1Department of Fundamental Microbiology, University of Lausanne
Integrative and conjugative elements (or ICE) form an important new class of mobile DNA that largely contribute to bacterial genome evolution and adaptation. ICE are normally integrated in the host chromosome but can excise at low frequencies, upon which they transfer by conjugation to a new recipient cell and reintegrate. Here we study transfer of a mobile DNA named ICEclc in the host Pseudomonas knackmussii B13, in which it is present in two identical copies. ICEclc permits its host to use 3-chlorobenzoate as unique carbon and energy source.
By using single cell analysis we find that ICEclc becomes active and excises in only a few percent of all cells in a population during stationary phase conditions (Sentchilo, 2009). Excision leads to a covalently closed circular DNA, that is processed for DNA transfer by a relaxase at the origin of transfer (oriT). ICEclc is unusual in that it carries two independently functioning oriT (Miyazaki, 2011). The trigger for this selective and bistable activation is not known yet, but we find that noise in expression of the stationary phase sigma factor RpoS and a set of specific regulatory proteins control the proportion of cells, which enter in the ICEclc transfer program (Miyazaki, 2012; Pradervand, 2014). Cells that activate ICEclc become totally committed to the transfer process (Minoia, 2008; Gaillard, 2010; Reinhard, 2013). Quite astonishingly, active donor cells face severe growth defects compared to non-active donor cells when presented with new carbon substrate. This growth defect appears to be caused by factors encoded on ICEclc itself. Thus, ICEclc induces a cellular differentiation in donor cells to become transfer competent, which for those cells is a dead-end road. (Reinhard, 2013). The ecological or evolutionary advantage of the cell killing process is unclear.