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Applied and Environmental Microbiology, October 2008, p. 6447-6451, Vol. 74, No. 20
0099-2240/08/$08.00+0     doi:10.1128/AEM.01024-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Extensive Genome Rearrangements and Multiple Horizontal Gene Transfers in a Population of Pyrococcus Isolates from Vulcano Island, Italy ^,

James R. White,¹ Patricia Escobar-Paramo,¹ Emmanuel F. Mongodin,^2, Karen E. Nelson,² and Jocelyne DiRuggiero^1*

University of Maryland, College Park, Maryland,¹ J. Craig Venter Institute, Rockville, Maryland²

Received 7 May 2008/ Accepted 14 August 2008

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The extent of chromosome rearrangements in Pyrococcus isolates from marine hydrothermal vents in Vulcano Island, Italy, was evaluated by high-throughput genomic methods. The results illustrate the dynamic nature of the genomes of the genus Pyrococcus and raise the possibility of a connection between rapidly changing environmental conditions and adaptive genomic properties.

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Environmental genomic surveys have increasingly demonstrated the remarkable diversity of natural microbial communities and have had a significant impact on our understanding of microbial ecology and evolution (10, 34). For example, data sets of environmental microbial sequences and fine-scale population studies revealed the high level of genetic diversity between strains or ribotypes in natural populations, as seen at the levels of sequence divergence, gene content, and genome organization (1, 27, 31, 38). Genomic recombination and the presence of mobile genetic elements resulting in insertion, deletion, and rearrangements have been reported as important mechanisms for genotypic diversity in natural populations of both Bacteria and Archaea (2, 12, 37, 40). In a previous study, we showed that Pyrococcus sp. strains isolated from the same shallow hydrothermal vent system, with <1% divergence at the level of their 16S rRNA sequences, showed extensive gene rearrangements and insertions/deletions associated with transposition events (12). These results raise the question of the role of nucleotide substitutions and genome rearrangements as driving forces in generating genetic diversity and in altering physiological properties and, therefore, in microbial adaptation to available resources.

We used comparative genomic hybridization (CGH) with Pyrococcus furiosus and seven isolates from Vulcano Island (Table 1) (12) to determine the extent of chromosome rearrangements within the Pyrococcus genus. Genomic DNAs from Pyrococcus sp. environmental isolates were hybridized to a microarray consisting of 2,060 unique PCR products, representing the genome of P. furiosus DSM 3638 (GenBank accession no. AE009950) as described before (23). Statistical analysis of the data was performed using GACK analysis software (17a). Genomic DNA from all environmental Pyrococcus isolates hybridized against most of the open reading frames (ORFs) present on the P. furiosus whole-genome array, reflecting a high level of sequence identity and genome conservation across these isolates. A significant finding from this study is the identification of six chromosomal regions, also called genomic islands (GIs), that were highly variable among the eight Pyrococcus genomes (Table 2). These regions contained between 5 and 126 ORFs, with 70% of the ORFs annotated as hypothetical or conserved hypothetical proteins in the genome of P. furiosus. No known essential or housekeeping genes were found among the variable ORFs, and the G+C content was not different than that of the rest of the genome of P. furiosus. Whereas CGH identifies ORFs from P. furiosus that are present or absent in the query isolates, it does not give information regarding genome location or gene order in the query isolates. This is particularly true for transposase sequences with nearly identical nucleotide sequences. However, an alignment of the GIs between P. furiosus, Pyrococcus abyssi, and Pyrococcus horikoshii substantiated the authenticity of those genomic regions (Fig. 1) (see Table S1 in the supplemental material). Patterns of presence and absence of groups of ORFs within these regions of synteny between the three Pyrococcus reference genomes, and the absence of those ORFs from the Vulcano isolates, characterized those regions as potential hot spots for chromosomal rearrangements. Two GIs, regions IV and VI, were associated with two to five insertion sequence (IS) elements of the IS6 family that is typically found in the P. furiosus genome (21), indicating possible mobile element-mediated horizontal gene transfer (HGT). This hypothesis was strengthened by the discovery of a 16-kb fragment in region V that is present only in P. furiosus and the closely related hyperthermophile Thermococcus litoralis but is absent from P. abyssi, P. horikoshii, and all the other Vulcano isolates (12). This genomic fragment was flanked by IS elements in P. furious.

