A Microbial Biorealm page on the genus Frankia alni
Higher order taxa
Bacteria; Actinobacteria; Actinobacteria (class); Actinobacteridae; Actinomycetales; Frankineae; Frankiaceae;
Description and significance
"The actinomycete Frankia is of fundamental and ecological interests for several reasons including its wide distribution, its ability to fix nitrogen, differentiate into sporangium and vesicles (specialized cell for nitrogen-fixation), and to nodulate plants from about 24 genera." 
Species of the Frankia Genus are Gram-positive bacteria.Frankia sp. are filamentous nitrogen-fixing bacterium that grow by branching and tip extension and thus resemble the antibiotic-producing Streptomyces sp. . They live in the soil and have a symbiotic relationship with certain woody angiosperms, called actinorhizal plants. During growth, the Frankia sp. produce three cell types: sporangiospores, hyphae, and diazo-vesicles (spherical, thick walled, lipid-enveloped cellular structures). The diazo-vesicles are responsible for supplying sufficient nitrogen to the host plant during symbiosis. Frankia supplies most or all of the host plant nitrogen needs without added nitrogen and thus can establish a nitrogen-fixing symbiosis with host plants where nitrogen is the limiting factor in the growth of the host. Therefore, actinorhizal plants colonize and often prosper in soils that are low in combined nitrogen. Symbiosis of this kind adds a large proportion of new nitrogen to several ecosystems such as temperate forests, dry chaparral, sand dunes, mine wastes, etc.
The Frankia alni ACN14a was first isolated in Tadoussac, Canada from a green alder (Alnus crispa). Except for Australia and Antarctica, Frankia alni can be isolated from soils on all continents. Frankia alni causes root hair deformation in a way that it enters the cortical cells and induces the nodule formations, which look like those induced by Rhizobium in legumes. Then, the nodules are colonized by vegetative hyphae (mycelium filaments) that differentiate into diazo-vesicles. Reductive nitrogen fixation takes place in the diazo-vesicles and this process is protected from molecular oxygen by many layers of tightly stacked hopanoid lipids.
The genome sequencing project of Frankia alni ACN14a was done at the Genoscope sequencing center and was completed on 03/08/2006. The genome of Frankia alni ACN14a contains a circular chromosome that is 7,497,934 necleotides long. It also contains plasmids, but the amount was not clear based on my research . It has a high G-C content of 72.8% and contains 88% coding regions. The genome has 6,786 genes, comprising of 6,711 protein coding genes, 63 structural RNAs, and 12 pseudo genes.
Cell Structure and Metabolism
Three different structural forms characterize Frankia: hyphae, sporangia, and vesicles. In free-living Frankia cells, the hyphae are septate and tightly interwoven in culture and produce terminal or intercalary multilocular sporangia. Vesicles are normally initiated when the nitrogen source in the medium is withdrawn or when growth occurs on amino acids or other nitrogen sources that cannot be degraded to ammonia.
Frankia alni undergoes glycerolipid metabolism, which includes glycolysis, and fatty acid metabolism. In symbiosis, when provided with NADH, succinate, phosphorylated hexoses plus pyridine nucleotides in various combinations, trehalose and other sugars, or a combination of malate, glutamate, and NAD+, vesicle cluster suspensions from Alnus species respire. Frankia alni also under goes nitrogen metabolism through the vesicles that are the main site of nitrogenase proteins. Vesicle appearance correlates with nitrogenase activity and degeneration of vesicles and nodules coincides with loss of nitrogenase activity. Optimal nitrogenase activity occurs at 20 kPa of O2, and is inhibited at any kPa O2 of above 25, therefore, a variety of mechanisms are used to limit the O2 diffusion to nitrogenase in symbiosis.
