Open Access

Complete genome sequence of Mesorhizobium opportunistum type strain WSM2075T

  • Wayne Reeve
  • , Kemanthi Nandasena
  • , Ron Yates,
  • , Ravi Tiwari
  • , Graham O’Hara
  • , Mohamed Ninawi
  • , Olga Chertkov
  • , Lynne Goodwin
  • , David Bruce
  • , Chris Detter
  • , Roxanne Tapia
  • , Shunseng Han
  • , Tanja Woyke
  • , Sam Pitluck
  • , Matt Nolan
  • , Miriam Land
  • , Alex Copeland
  • , Konstantinos Liolios
  • , Amrita Pati
  • , Konstantinos Mavromatis
  • , Victor Markowitz
  • , Nikos Kyrpides
  • , Natalia Ivanova
  • , Lynne Goodwin
  • , Uma Meenakshi
  • and John Howieson
Corresponding author

DOI: 10.4056/sigs.4538264

Received: 15 December 2013

Accepted: 15 December 2013

Published: 20 December 2013


Mesorhizobium opportunistum strain WSM2075T was isolated in Western Australia in 2000 from root nodules of the pasture legume Biserrula pelecinus that had been inoculated with M. ciceri bv. biserrulae WSM1271. WSM2075T is an aerobic, motile, Gram negative, non-spore-forming rod that has gained the ability to nodulate B. pelecinus but is completely ineffective in N2 fixation with this host. This report reveals that the genome of M. opportunistum strain WSM2075T contains a chromosome of size 6,884,444 bp, encoding 6,685 protein-coding genes and 62 RNA-only encoding genes. The genome contains no plasmids, but does harbor a 455.7 kb genomic island from Mesorhizobium ciceri bv. biserrulae WSM1271 that has been integrated into a phenylalanine-tRNA gene.


root-nodule bacterianitrogen fixationevolutionlateral gene transferintegrative and conjugative elementssymbiosisAlphaproteobacteria


Biserrula pelecinus L. is an autogamous annual legume species that is common, though never dominant, on coarse textured and acidic Mediterranean soils [1] and can often be found with other annual legumes including subterranean clover (Trifolium subterraneum) and serradella (Ornithopus) [2]. This reseeding legume was introduced to Western Australia in 1993 in a pasture legume breeding and selection program that sought to develop new pasture legume options for the sandy surfaced duplex, acidic soils in Western Australia, to improve soil fertility and farming system flexibility [1]. At the time of introduction, the Australian resident rhizobial populations were not capable of nodulating B. pelecinus [1,3] and a Mediterranean strain Mesorhizobium ciceri bv. biserrulae WSM1271 had to be used as an inoculant to establish an effective nitrogen fixing symbiosis. After 6 years of cultivation of B. pelecinus under field conditions, an isolate (designated WSM2075) was recovered from root nodules of plants grown near Northam, Western Australia that displayed an ineffective symbiotic phenotype [4]. Accumulated evidence revealed that WSM2075 had gained the ability to nodulate (but not fix with) B. pelecinus by acquiring symbiotic genes from the original inoculant strain Mesorhizobium ciceri bv. biserrulae WSM1271 following a lateral gene transfer event [5]. Strain WSM2075 has now been designated as strain WSM2075T (= LMG 24607 = HAMBI 3007) and is the type strain for a new species described as Mesorhizobium opportunistum [6]. The species name op.por.tu.nis’tum. L. neut. adj. opportunistum reflects the opportunistic behavior of the organism to nodulate a range of legume hosts by acquiring symbiotic genes [4,5]. M. opportunistum WSM2075T is competitive for nodulation of B. pelecinus but cannot fix nitrogen [4] and the finding of such strains that have rapidly evolved in the soil presents a threat to the successful establishment of this valuable pasture species in Australia [5].

Here we present a summary classification and a set of general features for M. opportunistum strain WSM2075T together with the description of the complete genome sequence and annotation. Here we reveal that a 455.7 kb genomic island from the inoculant Mesorhizobium ciceri bv. biserrulae WSM1271 has been horizontally transferred into M. opportunistum strain WSM2075T and integrated into the phenylalanine-tRNA gene.

Classification and general features

M. opportunistum strain WSM2075T is a motile, Gram-negative, non-spore-forming rod (Figure 1A and Figure 1B in the order Rhizobiales of the class Alphaproteobacteria. They are moderately fast growing, forming 2-4 mm diameter colonies within 3-4 days and have a mean generation time of 4-6 h when grown in half Lupin Agar (½LA) broth [7] at 28°C. Colonies on ½LA are white-opaque, slightly domed, moderately mucoid with smooth margins (Figure 1C).

Figure 1A

Image of Mesorhizobium opportunistum strain WSM2075T using scanning electron microscopy

Figure 1B

Image of Mesorhizobium opportunistum strain WSM2075T using transmission electron microscopy

Figure 1C

Image of Mesorhizobium opportunistum strain WSM2075T colony morphology on a solid medium (C).

Strains of this organism are able to tolerate a pH range between 5.5 and 9.0. Carbon source utilization and fatty acid profiles have been described previously [6]. Minimum Information about the Genome Sequence (MIGS) is provided in Table 1.