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TABLE 1. Strains used in this study

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TABLE 2. ORF contents for highly variable chromosome regions I to VI

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FIG. 1. Highly variable chromosomal regions in Pyrococcus strains and environmental isolates. (A) Region I (PF0025 to PF0042; 18 ORFs). Colored arrows represent ORFs present in the three Pyrococcus genomes and VB112, gray arrows represent ORFs present only in P. furiosus and VB112, and striped arrows represent ORFs present in some environmental isolates. (B) Region V (PF1337 to PF1341; five ORFs) (boxed). Colored arrows represent ORFs present in the three Pyrococcus genomes and VB112, black arrows represent ORFs not in synteny in region V of P. horikoshii, and striped arrows represent ORFs present in some environmental isolates. (C) Region IV (PF0681 to PF0807, 126 ORFs). Colored bars indicates ORFs present (red bars) or absent (green bars); black bars indicate transposase genes that are present in P. furiosus and putatively present in the isolates.

These GIs have also been reported for Prochlorococcus marinus, a marine cyanobacterium, and Haloquadratum walsbyi, a halophilic archaeon (8, 9). In those organisms, GIs were associated with phage-like sequences, indicating possible phage-mediated HGTs. The gene pool associated with the Pyrococcus GIs is highly variable compared to those described in previous studies of GIs from environmental isolates or metagenomes (8, 9). One constant is the high number of hypothetical genes within those regions and the association with sequences from mobile genetic elements. In Pyrococcus, a small fraction of ORFs were involved in transport systems for carbohydrates (ABC maltose transport system, region VI), metals or inorganic ions (sulfate transport, region VI), detoxification systems (peroxidases, region IV; daunorubicin membrane protein and ATPase, region I), peptide degradation (region IV), all functions potentially conferring an ecological advantage or a specific adaptation to a new source of nutrients. Similar findings were reported for H. walsbyi and P. marinus (8, 9), in particular at the level of transporters, supporting the idea that, in a dynamic natural environment, adaptive genomic properties such as chromosomal rearrangements are responsible for the maintenance of the microbial phenotypic diversity necessary to respond to rapidly changing environmental conditions. Many pathogenic bacteria have evolved to have phase variation, a spontaneous stochastic switching through genetic reorganization, to cope with unpredictably changing environments (18, 25). This strategy of phenotypic randomization is well known to ecologists as bet-hedging (33). Similarly, in a dynamic natural environment, the presence of different subsets of the microbial population that are well adapted to different types of environments will increase the adaptive success of the overall population.

The association of genomic "hot spots" for genome rearrangements with mobile genetic elements suggests a role for these elements in HGT. Evidence of HGT in the Pyrococcus genus has been reported previously by our group and others, often in association with the presence of IS elements (11, 12, 17, 26). Differential patterns of distribution of IS elements among the Vulcano Pyrococcus isolates (12, 17) suggest that these IS elements are potentially mobile, as recently demonstrated for Sulfolobus solfataricus (22, 30). Viruses and plasmids have also been isolated from Pyrococcus species, suggesting a variety of processes for the transfer of genetic material (29, 39). In the present study, we used suppressive subtractive hybridization (SSH) (14, 28) to identify the complements of genes found in the genome of four Pyrococcus isolates but absent from the genome of P. furiosus, thus indicating that those genes might be the result of HGT events (Table 3). The genomic DNA from each Pyrococcus environmental isolate was hybridized against the P. furiosus genomic DNA by using a PCR-Select bacterial genome subtraction kit (Clontech, Mountain View, CA). High-throughput automated sequencing of clone inserts was performed at The Institute for Genomic Research, now the J. Craig Venter Institute, as previously described (23). Sequences have been made available in GenBank. We obtained 1,013 sequences read from four SSH libraries, totaling 564,139 bp of sequence (Table 3). BLASTX was used to align all reads to a local version of the NCBI nonredundant (nr) protein database for taxonomic and functional assignment (3). Hypothetical functional annotation also included a database of 4,873 clusters of orthologous groups (COGs). We observed a significant number of false positives (25%) among the SSH clones we sequenced, i.e., clone sequences with BLASTX hits to Pyrococcus sequences, requiring a large number of clones to be sequenced. Another considerable number of clones were similar to sequences from other Pyrococcus species (63%), indicating a significant sequence divergence of the isolates' nucleotide sequences from that of P. furiosus. Although the large number of SSH fragments matching Pyrococcus species sequences might be a limitation of the method, our analysis resulted in the characterization of 38 sequences unique to the Pyrococcus isolates, with no hit to any known Pyrococcus sequences currently available in public databases (Table 3) (see Table S2 in the supplemental material). We also found 80 sequences that had no known matches in existing databases, underlying the ability of the method to enrich for unique DNA fragments. A large number of unknown sequences was also reported in other studies, such as the Sorcerer II global ocean sampling expedition (31) and a study of fossil genes from ancient ice (5), reflecting the limited representation of environmental sequences in current databases.

