Recent Advances in Discovery, Biosynthesis and Genome Mining of Medicinally Relevant Polycyclic Tetramate Macrolactams

Abstract

Polycyclic tetramate macrolactams (PTMs), a widely distributed class of structurally complex natural products exhibiting diverse biological activities, share a tetramate-containing macrocyclic lactam ring fused to a subset of carbocyclic rings. More than 30 naturally occurring PTM members have been reported. Representative members include ikarugamycin, HSAF, and alteramides. The emerging significance of PTMs in medicinal applications has raised attentions on their biosynthetic studies. These studies have unveiled the unexpected conservation of compact PTM biosynthetic loci in phylogenetically diverse bacteria and elucidated mechanisms for key steps in PTM biosynthesis. PTMs were demonstrated to be derived from the common origin of a hybrid polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) pathway, in which the PKS portion was iteratively used to generate two separate polyketide chains. A common tetramate-containing polyene intermediate was proposed to be the final product of all PTM PKS/NRPS assembly lines. Subsequently, a set of oxidoreductases acted in a not yet clearly understood way to dictate the manner of cyclizations to yield different polycycle ring systems in PTMs. The only well studied example was the formation of the inner fivemembered ring in ikarugamycin, which was catalyzed by an alcohol dehydrogenase via a [1 + 6] Michael addition. Nonetheless, these studies have illustrated the extraordinary simplicity of nature's art in the biosynthesis of PTMs with complex structures and paved the way to further expand the structural diversity of the family of medicinally relevant PTMs by genome mining and combinatorial biosynthesis.

Activation and characterization of a cryptic gene cluster reveals a cyclization cascade for polycyclic tetramate macrolactams

“…1), 1 such as the 5/5 ring system in alteramides ( 1 , 2 ), 2 5/5/6 ring system in HSAF (heat-stable antifungal factor, 3 ), lysobacteramide B ( 4 ), 3,4 and frontalamides ( 5 , 6 ), 5 5/6/5 ring system in ikarugamycin ( 7 ) 6 and butremycin ( 8 ), 7 and 5/4/6 ring system in 9 . 8 PTMs exhibit a wide range of antifungal, antibacterial, antiprotozoal, antiulcer, antiviral, cytotoxic, and apoptosis-inducing biological activities.…”

“…8 PTMs exhibit a wide range of antifungal, antibacterial, antiprotozoal, antiulcer, antiviral, cytotoxic, and apoptosis-inducing biological activities. 1 As such, PTMs have shown significant potential in both agricultural and medicinal applications. 1 …”

“…9–12 In sharp contrast, nature has developed a conserved and compact biosynthetic pathway to construct the complex structures of PTMs. 1,13 PTMs were revealed to have originated from a conserved hybrid polyketide synthase (PKS)/non-ribosomal peptide synthetase (NRPS) pathway. 5 The PKS was iteratively used to generate two separate polyketide chains that were respectively condensed to the α- and δ-amino groups of an l -ornithine tethered in the NRPS, leading to a common tetramate-containing polyene precursor 10 (Fig.…”

“…15,16 However, the enzymatic machinery that dictates the manner of the divergent cyclization cascades that convert 10 into diverse ring systems in PTMs remains mysterious. 1 The only well studied example was the formation of the inner five-membered ring in 7 , which is catalyzed by the alcohol dehydrogenase IkaC in the manner of a Michael addition reaction. 15 …”

“…5 However, most of them are silent. 1,5 Recently, different strategies were employed to successfully activate silent PTM gene clusters for the production of new PTMs, including promoter engineering 18 and the heterologous expression of a reconstituted PTM gene cluster via synthetic biology. 8 The success of these strategies highlights the potential for the discovery of new PTMs by the genome mining-based activation of PTM gene clusters.…”