Frankia alni is a nitrogen fixing bacteria that assists actinorhizal plants. Certain plants in certain environment require organisms to be in symbiotic relationship with them. Frankia supplies most or all of the host plant nitrogen needs without added nitrogen and thus can establish a nitrogen-fixing symbiosis with host plants where nitrogen is the limiting factor in the growth of the host. Therefore, actinorhizal plants colonize and often prosper in soils that are low in combined nitrogen. Symbiosis of this kind adds a large proportion of new nitrogen to several ecosystems such as temperate forests, dry chaparral, sand dunes, mine wastes, etc. Frankia alni causes root hair deformation in a way that it enters the cortical cells and induces the nodule formations, which look like those induced by Rhizobium in legumes. Then, the nodules are colonized by vegetative hyphae (mycelium filaments) that differentiate into diazo-vesicles. Reductive nitrogen fixation takes place in the diazo-vesicles and this process is protected from molecular oxygen by many layers of tightly stacked hopanoid lipids. 
Studies have not been able to identify Frankia as a pathogen, causing any disease.
Sample of some current research as of June 5th, 2007.
"Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography" Summary- Three closely related faculative symbiotic Frankia sp strains which each infect a subset of plants among eight angiosperm families were used. Their genomes were sequenced and their sized varied. The biogeographic history of symbiosis and host plant speciation, were in agreement with the extent of the gene acquisition, deletion and duplication. As the host plant was more diverse, it favored genome expansion and host plant isolation favored genome contraction. The results supported the idea that in faculatative symbiotic soil bacteria, genome expansions and reductions can occur as a response to new environments. 
 Lavire, C., Cournoyer, B. "Progress on the Genetics of the N2-fixing actnorhizal symbiont Frankia ". Plant and Sil. July 2003, Volume 254, Number 1. p.125-137. 
 Benson, D., Silvester, W., "Biology of Frankia Strains, Actinomycete Symbionts of Actinorhizal Plants". Microbiological Reviews, June 1993. p.293-319 
 Genoscope: Frankia alni: A symbiotic nitrogen-fixing actinobacterium
 Tjepkema,J. D., Ormerod, W. O.,Torrey, J. G.," Vesicle Formation and acetylene reduction (nitrogenase activity) in Frankia sp. CpI1 in Culture". Microbiological Reviews. 1980, Volume 27. p.815-823.
 Zhang, X.,Benson, D. R., "Utilization of Amino Acids by Frankia sp. strain CpI1". Arch. Microbiol. 1992, Volume 158. p256-261.
 Normand P, Lapierre P, Tisa LS, Gogarten JP, Alloisio N, Bagnarol E, Bassi CA, Berry AM, Bickhart DM, Choisne N, Couloux A, Cournoyer B, Cruveiller S, Daubin V, Demange N, Francino MP, Goltsman E, Huang Y, Kopp OR, Labarre L, Lapidus A, Lavire C, Marechal J, Martinez M, Mastronunzio JE, Mullin BC, Niemann J, Pujic P, Rawnsley T, Rouy Z, Schenowitz C, Sellstedt A, Tavares F, Tomkins JP, Vallenet D, Valverde C, Wall LG, Wang Y, Medigue C, Benson DR. "Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography". Genome Res. 2007 Jan;17(1):7-15. Epub 2006 Dec 6.PMID: 17151343 [PubMed - indexed for MEDLINE]
 Huguet, V., Batzli, J. M., Zimpfer, J. F., Normand, P., Dawson, J. O., Fernandez, M. P. "Diversity and Specificity of Frankia Strains in Nodules of Sympatric Myrica gale, Alnus incana, and Shepherdia canadenis Determined by rrs Gene Polymorphism" Applied and Environmental Microbiology, May 2001. p. 2116-2122.
 Xu, X., Kong, R., de Bruijn, F. J., He, S. Y., Murry, M. A., Newman, T., Wolk, C. P. " DNA sequence and genetic characterization of plasmid mFQ11 from Frankia alni strain Cpl1". Jan. 2002. 22;207(1): 103-7. PIMD: 11886759 [PubMed-indexed for MEDLINE]
Edited by Mandana Farahani student of Rachel Larsen and Kit Pogliano at UCSD.