Aspergillus has distinct fatty acid synthases for primary and secondary metabolism

Abstract

Aspergillus nidulans contains two functionally distinct fatty acid synthases (FASs): one required for primary fatty acid metabolism (FAS) and the other required for secondary metabolism (sFAS). FAS mutants require long-chain fatty acids for growth, whereas sFAS mutants grow normally but cannot synthesize sterigmatocystin (ST), a carcinogenic secondary metabolite structurally and biosynthetically related to aflatoxin. sFAS mutants regain the ability to synthesize ST when provided with hexanoic acid, supporting the model that the ST polyketide synthase uses this short-chain fatty acid as a starter unit. The characterization of both the polyketide synthase and FAS may provide novel means for modifying secondary metabolites.

Figure 1

Sterigmatocystin/AF pathway. The aliphatic moiety of norsolorinic acid, the first intermediate in the pathway, is enclosed in a box.

Figure 2

Growth requirements of the strains disrupted in the two FASs. From the top clockwise: TTAARG (wild type), TDB57.1 (fasB⁻), TDB5 (stcK⁻), TDB1 (stcJ⁻), and TDB51.1 (fasA⁻). Minimal media (B) and minimal media plus 2 mM myristic acid (n-tetradecanoic acid, Sigma) (A).

Figure 3

ST analysis of oatmeal culture extracts by TLC. Approximately 20 times as much material from stcJ⁻ and stcK⁻ mutants was loaded as compared with wild type and samples were resolved with benzene/glacial acetic acid (95:5 vol/vol). The nature of the band above ST is unknown.

Figure 4

Amino acid alignments of two representative FAS domains. (A) FAS β-keto acyl synthase domain of the α subunit. Ca, C. albicans (23); Sc, S. cerevisiae (25); Pp, P. patulum (28); and FasA and StcJ, A. nidulans. The constitutive FAS a peptides share 78% identity within this domain in contrast to StcJ that shares only 57% identity. (B) FAS malonyl/palmitoyl transferase domain of the β subunit. Ca, C. albicans (22); Sc, S. cerevisiae (24); Yl, Y. lipolytica (26); and FasB and StcK, A. nidulans. The constitutive FAS β peptides share 72% identity in contrast to StcK that shares 44% identity.