Identification of the Biosynthetic Gene Cluster for Himeic Acid†A: A Ubiquitin-Activating Enzyme (E1) Inhibitor in Aspergillus japonicus MF275.

Himeic acid A, which is produced by the marine fungus Aspergillus japonicus MF275, is a specific inhibitor of the ubiquitin-activating enzyme E1 in the ubiquitin-proteasome system. To elucidate the mechanism of himeic acid biosynthesis, feeding experiments with labeled precursors have been performed. The long fatty acyl side chain attached to the pyrone ring is of polyketide origin, whereas the amide substituent is derived from leucine. These results suggest that a polyketide synthase-nonribosomal peptide synthase (PKS-NRPS) is involved in himeic acid biosynthesis. A candidate gene cluster was selected from the results of genome sequencing analysis. Disruption of the PKS-NRPS gene by Agrobacterium-mediated transformation confirms that HimA PKS-NRPS is involved in himeic acid biosynthesis. Thus, the him biosynthetic gene cluster for himeic acid in A. japonicus MF275 has been identified.

pH-dependent production of himeic acid A and its non-enzymatic conversions to himeic acids B and C.

The fungus Aspergillus japonicus MF275 produces himeic acid A (1), containing a 4-pyrone ring, along with its congeners, himeic acids B (2) and C (3). During culture, 1 was gradually converted to 3, the corresponding 4-pyridone derivative. A study of the relationship between the culture pH and the fungal metabolites showed that a decrease from pH 6.5 to pH 2 is essential for production of 1, while a subsequent increase to pH 5 is necessary for production of 3. In addition, we revealed that 1 was non-enzymatically converted to 3 by the incorporation of an ammonium nitrogen atom in a pH 5 buffer, and that 1 was converted to 2 at a conversion ratio of 50% during incubation in MeOH for five days.