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Open Access Research article

A bacterial genome in transition - an exceptional enrichment of IS elements but lack of evidence for recent transposition in the symbiont Amoebophilus asiaticus

Stephan Schmitz-Esser13*, Thomas Penz1, Anja Spang2 and Matthias Horn1

Author Affiliations

1 Department of Microbial Ecology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria

2 Department of Genetics in Ecology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria

3 Institute for Milk Hygiene, University of Veterinary Medicine Vienna Veterinärplatz 1, 1210 Vienna, Austria

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BMC Evolutionary Biology 2011, 11:270  doi:10.1186/1471-2148-11-270

Published: 26 September 2011

Abstract

Background

Insertion sequence (IS) elements are important mediators of genome plasticity and are widespread among bacterial and archaeal genomes. The 1.88 Mbp genome of the obligate intracellular amoeba symbiont Amoebophilus asiaticus contains an unusually large number of transposase genes (n = 354; 23% of all genes).

Results

The transposase genes in the A. asiaticus genome can be assigned to 16 different IS elements termed ISCaa1 to ISCaa16, which are represented by 2 to 24 full-length copies, respectively. Despite this high IS element load, the A. asiaticus genome displays a GC skew pattern typical for most bacterial genomes, indicating that no major rearrangements have occurred recently. Additionally, the high sequence divergence of some IS elements, the high number of truncated IS element copies (n = 143), as well as the absence of direct repeats in most IS elements suggest that the IS elements of A. asiaticus are transpositionally inactive. Although we could show transcription of 13 IS elements, we did not find experimental evidence for transpositional activity, corroborating our results from sequence analyses. However, we detected contiguous transcripts between IS elements and their downstream genes at nine loci in the A. asiaticus genome, indicating that some IS elements influence the transcription of downstream genes, some of which might be important for host cell interaction.

Conclusions

Taken together, the IS elements in the A. asiaticus genome are currently in the process of degradation and largely represent reflections of the evolutionary past of A. asiaticus in which its genome was shaped by their activity.

Keywords:
insertion sequence element; endosymbiont; Bacteroidetes; genome evolution