posted on 2021-03-03, 15:36authored byTrevor
N. Purdy, Min Cheol Kim, Reiko Cullum, William Fenical, Bradley S. Moore
Ortho-quinone methides (o-QMs)
are reactive intermediates in biosynthesis that give rise to a variety
of intra- and intermolecular cyclization/addition products in bacteria,
fungi, and plants. Herein, we report a new metabolic deviation of
an o-QM intermediate in a benzylic dehydrogenation
reaction that links the newly described marine bacterial natural products
dihydrotetrachlorizine and tetrachlorizine. We discovered these novel
dichloropyrrole-containing compounds from actinomycete strain AJS-327
that unexpectedly harbors in its genome a biosynthetic gene cluster
(BGC) of striking similarity to that of chlorizidine, another marine
alkaloid bearing a different carbon skeleton. Heterologous expression
of the homologous flavin-dependent oxidoreductase enzymes Tcz9 and
Clz9 revealed their native functions in tetrachlorizine and chlorizidine
biosynthesis, respectively, supporting divergent oxidative dehydrogenation
and pyrrolizine-forming reactions. Swapping these berberine bridge
enzyme-like oxidoreductases, we produced cyclized and dehydrogenated
analogs of tetrachlorizine and chlorizidine, including a dearomatized
chlorizidine analog that stabilizes an o-QM via conjugation
with a 3H-pyrrolizine ring.