them harbor large numbers of gene clusters predicted to encode the proteins
of secondary metabolite (SM) biosynthetic pathways. The numbers of clusters
far exceed the numbers of secondary metabolites that these fungi are known to
produce. Most of these SM clusters are, thus, cryptic, probably because the clusters
are not expressed under normal laboratory culture conditions. It has been possible
to use the well-developed molecular genetic system of Aspergillus nidulans to up-
regulate SM clusters by various strategies and this has allowed the identification of
more than 100 SMs and the elucidation of many SM pathways. These approaches
cannot be used for the vast majority of fungi, however, because they lack adequate
molecular genetic systems. We have developed an alternative approach in which
we express SM genes from other fungi in A. nidulans. This approach uses PCR to
amplify target genes, place them under control of a regulatable A. nidulans promoter
and fuse them to selectable markers. Furthermore, the approach uses the efficient
homologous recombination system of A. nidulans to assemble fragments of large
genes in vivo, creating functional genes many thousand base pairs in length. The
approach uses selectable marker recycling so that, in principle, an indefinite number
of genes can be transferred into A. nidulans. This approach allows all genes, or any
desired combination of genes, of SM clusters to be expressed in order to facilitate
elucidation of SM biosynthetic pathways.