Frankia Symbioses

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The genome of Frankia alni ACN14a contains a circular chromosome that is 7,, necleotides long. It also cantains plasmids, but the amount was not clear based on my research. It has a high G-C content of The genome has 6, genes, comprising of 6, protein coding genes, 63 structural RNAs, and 12 pseudo genes. Their goal is to gain more knowledge on the mechanisms behind Frankia' s symbiotic relationships.

Currently there are three projects taking place at the DOE Joint Genome Institute, two are in the draft assembly stage and one is in progress. Mbj2 , Frankia sp. EAN1pec , Frankia sp. Frankia are Gram-positive, aerobic, nitrogen-fixing bacteria.

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The membranes of Frankia , as well as the membranes of some other bacteria like Bradyrhizobium , Rhizobium , and Streptomyces , contain lipid components called hopanoids. Hopanoids, which are amphiphilic, pentacylic triterpenoid lipids, condense membrane lipids therefore stabilizing the membranes in a similar way to which sterols do in higher organisms.

There are many structural variants of hopanoids like polyol- and glyco-derivatives that can be found within various bacteria; the specific functions of each hopanoid derivative have not been found. Some people hypothesize that hopanoids may have more refined functions than just membrane stabilizing. In Frankia specifically, membranes containing hopanoids surround vesicles called diazovesicles that contain nitrogenase, an oxygen-sensitive enzyme. While hopaniods thicken and stabilize the walls which help to keep oxygen away from the nitrogenase, some suggest that the hopanoids themselves play a more specific and molecular role in the oxygen protection mechanism of nitrogenase.

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Frankia both Frankia that lives in a symbiotic relationship with plants and free-living Frankia strains secrete extracellular proteins that might be involved in processes like bacteriolysis, hydrolysis, and virulence. Some Frankia strains have been known to secrete extracellular cellulases, pectinases, and proteinases. It is not totally understood how these secretions impact Frankia's symbiotic systems, but they are being studied along with other common enzymes and proteins, like uptake hydrogenase, that are found in Frankia's symbiotic relationships.

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen H 2. Hydrogen uptake is also present in free-living Frankia. Organisms in symbiotic relationships with plant hosts are necessary for the health and survival of certain plants. They assist in creating and transporting certain root hormones, controlling pathogens and nematodes, root exploration, water retention, mineral uptake, and resource sharing.

Frankia specifically fixes nitrogen in the air and produces molecules that other plants can use. One type of symbiotic relationship including plant, mycorrhiza, and Frankia is called a tripartite relationship and is a complex, multi-layered community of organisms that protect and support each other. Other symbiotic relationships include Frankia , one or multiple plants, and other bacteria. Frankia alni's specific role in these relationships is to infect the roots of plants: it deforms root hairs of its plant host by going into the cortical cells and causing the formation of nodules.

Vegetative hyphae colonize the nodules and then differentiate into diazovesicles. These structures can be seen on the bottom right-hand picture. Primer sequences are given in S1 Table.


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Assay performance was evaluated with a standard curve. PCR products were validated by dissociation curve analysis and agarose gel electrophoresis. Assays were analyzed in triplicate with the standard curve method [ 48 ]. PCR efficiency was calculated with the Eco software with data obtained from the exponential phase of each amplification plot. Gene expression data was normalized against expression of the translation initiation factor gene IF Data analysis including data pre-processing and normalization was performed with GenEx version 5.

Repeat units between 8 and 11 bases with up to a 3 base gap. Repeat units between 12 and 19 bases with up to a 7 base gap. Repeat units between 20 and 90 bases with up to a 20 base gap. Repeat units of less than 12 bases must occur at least 10 times in the genome. Repeat units of less than 20 bases must occur twice in the genome.

Tandem repeats was analyzed with the MUMmer 3.

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It is assumed that one copy of a tandem repeat in a genome is not very significant unless it is long. Therefore, a genome-wide screen for the repeat used was added. The total number of bases incorporated into repeats for a particular repeat unit must total 50 or more bases. Repeats associated to non-mobile repeated genes, e. This yielded putative IS elements. Second, likely ISs identified in a previous investigation [ 54 ] were added to the database. This database included repeats from the ISfinder site [ 55 ], a web repository of known ISs, as well as repeats from cyanobacteria, Frankia genomes available in and other known symbiotic bacteria.