Table 1

Classification and general features of Mesorhizobium opportunistum strain WSM2075T according to the MIGS recommendations [8,9].




    Evidence code

    Current classification

   Domain Bacteria

    TAS [9]

   Phylum Proteobacteria

    TAS [10]

   Class Alphaproteobacteria

    TAS [11,12]

   Order Rhizobiales

    TAS [12,13]

   Family Phyllobacteriaceae

    TAS [12,14]

   Genus Mesorhizobium

    TAS [15]

   Species Mesorhizobium opportunistum

    TAS [6]

    Gram stain


    TAS [6]

    Cell shape


    TAS [6]



    TAS [6]



    TAS [16]

    Temperature range


    TAS [16]

    Optimum temperature


    TAS [6]





    Oxygen requirement


    TAS [16]

    Carbon source

   Arabinose, β-gentibiose, glucose, mannitol & melibiose

    TAS [6]

    Energy source


    TAS [16]



   Soil, root nodule, host

    TAS [6]


    Biotic relationship

   Free living, Symbiotic

    TAS [6]





    Biosafety level


    TAS [17]


   Root nodule of Biserrula pelecinus L.

    TAS [6,18]


    Geographic location

   Northam, Western Australia

    TAS [6,18]


    Nodule collection date

   August 2000

    TAS [4]




    TAS [4]




    TAS [4]



   10 cm




   160 m


Evidence codes - TAS: Traceable Author Statement (i.e., a direct report exists in the literature); NAS: Non-traceable Author Statement (i.e., not directly observed for the living, isolated sample, but based on a generally accepted property for the species, or anecdotal evidence). These evidence codes are from the Gene Ontology project [19].

Figure 2 shows the phylogenetic neighborhood of Mesorhizobium opportunistum strain WSM2075T in a 16S rRNA sequence based tree. This strain clusters in a tight group which included M. amorphae, M. huakuii, M. plurifarium and M. septentrionale and has >99% sequence identity with all four type strains. However, based on a polyphasic taxonomic study we have identified that this strain belongs to a new species [6].

Figure 2

Phylogenetic tree showing the relationships of Mesorhizobium opportunistum WSM2075T with other root nodule bacteria in the order Rhizobiales based on aligned sequences of the 16S rRNA gene (1,290 bp internal region). All positions containing gaps and missing data were eliminated. Phylogenetic analyses were performed using MEGA, version 3.1 [20]. The tree was built using the Maximum-Likelihood method with the General Time Reversible model and bootstrap analysis [21] with 500 replicates to construct a consensus tree. Type strains are indicated with a superscript T. Brackets after the strain name contain a DNA database accession number and/or a GOLD ID (beginning with the prefix G) for a sequencing project registered in GOLD [22]. Published genomes are indicated with an asterisk.


M. opportumistum strain WSM2075T forms an ineffective (non-N fixing) symbiosis with its original host of isolation, B. pelecinus L., as well as with Astragalus adsurgens, A. membranaceus, Lotus peregrinus and Macroptilium atropurpureum [4,6]. In all cases the root nodules formed are small, white and seem incapable of fixing nitrogen [6]. Strain WSM2075T has a broader host range for nodulation than Mesorhizobium ciceri bv. biserrulae WSM1271 [6].

Genome sequencing and annotation

Genome project history

This organism was selected for sequencing on the basis of its environmental and agricultural relevance to issues in global carbon cycling, alternative energy production, and biogeochemical importance, and is part of the Community Sequencing Program at the U.S. Department of Energy, Joint Genome Institute (JGI) for projects of relevance to agency missions. The genome project is deposited in the Genomes OnLine Database [22] and the complete genome sequence in GenBank. Sequencing, finishing and annotation were performed by the JGI. A summary of the project information is shown in Table 2.

Table 2

Genome sequencing project information for Mesorhizobium opportunistum WSM2075T.





    Finishing quality



    Libraries used

    Illumina GAii shotgun library, 454 Titanium standard library and paired end 454 libraries


    Sequencing platforms

    Illumina and 454 technologies


    Sequencing coverage

    454 std (63.8×), 454 paired end (91.5×) and Illumina (1×), total 146.9×



    Velvet, Newbler, phred/Phrap/Consed


    Gene calling method

    Prodigal, GenePRIMP

    Genbank ID


    Genbank Date of Release

    January 21, 2011



    NCBI project ID


    Database: IMG


    Project relevance

    Symbiotic nitrogen fixation, agriculture

Growth conditions and DNA isolation

M. opportunistum strain WSM2075T was grown to mid logarithmic phase in TY rich medium [23] on a gyratory shaker at 28°C. DNA was isolated from 60 mL of cells using a CTAB (Cetyl trimethyl ammonium bromide) bacterial genomic DNA isolation method [24].

