Arabidopsis camelliol C synthase evolved from enzymes that make pentacycles.

We establish by heterologous expression that the Arabidopsis thaliana oxidosqualene cyclase At1g78955 (CAMS1) makes camelliol C (98%), achilleol A (2%), and beta-amyrin (0.2%). CAMS1 is the first characterized cyclase that generates predominantly a monocyclic triterpene alcohol. Phylogenetic analysis shows that CAMS1 evolved from enzymes that make pentacycles, thus revealing that its pentacyclic beta-amyrin byproduct is an evolutionary relic. Sequence alignments support prior suggestions that decreased steric bulk at a key active-site residue promotes monocycle formation.