Modified oxidosqualene cyclases in the formation of bioactive secondary metabolites: Biosynthesis of the antitumor clavaric acid

Hypholoma sublateritium is a basidiomycete that produces the antitumor compound clavaric acid. We cloned a gene encoding an oxidosqualene cyclase (occ) that is involved in the conversion of oxidosqualene to clavaric acid. Mutants disrupted in occ by gene replacement were unable to synthesize clavaric acid, but did not require sterols for growth. Amplification of the occ gene produced a 35–67% increase in clavaric acid yield. Northern analysis of occ gene expression confirmed that there is a clear correlation of occ expression and clavaric acid biosynthesis. Analysis of the occ-encoded oxidosqualene cyclase revealed that it has VSDCVGE motif instead of the consensus VSDCTAE sequence of the active center. In summary, there is an oxidosqualene cyclase specific for secondary metabolite biosynthesis; this is in agreement with the finding of two squalene cyclases in the sequenced genomes of basidiomycetes.