Sporulenes, three new polycyclic terpenoids from Bacillus subtilis spores

Abstract

Hopanoids are ubiquitous polycyclic isoprenoids derived from terpene metabolism. While hopanoids are believed to regulate membrane fluidity in prokaryotes, the significance of their structural diversity is not understood. Analysis of numerous bacterial species has revealed this class of compounds to be widespread among diverse taxonomic groups [1], but polycyclic terpenoids have yet to be detected in a large proportion of bacterial species. Previous investigations have failed to detect hopanoid-like compounds produced by Bacillus subtilis, for example, despite the presence of a putative squalene-hopene cyclase enzyme (SqhC) in its genome. By comparing lipid profiles of wild-type B. subtilis and an SqhC-deficient strain, we identified three novel tetracyclic terpenoids, the sporulenes, believed to be products of the B. subtilis SqhC. Structures of these compounds were deduced by NMR and mass spectroscopic methods. The unusual scalarane backbone of the sporulenes arises from cyclization of a regular heptaprenyl precursor rather than the more common substrate of terpenoid cyclases, squalene. The discovery of these monoaromatic terpenoids in bacterial spores highlights the structural diversity of terpenoids and underscores our modest understanding of their physiological functions.