Transient Replication in Specialized Cells Favors Conjugative Transfer of a Selfish DNA Element

Bacterial evolution is driven to a large extent by horizontal gene transfer (HGT) - the processes that distribute genetic material between species rather than by vertical descent. HGT is mostly mediated by an assortment of different selfish DNA elements, several of which have been characterized in great molecular detail. In contrast, very little is known on adaptive features optimizing horizontal fitness. By using single DNA molecule detection and time-lapse microscopy, we analyze here the fate of an integrative and conjugative element (ICE) in individual cells of the bacterium Pseudomonas putida. We uncover how the ICE excises and irregularly replicates, exclusively in a sub-set of specialized host cells. As postulated, ICE replication is dependent on its origin of transfer and its DNA relaxase. Rather than being required for ICE maintenance, however, we find that ICE replication serves more effective conjugation to recipient cells, providing selectable benefit to its horizontal transfer.