Structural insight into MtmC, a bifunctional ketoreductase-methyltransferase involved in the assembly of the mithramycin trisaccharide chain

Post-PKS enzyme complexes

This review highlights the protein–protein interactions between type II post-PKS tailoring enzymes with an emphasis on gilvocarcin and mithramycin.

Structural Basis of the Selectivity of GenN, an Aminoglycoside N-Methyltransferase Involved in Gentamicin Biosynthesis

Gentamicins are heavily methylated, clinically valuable pseudotrisaccharide antibiotics produced by Micromonospora echinospora. GenN has been characterized as an S-adenosyl-l-methionine-dependent methyltransferase with low sequence similarity to other enzymes. It is responsible for the 3″-N-methylation of 3″-dehydro-3″-amino-gentamicin A2, an essential modification of ring III in the biosynthetic pathway to the gentamicin C complex. Purified recombinant GenN also efficiently catalyzes 3″-N-methylation of related aminoglycosides kanamycin B and tobramycin, which both contain an additional hydroxymethyl group at the C5″ position in ring III. We have obtained eight cocrystal structures of GenN, at a resolution of 2.2 Å or better, including the binary complex of GenN and S-adenosyl-l-homocysteine (SAH) and the ternary complexes of GenN, SAH, and several aminoglycosides. The GenN structure reveals several features not observed in any other N-methyltransferase that fit it for its role in gentamicin biosynthesis. These include a novel N-terminal domain that might be involved in protein:protein interaction with upstream enzymes of the gentamicin X2 biosynthesis and two long loops that are involved in aminoglycoside substrate recognition. In addition, the analysis of structures of GenN in complex with different ligands, supported by the results of active site mutagenesis, has allowed us to propose a catalytic mechanism and has revealed the structural basis for the surprising ability of native GenN to act on these alternative substrates.

Evolutionary analysis of a novel zinc ribbon in the N-terminal region of threonine synthase

Threonine synthase (TS) catalyzes the terminal reaction in the biosynthetic pathway of threonine and requires pyridoxal phosphate as a cofactor. TSs share a common catalytic domain with other fold type II PALP dependent enzymes. TSs are broadly grouped into two classes based on their sequence, quaternary structure, and enzyme regulation. We report the presence of a novel zinc ribbon domain in the N-terminal region preceding the catalytic core in TS. The zinc ribbon domain is present in TSs belonging to both classes. Our sequence analysis reveals that archaeal TSs possess all zinc chelating residues to bind a metal ion that are lacking in the structurally characterized homologs. Phylogenetic analysis suggests that TSs with an N-terminal zinc ribbon likely represents the ancestral state of the enzyme while TSs without a zinc ribbon must have diverged later in specific lineages. The zinc ribbon and its N- and C-terminal extensions are important for enzyme stability, activity and regulation. It is likely that the zinc ribbon domain is involved in higher order oligomerization or mediating interactions with other biomolecules leading to formation of larger metabolic complexes.

Elucidation of the glycosylation steps during biosynthesis of antitumor macrolides PM100117 and PM100118 and engineering for novel derivatives

BACKGROUND

Antitumor compounds PM100117 and PM100118 are glycosylated polyketides derived from the marine actinobacteria Streptomyces caniferus GUA-06-05-006A. The organization and characterization of the PM100117/18 biosynthesis gene cluster has been recently reported.

RESULTS

Based on the preceding information and new genetic engineering data, we have outlined the pathway by which PM100117/18 are glycosylated. Furthermore, these genetic engineering experiments have allowed the generation of novel PM100117/18 analogues. Deletion of putative glycosyltranferase genes and additional genes presumably involved in late biosynthesis steps of the three 2,6-dideoxysugars appended to the PM100117/18 polyketide skeleton, resulted in the generation of a series of intermediates and novel derivatives.

CONCLUSIONS

Isolation and identification of the novel compounds constitutes an important contribution to our knowledge on PM100117/18 glycosylation, and set the basis for further characterization of specific enzymatic reactions, additional genetic engineering and combinatorial biosynthesis approaches.

Selvamicin, an atypical antifungal polyene from two alternative genomic contexts

The bacteria harbored by fungus-growing ants produce a variety of small molecules that help maintain a complex multilateral symbiosis. In a survey of antifungal compounds from these bacteria, we discovered selvamicin, an unusual antifungal polyene macrolide, in bacterial isolates from two neighboring ant nests. Selvamicin resembles the clinically important antifungals nystatin A₁ and amphotericin B, but it has several distinctive structural features: a noncationic 6-deoxymannose sugar at the canonical glycosylation site and a second sugar, an unusual 4-O-methyldigitoxose, at the opposite end of selvamicin's shortened polyene macrolide. It also lacks some of the pharmacokinetic liabilities of the clinical agents and appears to have a different target. Whole genome sequencing revealed the putative type I polyketide gene cluster responsible for selvamicin's biosynthesis including a subcluster of genes consistent with selvamicin's 4-O-methyldigitoxose sugar. Although the selvamicin biosynthetic cluster is virtually identical in both bacterial producers, in one it is on the chromosome, in the other it is on a plasmid. These alternative genomic contexts illustrate the biosynthetic gene cluster mobility that underlies the diversity and distribution of chemical defenses by the specialized bacteria in this multilateral symbiosis.