Bacterial Genome Plasticity - CNRS URA2171  


  HEADDr MAZEL Didier / mazel@pasteur.fr
  MEMBERSDr FRUMERIE Clara / Dr GUEROUT Anne-Marie / Dr LE ROUX Frédérique / Dr LOOT Céline


  Annual Report

We study the mechanisms responsible of the bacterial genome variability, with a special interest for those involved in exogenous gene acquisition – the horizontal gene transfer. Our model system is the integron, a natural genetic engineering system involved in the development and dissemination of antibiotic resistance genes among Gram-negative species. We are also investigating other factors playing a role in the plasticity of the Vibrio species genomes, which are all constituted of two circular chromosomes that could have distinct dynamic characteristics.

Integrons. We are studying different aspects of this gene capture system: their distribution, their contribution to the adaptive capacity of their host and their recombination processes.

This natural genetic system is composed of two basic elements: a gene coding an integrase of the site-specific tyrosine recombinase family and a primary recombination site, attI. The integrase activity allows the insertion of open reading frames, in the form of a circular cassette, at the recombination site. All these cassettes are composed of a single gene associated to a recombination site, the attC site, indispensable for the integrase recognition and recombination with attI.

We have shown that the resistance integrons derived from sedentary super-integrons carried by environmental species, such as the different Vibrio.

Recently, the structural characteristics of the attC sites led us to propose a new model for the recombination in integrons, which only involved the attC bottom strand folded in a stem-and loop, based on its symmetrical structure, and a canonical double-strand (ds) attI site. Recognition and recombination by the IntI integrase of such a structure with a canonical ds-attI site would lead to a Holliday junction (HJ) intermediate which may be resolved by a replication step. We have now sustained this model with in vivo experiments, but also through the resolution of the 3D structure of integron integrase tetramer bound to single stranded substrates (collaboration with D. Gopaul).

Plasticity of the Vibrio species genomes. The second project is to investigate other factors involved in genome plasticity of the complex genome of Vibrio species. The Vibrio group includes a large number of pathogenic species whose hosts range from human to aquatic animals. The few species so far characterized have been found to carry two circular chromosomes showing a high variability. The selective advantage conferred by such an organization is unknown.

To increase our knowledge, we have sequenced the genome of V. splendidus LGP 32 in collaboration with C. Bouchier, a strain which is only remotely related to the Vibrio species sequenced so far. Together with comparative analyses with the other sequenced Vibrio genome, we have undertaken different in vivo and in silico genome subtraction approaches to identify the hot spot of variability. We expect better understanding of the rules governing the overall organization and the gene partition between the two chromosomes in Vibrio.



  Publications

Publications 2006 of the unit on Pasteur's references database




Activity Reports 2006 - Institut Pasteur
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