Hopanoid lipids in Frankia: identification of squalene-hopene cyclase gene sequences

In Frankia, the microsymbiont in actinorhizal root nodules, nitrogen fixation takes place in specialized structures called vesicles. The lipidic vesicle envelope forms a barrier to oxygen diffusion, an essential part of the nitrogenase oxygen protection system. We have shown previously that the vesicle envelope is composed primarily of two species of hopanoid lipids, sterol-like molecules that are synthesized in a wide range of bacteria, including Frankia, several cyanobacteria, and rhizobia. The levels of hopanoid found in Frankia are among the highest of any organism known to date. Here we report that short (328-bp) DNA sequences from several strains of Frankia spp. have been identified that are homologous to a portion of the coding region of squalene-hopene cyclase (shc) genes. The fragments and corresponding polymerase chain reaction (PCR) primers can be used in phylogenetic comparisons of Frankia, both within Frankiaceae and among bacteria that synthesize hopanoids.