Hexanoate synthase, a specialized type I fatty acid synthase in aflatoxin B-1 biosynthesis

In fungi, fatty acids are biosynthesized by large multifunctional enzyme co mplexes, the fatty acid synthases (FASs), which catalyze chain assembly in an iterative manner. Many fungal secondary metabolites contain fatty acid m oieties, and it is often unclear whether they are recruited from primary me tabolism or are biosynthesized de novo by secondary metabolic FASs. The mos t convincing evidence of such a dedicated FAS comes from the biosyntheses o f aflatoxin (AF) and sterigmatocystin (ST) in certain species of the filame ntous fungus Aspergillus. Incorporation studies in AF and genetic analyses of ST and AF biosynthesis strongly suggest that their biosyntheses be-in wi th the production of a C-6 fatty acid by a specialized FAS. The genes encod ing the alpha (hexA) and beta (hexB) subunits of this hexanoate synthase (H exS) from the AF pathway in Aspergillus parsiticus SU-1 were cloned and bot h their gDNAs and cDNAs were sequenced and their transcriptional ends analy zed. Translated amino acid sequences are predicted to result in proteins of 181.3 and 210.5 k-Da, for HexA and HexB, respectively. Comparison of the H exA and HexB sequences with those of the ST FAS subunits and primary metabo lic FASs indicated that the secondary metabolic enzymes are members of a we ll-defined subclass of the FAS family. Phylogenetic predictions and an anal ysis of GC-bias in AF and ST pathway genes compared with primary metabolic Aspergillus genes were used as a basis to propose a route for the evolution of the AF and ST clusters. (C) 2001 Academic Press.