An Operon of Three Transcriptional Regulators Controls Horizontal Gene Transfer of the Integrative and Conjugative Element ICEclc in Pseudomonas knackmussii B13

The integrative and conjugative element ICE clc is a mobile genetic element in Pseudomonas knackmussii B13, and an experimental model for a widely distributed group of elements in Proteobacteria . ICE clc is transferred from specialized transfer competent cells, which arise at a frequency of 3-5% in a population at stationary phase. Very little is known about the different factors that control the transfer frequency of this ICE family. Here we report the discovery of a three-gene operon encoded by ICE clc , which exerts global control on transfer initiation. The operon consists of three consecutive regulatory genes, encoding a TetR-type repressor MfsR, a MarR-type regulator and a LysR-type activator TciR. We show that MfsR autoregulates expression of the operon, whereas TciR is a global activator of ICE clc gene expression, but no clear role was yet found for MarR. Deletion of mfsR increases expression of tciR and marR , causing the proportion of transfer competent cells to reach almost 100% and transfer frequencies to approach 1 per donor. mfsR deletion also caused a two orders of magnitude loss in population viability, individual cell growth arrest and loss of ICE clc . This indicates that autoregulation is an important feature maintaining ICE transfer but avoiding fitness loss. Bioinformatic analysis showed that the mfsR-marR-tciR operon is unique for ICE clc and a few highly related ICE, whereas tciR orthologues occur more widely in a large variety of suspected ICE among Proteobacteria .