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TABLE 3. BLASTX analysis of four SSH libraries

SSH fragments from both Archaea (75%) and Bacteria (25%), but none from Eukarya, were identified from the four SSH libraries. Most hits to the Archaea were to members of the order Thermococcales, followed by methanogens, bacteria mostly from the firmicutes and gammaproteobacterium phyla, halophilic archaea, and various anaerobic/high-temperature archaea. ORF functions were again dominated by hypothetical genes and by so-called operational genes, which are involved in cell growth and maintenance (see Table S2 in the supplemental material). Although recent work (36) showed that there is no metabolic category of genes resilient to transfer, the authors of the study also found that the frequency of informational gene transfer was much lower than that of operational gene transfer. This might explain the small number of those gene functions in our SSH fragments and those of others (14). Sequence reads were inspected for clustered regularly interspaced short palindromic repeat (CRISPR) elements using the program PilerCR (11a). CRISPRs have been found in 50% of the bacterial and most of the archaeal genomes sequenced so far and have been implicated in an RNA interference type of defense mechanism directly primarily against viruses (4, 20, 35). Several of the SSH clones we analyzed contained CRISPR sequence elements, suggesting a possible mode of propagation for those sequences between strains and across lineages (15). A CGH study that compared nine Thermotoga strains from different geographic locations to the sequenced Thermotoga maritima strain MSB8 also revealed significant differences across the strains that could be related to substrate utilization, and there were many examples of deletion/insertion events with complete cassettes of genes and gene rearrangements (24).

In the present study of the genotypic diversity in a population of Pyrococcus species, hyperthermophilic archaea isolated from hydrothermal vents in Vulcano Island, Italy, we showed extensive genome rearrangements, revealing a possible mechanism by which microbial phenotypic diversity is maintained in this ecosystem. The shallow vents of Vulcano Island are characterized by extensive variations in physicochemical conditions (6, 32), raising the question of whether adaptive genomic properties, such as genome rearrangements and transposition and lateral gene transfer events, are a necessary adaptation to rapidly changing environmental conditions.

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Sequences are available under GenBank accession numbers EU922594 to EU923604.

 
This research was funded under a grant from the U.S. Department of Energy (DE-FG02-01ER63133) to K.E.N. and J.D.

We thank Patrick Forterre and Evelyne Marguet for providing strains 12/1 and 32/4.

 
* Corresponding author. Present address: Johns Hopkins University, Department of Biology, 3400 N. Charles Street, 127 Mudd Hall, Baltimore, MD 21218. Phone: (410) 516-8498. Fax: (410) 516-5213. E-mail: jdiruggiero{at}jhu.edu

Published ahead of print on 22 August 2008.

Supplemental material for this article may be found at http://aem.asm.org/.

Present address: University of Maryland School of Medicine, Institute for Genome Sciences, Baltimore, MD.

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Applied and Environmental Microbiology, October 2008, p. 6447-6451, Vol. 74, No. 20
0099-2240/08/$08.00+0     doi:10.1128/AEM.01024-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Supplemental material Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by White, J. R. Articles by DiRuggiero, J. Search for Related Content PubMed PubMed Citation Articles by White, J. R. Articles by DiRuggiero, J. Agricola Articles by White, J. R. Articles by DiRuggiero, J.