Genetic diversity among Elaeagnus compatible Frankia strains and sympatric-related nitrogen-fixing actinobacteria revealed by nifH sequence analysis

Elaeagnus compatible Frankia isolates from Tunisian soil have been previously clustered with Frankia, colonizing Elaeagnaceae and Rhamnaceae in two different phylogenetic subgroups, while strain BMG5.6 was described as a new lineage closely related to Frankia and Micromonospora genera. In this study we further assess the diversity of captured Frankia and the relationship with BMG5.6-like actinobacteria, by using nifH gene sequences. Using PCR-RFLP screening on DNA extracted from lobe nodules, additional microsymbionts sharing BMG5.6 features have been detected proving a widespread occurrence of these actinobacteria in Elaeagnus root nodules. Neighbour-Joining trees of Frankia nifH sequences were consistent with previously published 16S rRNA and GlnII phylogenetic trees. Although four main clades could be discerned, actinobacterial strain BMG5.6 was clustered with Frankia strains isolated from Elaeagnus. The present study underscored the emanation of new diazotrophic taxon isolated from actinorhizal nodules occupying intermediate taxonomic position between Frankia and Micromonospora. Moreover, its aberrant position in nifH phylogeny should open network investigations on the natural history of nitrogen-fixing gene among actinobacteria.