biosynthetic gene cluster for hitachimycin in Streptomyces scabrisporus was identified by genome mining. The identified gene cluster contains a putative phenylalanine-2,3-aminomutase (PAM), five polyketide synthases, four β-amino-acid-carrying enzymes, and a characteristic amidohydrolase. A hitA knockout mutant showed no hitachimycin production, but antibiotic production was restored by feeding with (S)-β-Phe. We also confirmed the enzymatic activity of the HitA PAM. The results suggest that the identified gene cluster is responsible for the biosynthesis of hitachimycin. A plausible biosynthetic pathway for hitachimycin, including a unique polyketide skeletal transformation mechanism, is proposed.

Butirosin is an aminoglycoside antibiotic consisting two epimers at C-3′′ of ribostamycin/xylostasin with a unique 4-amino-2-hydroxybutyrate moiety at C-1 of the aminocyclitol 2-deoxystreptamine (2DOS). To date, most of the enzymes encoded in the biosynthetic gene cluster for butirosin, from the producing strain Bacillus circulans, have been characterized. A few unknown functional proteins, including nicotinamide adenine dinucleotide cofactor-dependent dehydrogenase/reductase (BtrE and BtrF), are supposed to be involved in the epimerization at C-3′′ of butirosin B/ribostamycin but remain to be characterized. Herein, the conversion of ribostamycin to xylsostasin by BtrE and BtrF in the presence of NAD⁺ and NADPH was demonstrated. BtrE oxidized the C-3′′ of ribostamycin with NAD⁺ to yield 3′′-oxoribostamycin. BtrF then reduced the generated 3′′-oxoribostamycin with NADPH to produce xylostasin. This reaction step was the last piece of butirosin biosynthesis to be described.

Cremimycin is a 19-membered macrolactam glycoside antibiotic based on three distinctive substructures: 1) a β-amino fatty acid starter moiety, 2) a bicyclic macrolactam ring, and 3) a cymarose unit. To elucidate the biosynthetic machineries responsible for these three structures, the cremimycin biosynthetic gene cluster was identified. The cmi gene cluster consists of 33 open reading frames encoding eight polyketide synthases, six deoxysugar biosynthetic enzymes, and a characteristic group of five β-amino-acid-transfer enzymes. Involvement of the gene cluster in cremimycin production was confirmed by a gene knockout experiment. Further, a feeding experiment demonstrated that 3-aminononanoate is a direct precursor of cremimycin. Two characteristic enzymes of the cremimycin-type biosynthesis were functionally characterized in vitro. The results showed that a putative thioesterase homologue, CmiS1, catalyzes the Michael addition of glycine to the β-position of a non-2-enoic acid thioester, followed by hydrolysis of the thioester to give N-carboxymethyl-3-aminononanoate. Subsequently, the resultant amino acid was oxidized by a putative FAD-dependent glycine oxidase homologue, CmiS2, to produce 3-aminononanoate and glyoxylate. This represents a unique amino transfer mechanism for β-amino acid biosynthesis.

FD-891 is a 16-membered cytotoxic antibiotic macrolide that is especially active against human leukemia such as HL-60 and Jurkat cells. We identified the FD-891 biosynthetic (gfs) gene cluster from the producer Streptomyces graminofaciens A-8890 by using typical modular type I polyketide synthase (PKS) genes as probes. The gfs gene cluster contained five typical modular type I PKS genes (gfsA, B, C, D, and E), a cytochrome P450 gene (gfsF), a methyltransferase gene (gfsG), and a regulator gene (gfsR). The gene organization of PKSs agreed well with the basic polyketide skeleton of FD-891 including the oxidation states and α-alkyl substituent determined by the substrate specificities of the acyltransferase (AT) domains. To clarify the involvement of the gfs genes in the FD-891 biosynthesis, the P450 gfsF gene was inactivated; this resulted in the loss of FD-891 production. Instead, the gfsF gene-disrupted mutant accumulated a novel FD-891 analogue 25-O-methyl-FD-892, which lacked the epoxide and the hydroxyl group of FD-891. Furthermore, the recombinant GfsF enzyme coexpressed with putidaredoxin and putidaredoxin reductase converted 25-O-methyl-FD-892 into FD-891. In the course of the GfsF reaction, 10-deoxy-FD-891 was isolated as an enzymatic reaction intermediate, which was also converted into FD-891 by GfsF. Therefore, it was clearly found that the cytochrome P450 GfsF catalyzes epoxidation and hydroxylation in a stepwise manner in the FD-891 biosynthesis. These results clearly confirmed that the identified gfs genes are responsible for the biosynthesis of FD-891 in S. graminofaciens.