Characterization of a gene locus containing squalene-hopene cyclase (shc) in Frankia alni ACN14a, and an shc homolog in Acidothermus cellulolyticus

Hopanoid lipids are pentacyclic triterpenes that are synthesized by eubacterial prokaryotes, including the two phylogenetically-related actinobacteria Frankia and Acidothermus, and that function in protection against environmental stress. In the nitrogen-fixing Frankia, hopanoids in the vesicle envelope reduce oxygen diffusion to nitrogenase, while in the thermophile Acidothermus, hopanoids may play a role in adaptation to high temperature and low pH. A 26,335 nucleotide region of the Frankia alni genome encoding the squalene-hopene cyclase gene (shc) was cloned and sequenced. Contrary to several characterized shc genes in other organisms, the Frankia shc gene was not part of a hopanoid biosynthetic cluster. Rather, the neighboring genes included several regulators, oxidoreductases, transmembrane proteins as well as multidrug transporters. A second copy of the shc gene, detected by Southern hybridization, is likely to be present in F. alni. A homologous gene was amplified and sequenced in Acidothermus cellulolyticus and found to have a sequence distance of 36.9% from that of Frankia. Within the actinobacteria, phylogenetic analysis shows a close relationship between the shc gene sequences of F. alni and A. cellulolyticus.