Dickeya
solani, a plant-pathogenic
bacterium, produces solanimycin, a potent hybrid polyketide/nonribosomal
peptide (PKS/NRPS) anti-fungal compound. The biosynthetic gene cluster
responsible for synthesis of this compound has been identified. Because
of instability, the complete structure of the compound has not yet
been elucidated, but LC-MS2identified that the cluster
produces two main compounds, solanimycin A and B, differing by a single
hydroxyl group. The fragmentation pattern revealed that the central
part of solanimycin A is a hexapeptide, Gly-Dha-Dha-Dha-Dha-Dha (where
Dha is dehydroalanine). This is supported by isotopic labeling studies
using labeled serine and glycine. The N-terminal group is a polyketide-derived
C16acyl group containing a conjugated hexaene, a hydroxyl,
and an amino group. The additional hydroxyl group in solanimycin B
is on the α-carbon of the glycine residue. The incorporation
of five sequential Dha residues is unprecedented because there is
only one NRPS module in the cluster that is predicted to activate
and attach serine (which is subsequently dehydrated to Dha), meaning
that this NRPS module must act iteratively. While a few other iterative
NRPS modules are known, they all involve iteration of two or three
modules. We believe that the repetitive use of asinglemodule makes the solanimycin biosynthetic pathway unique among NRPSs
so far reported.