Operons are clusters of unrelated genes with related functions that are a 
feature of prokaryotic genomes. Here, we report on an operon-like gene cluster 
in the plant Arabidopsis thaliana that is required for triterpene synthesis (the 
thalianol pathway). The clustered genes are coexpressed, as in bacterial 
operons. However, despite the resemblance to a bacterial operon, this gene 
cluster has been assembled from plant genes by gene duplication, 
neofunctionalization, and genome reorganization, rather than by horizontal gene 
transfer from bacteria. Furthermore, recent assembly of operon-like gene 
clusters for triterpene synthesis has occurred independently in divergent plant 
lineages (Arabidopsis and oat). Thus, selection pressure may act during the 
formation of certain plant metabolic pathways to drive gene clustering.