A. VUCICEVIC¹, S.D. SULSER¹, F. DELAVAT¹, V. SENTCHILO¹, J.R. VAN DER MEER<sup>1

1</sup>UNIL, FBM, DMF - Quartier Sorge, Batiment Biophore CH-1015 Lausanne, Switzerland

Integrative and conjugative elements (ICEs) are mobile DNA elements integrated in the bacterial host chromosome, but able to excise and transfer themselves into a new host cell. ICEclc of Pseudomonas knackmussii B13 is a model ICE for a family of elements widespread in Gamma- and Betaproteobacteria. Despite several studies, the role of most genes encoded on ICEclc in transfer remains obscure. One such example is comprised by a 14 kb cluster on ICEclc (provisorily named the 81655 operon), whose expression is among the highest detected so far for ICEclc genes. Here we investigate the role of the 81655 operon on ICEclc transfer and try to understand the nature of its extremely high expression. Using gene replacement techniques, we find that several of the genes within the 81655 operon are directly involved in ICEclc transfer, albeit deletion of others (notably of the first gene of the operon itself) has no discernable effect on transfer rates. Expression of the 81655 operon studied by micro-array hybridizations and RNA sequencing indicated specific high transcription in stationary phase but not in exponentially growing cells. Reporter gene fusions to the 81655 promoter region introduced in Pseudomonas putida with or without ICEclc showed expression only in cells containing ICEclc and only in a small fraction of cells. Expression coincided in cells which simultaneously expressed other ICEclc promoters, such as the intB13 integrase promoter.