Hopanoid lipids in Frankia: identification of squalene-hopene cyclase genesequences

In Frankia, the microsymbiont in actinorhizal root nodules, nitrogen fixati on takes place in specialized structures called vesicles. The lipidic vesic le envelope forms a barrier to oxygen diffusion, an essential part of the n itrogenase oxygen protection system. We have shown previously that the vesi cle envelope is composed primarily of two species of hopanoid lipids, stero l-like molecules that are synthesized in a wide range of bacteria, includin g Frankia, several cyanobacteria, and rhizobia. The levels of hopanoid foun d in Frankia are among the highest of any organism known to date. Here we r eport that short (328-bp) DNA sequences from several strains of Frankia spp . have been identified that are homologous to a portion of the coding regio n of squalene-hopene cyclase (shc) genes. The fragments and corresponding p olymerase chain reaction (PCR) primers can be used in phylogenetic comparis ons of Frankia, both within Frankiaceae and among bacteria that synthesize hopanoids.