FIGURE 6.

Proposed biosynthetic pathway of furaquinocin. Compound 4 was identified as a genuine substrate for the Fur7 prenyltransferase in the present study. The formation of 4 appears to involve successive methylations of 1 by the Fur4 and Fur6 methyltransferases. Substrate 4 is attached to a 10-carbon geranyl group by the Fur7 prenyltransferase to yield Fur-P1. Fur-P1 is then presumably converted into 3 by an unidentified cyclase. Finally, 3 is most likely converted into 2 by the action of cytochrome P450, which is encoded by the fur8 gene. The present data were also used to construct the late stage of the furanonaphthoquine biosynthetic pathway.