Genome sequencing and assembly

The genome of Mesorhizobium opportunistum WSM2075T was sequenced at the Joint Genome Institute (JGI) using a combination of Illumina [25] and 454 technologies [26]. An Illumina GAii shotgun library comprising 370 Mb in reads of 36 bases, a 454 Titanium library with read length of 480-495 bases containing approximately 1.05 million reads, and a paired end 454 library containing 63840 reads with average insert size of 39 Kb were generated for this genome. All general aspects of library construction and sequencing performed at the JGI can be found at [24]. Illumina sequencing data was assembled with VELVET [27], and the consensus sequences were shredded into 1.5 Kb overlapped fake reads and assembled together with the 454 data. Draft assemblies were based on 375 Mb 454 standard data, and all of the 454 paired end data. Newbler parameters used were ‘-consed -a 50 -l 350 -g -mi 96 -ml 96’. The initial Newbler assembly contained 44 contigs in 1 scaffold. We converted the initial 454 assembly into a phrap assembly by making fake reads from the consensus, collecting the read pairs in the 454 paired end library. The Phred/Phrap/Consed software package was used for sequence assembly and quality assessment [28-30] in the subsequent finishing process. Illumina data was used to correct potential base errors and increase consensus quality using software developed at JGI (Polisher, Alla Lapidus, unpublished). After the shotgun stage, reads were assembled with parallel phrap (High Performance Software, LLC). Gaps were closed in silico using software developed at JGI (gapResolution, unpublished), and mis-assemblies were corrected using Dupfinisher [31], or sequencing cloned bridging PCR fragments. Remaining gaps between contigs were manually closed by editing in Consed, by PCR, and by Bubble PCR primer walks. A total of 464 additional reactions and 3 shatter libraries were necessary to close all gaps and to improve the quality of the finished sequence.

Genome annotation

Genes were identified using Prodigal [32] as part of the Oak Ridge National Laboratory genome annotation pipeline, followed by a round of manual curation using the JGI GenePrimp pipeline [33]. The predicted CDSs were translated and used to search the National Center for Biotechnology Information (NCBI) nonredundant database, UniProt, TIGRFam, Pfam, PRIAM, KEGG, COG, and InterPro databases. These data sources were combined to assert a product description for each predicted protein. Non-coding genes and miscellaneous features were predicted using tRNAscan-SE [34], RNAMMer [35], Rfam [36], TMHMM [37], and SignalP [38]. Additional gene prediction analyses and functional annotation were performed within the Integrated Microbial Genomes (IMG-ER) platform [39].

Genome properties

The genome is 6,884,444 nucleotides with 62.87% GC content (Table 3) and comprised of a single chromosome and no plasmids. From a total of 6,747 genes, 6,685 were protein encoding and 62 RNA only encoding genes. Within the genome, 177 pseudogenes were also identified. The majority of genes (71.11%) were assigned a putative function while the remaining genes were annotated as hypothetical. The distribution of genes into COGs functional categories is presented in Table 4 and Figure 3.

Table 3

Genome Statistics for Mesorhizobium opportunistum WSM2075T.



    % of Total

Genome size (bp)



DNA coding region (bp)



DNA G+C content (bp)



Number of replicons


Extrachromosomal elements


Total genes



RNA genes



Protein-coding genes



Genes with function prediction



Genes assigned to COGs



Genes assigned Pfam domains



Genes with signal peptides



Genes with transmembrane helices



Table 4

Number of protein coding genes of Mesorhizobium opportunistum WSM2075T associated with the general COG functional categories.








    Translation, ribosomal structure and biogenesis




    RNA processing and modification








    Replication, recombination and repair




    Chromatin structure and dynamics




    Cell cycle control, mitosis and meiosis




    Nuclear structure




    Defense mechanisms




    Signal transduction mechanisms




    Cell wall/membrane biogenesis




    Cell motility








    Extracellular structures




    Intracellular trafficking and secretion




    Posttranslational modification, protein turnover, chaperones




    Energy production conversion




    Carbohydrate transport and metabolism




    Amino acid transport metabolism




    Nucleotide transport and metabolism




    Coenzyme transport and metabolism




    Lipid transport and metabolism




    Inorganic ion transport and metabolism




    Secondary metabolite biosynthesis, transport and catabolism




    General function prediction only




    Function unknown




    Not in COGS



Figure 3

Graphical circular map of the chromosome of Mesorhizobium opportunistum WSM2075T. From outside to the center: Genes on forward strand (color by COG categories as denoted by the IMG platform), Genes on reverse strand (color by COG categories), RNA genes (tRNAs green, sRNAs red, other RNAs black), GC content, GC skew.



This work was performed under the auspices of the US Department of Energy’s Office of Science, Biological and Environmental Research Program, and by the University of California, Lawrence Berkeley National Laboratory under contract No. DE-AC02-05CH11231, Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344, and Los Alamos National Laboratory under contract No. DE-AC02-06NA25396. We gratefully acknowledge the funding received from Australian Research Council Discovery grant (DP0880896), Murdoch University Strategic Research Fund through the Crop and Plant Research Institute (CaPRI) and the Centre for Rhizobium Studies (CRS) at Murdoch University. The authors would like to thank the Australia-China Joint Research Centre for Wheat Improvement (ACCWI) and SuperSeed Technologies (SST) for financially supporting Mohamed Ninawi’s PhD project.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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