Evolutionary trends of the transposase-encoding open reading frames A and B (orfA and orfB) of the mycobacterial IS6110 insertion sequence
3rd International Congress on Bacteriology and Infectious Diseases
August 04-06, 2015 Valencia, Spain

Helmi Mardassi1, Sara Thabet1 and Amine Namouchi2

Posters-Accepted Abstracts: J Bacteriol Parasitol


The IS6110 insertion sequence, a member of the IS3 family of insertion sequences was found to be specific to the
Mycobacterium tuberculosis complex (MTBC) and contributes significantly to its pathogenicity. Here we explored the
evolution of the IS6110 sequence by analysing the genetic variability and the selective forces acting on its transposase-encoding
open reading frames (ORFs) A and B (orfA and orfB). For this purpose, we used a strain collection consisting of smooth
tubercle bacilli (STB), an early branching lineage of the MTBC and present day M. tuberculosis strains representing the full
breadth of genetic diversity in Tunisia. In each ORF, we found a major haplotype that dominated over a flat distribution of rare
descendent haplotypes consisting mainly of single- and double-nucleotide variant singletons. The predominant haplotypes
consisted of both ancestral and present-day strains suggesting that IS6110 acquisition predated the emergence of the MTBC.
There was no evidence of recombination and both ORFs were subjected to strict purifying selection as demonstrated by their
dN/dS ratios and Tajima’s D statistics. Strikingly, the purifying selection acting on orfA proved much more stringent suggesting
its critical role in regulating the transpositional process. Maximum likelihood analyses further excluded any possibility of
positive selection acting on single amino acid residues. Taken together our data fit with an evolutionary scenario according
to which the observed variability pattern of the IS6110 transposase-encoding ORFs is generated mainly through random
point mutations that accrued on a functionally optimal IS6110 copy, whose acquisition predated the emergence of the MTBC

Biography :

Helmi Mardassi after obtaining his Doctorate in Veterinary Medicine (1988) at the Tunisian National School of Veterinary Medicine, he moved to the University of
Montreal (Canada) where he completed a Master degree in Microbiology and Animal Pathology. In 1996, he obtained his PhD degree in Molecular Virology at the
Institut Armand-Frappier (University of Quebec, Canada) and then joined the Biotechnology Research Institute of Montreal for a Post Doctoral Training. Actually
he is leading a research unit focusing on molecular epidemiology, evolution and genetics of Mycobacterium tuberculosis. He has published more than 26 papers
in reputed journals.