Nonactin biosynthesis: the product of nonS catalyzes the formation of the furan ring of nonactic acid

Nonactin is the parent compound of a group of ionophore antibiotics, known as the macrotetrolides, produced by Streptomyces griseus subsp, griseus ETH A7796, Nonactin is a significant compound because of its inhibitory effect s on the P170 glycoprotein-mediated efflux of chemotherapeutic agents in mu ltiple-drug-resistant cancer cells. Nonactin is also significant in that it is a highly atypical polyketide. Very little is presently known about the genes of the nonactin biosynthesis cluster. In this paper we describe our e fforts to establish a connection between the product of a gene from the non actin biosynthesis cluster and a known biochemical transformation in nonact in biosynthesis. Nonactate synthase is the enzyme which catalyzes the forma tion of nonactic acid from an acyclic precursor in nonactin biosynthesis, W e have synthesized the substrate for this enzyme and have detected the in v itro cyclization activity of the substrate in cell-free preparations of S. griseus subsp, griseus ETH A7796, Previous studies by R, Plater and J. A. R obinson (Gene 112:117-122, 1992) had suggested, based on sequence homology, that the product of a partial open reading frame found close to the tetran actin resistance gene of S, griseus could be the nonactate synthase, We hav e therefore cloned, sequenced, and heterologously expressed this full gene (nonS), and we have shown that the gene product, NonS, does indeed catalyze the formation of the furan ring of nonactic acid as hypothesized.