ISfinder and RepeatScout sequences were collected into a single Fasta file. Redundancies, generated when ISfinder sequences were also detected by RepeatScout, were removed, with the ISfinder name taking precedence. Each of the collected nucleotide sequences was used as queries in a BlastN search against each of the genomes, one at a time. In the next step, each footprint was analysed to determine what repeats it consisted of. This was performed by using the footprints as queries against the database of repeats.

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The repeat sequence that received the highest score with the footprint as query was chosen as the most likely IS to occupy the footprint. The search was then repeated with the remaining footprint as query. The process was repeated until all parts of the footprints had been identified. The whole process yielded a GenBank file for each of the investigated genomes, where the verified and putative ISs are described with position, kind and quality of identification S2 Table.

The inoculum source for the SU plants was a ground nodule suspension containing Frankia originally from Pakistan [ 56 ]; the inoculum source for the UCD plants was soil from the rhizosphere of nodulated Ceanothus griseus in California.

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Sequences were trimmed and denoised [ 57 ] to avoid artificial inflation of rare diversity from homopolymer read errors, and processed in QIIME 1. Finally, 2, and 4, bacterial reads from the two SU nodule samples with mean and standard deviation , and 1, and 4, bacterial reads from the two UCD nodule samples with a mean of and a standard deviation of were included in analysis.

All positions were weighted equally and all gaps were treated as missing data. It was then introduced first into E. The R. Plasmid DNA isolation were done from several colonies. The correct gene fusion was confirmed by sequencing. Transfer to the R. The construct in R. The seedlings were then transferred into ml glass flasks containing ml of nitrogen-free FP medium solution [ 65 ] gelified with 0.

When the seedlings were 2 weeks old, they were inoculated with wild-type R. The effects of the rhizobia on the plant roots was followed under a stereomicroscope. After infection, the roots were observed twice per week over a period of two months, looking for root hair deformation and nodule formation, on two separate sets of sextuplicates for all genetic constructs.

After 31 days, photographs were taken of deformed root hairs. First, plants were observed under a Leica MZ8 stereomicroscope using lateral illumination to avoid light reflections. For photography, a Zeiss Axioskop was used with an Axiocam MRC5 camera, and the root systems with agar were removed from the glass flasks.

However, since similar auxotrophies were found in the genomes of cultured Frankia strains, the only safe conclusion to be drawn is that not all actinobacterial versions of common metabolic enzymes are characterized yet. For this reason, it is also difficult to compare the saprotrophic potential—i.


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  • In comparison with CcI3, EAN1pec and ACN14a which have a high capacity for the production of secondary metabolites [ 67 ], Dg1 had a lower potential for the biosynthesis of polyketides, non-ribosomal peptide synthases and terpenoids S4 Table. The gvp operon, responsible for the synthesis of gas vesicle proteins that are supposed to maintain bacteria at the top of the water table in the soil [ 68 ], which is present partly in CcI3, and completely in ACN14a and EAN1pec, is missing in Dg1.

    These features indicate that Dg1 possesses a comparatively low potential for the biosynthesis of siderophores for iron scavenging, and of compounds used against competitors in the soil habitat. In bacterial symbionts, proliferation of the mobilome is a key feature of early stages of genome reduction [ 69 , 70 , 71 ].

    The mobilome encompasses mobile genetic elements—genetic units that can move within a genome or from cell to cell. Intracellular mobile genetic elements include transposons and insertion sequenes ISs. The latter are defined as genomic sequences of mobile DNA, typically — bp in length, that encode the enzyme transposase.

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    In this context, it is interesting that a larger number of transposases was observed in Dg1 compared with the other studied Frankia genomes S5 Table. To further our understanding on genome reduction in Frankia strains, we examined the identity and distribution, as well as length distribution, of IS elements in the Frankia genomes published to date. We defined a particular DNA sequence encoding a transposase as an IS when it was present in multiple copies in a genome, taking into account that IS elements may have accumulated mutations leading to differences in sequence and length.