Equisetin is a filamentous fungal tetramic acid natural product synthesized
by a hybrid PKS-NRPS, EqiS, and by modifying enzymes. Several of the
protein-encoding genes have been knocked out, indicating that some of the
putative modifying genes may not be involved equisetin biosynthesis. The
peptide synthetase portion of the compound has been overexpressed in E. coli
and purified. In addition, related proteins from bacteria have been
overexpressed and analyzed. Taken together, these results shed light on the
mechanism of reductive NRPS function and tetramic acid biosynthesis in
fungi.