All the versions of this article:
Bacteria of the genus Frankia belong to the class Actinobacteria. These bacteria were originally linked to fungi, because of the mycelium-like filaments many of them form. They have Gram-positive cell walls and their genomes display a high percentage of bases G and C (high G+C%). Actinobacteria notably include bacteria of the genus Mycobacterium, the causative agents of tuberculosis and leprosy, and bacteria of the genus Streptomyces, which produce many antibiotics.
Twelve species of Frankia are recognized today. These are nitrogen-fixing bacteria which live in symbiosis with a large spectrum of dicot plants (24 genera belonging to 8 families). These plants, with their symbiotic bacteria, are collectively responsible for 15% of the biologically fixed nitrogen in the world. Actinobacteria are not the only group that includes nitrogen-fixing bacteria : some are also found among the alpha-proteobacteria - Azospirillum which promotes the growth of graminaceae or Rhizobium, which fixes nitrogen in nodules formed on the roots of legumes - and among cyanobacteria - one species of which lives in symbiosis with the leaves of a water fern Azolla.
The Frankia alni ACN14a strain was isolated from a green alder (Alnus crispa) growing in Tadoussac, Canada. This bacterium is able to establish a nitrogen-fixing symbiosis with alder (Alnus spp. ) and myrtle (Myrica spp. ), two pioneer plant genera of temperate regions, found on forest clearings, mine wastes, sand dunes and glacial moraines where nitrogen is the limiting factor. Frankia alni causes root hair deformation: it penetrates the cortical cells and induces the formation of nodules which resemble those induced by Rhizobium in legumes. These nodules are then colonized by vegetative hyphae (mycelium filaments) which differentiate into diazo-vesicles. It is in these specialized, spherical, thick-walled cells that reductive nitrogen fixation takes place, protected from molecular oxygen by numerous layers of tightly stacked hopanoid lipids. No genetic system exists so far for unraveling the workings of the symbiosis between Frankia and its hosts. However, sequencing the genome of the ACN14a strain will help us understand how this interaction takes place, the nature of secondary metabolites produced, and how evolution has proceeded. The Frankia alni genome sequence will be analyzed through comparison with other sequenced actinobacteria: Streptomyces, Mycobacterium, Corynebacterium, Tropheryma and Bifidobacterium.
The Frankia sequencing project, which takes the form of a collaborative project between Genoscope, the CNRS and the University of Lyon (Center for Microbial Ecology, UMR CNRS 5557), with help from numerous other teams, began in October 2003.