V SENTCHILO¹, L CHALUPOVA¹, J VAN DER MEER<sup>1

1</sup>Department of Fundamental Microbiology, University of Lausanne, Biophore, UNIL-Sorge, 1015 Lausanne

Bacterial genomes are dynamic structures prone to constant reshuffling of their genetic material via horizontal gene transfer and recombination. A particular class of mobile genetic elements (MGEs) coined Integrative Conjugative Elements (ICEs) is currently becoming more and more appreciated as an important mean for large scale gene mobilization and integration in the whole domain Bacteria. We have completely sequenced a genome of a sewage sludge isolate P. azelaica HBP-1 and discovered two new ICEs named ICEPaz1 (96 kbp)and ICEPaz2 (171 kbp). First of them confers resistance to mercury (II) and to arsenate whereas the second encodes catabolism of a range of aromatic substrates (2-hydroxybiphenyl, 4-hydroxybenzoate, vanillate, salicylate, catechol and protocatechuate) --thus both ICEs being involved in the host adaptation to the particular environment. The two ICEs in P. azelaica were found integrated into two non-identical copies of the glycine-t-RNA gene; they encoded their own tyrosine-type site-specific recombinase (integrase) and were flanked by 18 base pairs direct repeats. Pairwise comparison between the two ICEs from P. azelaica revealed that the region of structural similarity between them was limited to about 33 kbp with average DNA identity of 75% and was comprised of genes involved in conjugation and life-cycle control—the so called “ICE core-region”. When compared to a model ICEclc from P. knackmussii B13, ICE1 displayed higher degree of similarity over the whole “core” region (53 kb aligned at 86% nucleotide identity), whereas ICE2 - much lower (ca. 40 kb at 75% identity). Subsequent mating experiments with P. putida UWC1 as recipient revealed that both ICEs are currently active that is capable of self-transfer and self-integration and, interestingly, their integrases displayed both conservatism (integration site preferences) and flexibility (integration into less than 100% conserved DNA targets). The distinct and unrelated “cargo” genes carried on the two ICEs from P. azelaica and their mosaicism imply ICEs' divergent evolution trajectories on one hand and their access to the communal gene pool of several classes of Proteobacteria, on the other.