Orchestrated long-distance gene activation by a ParB-like BisD-CTP DNA clamp in low-frequency transfer competence development in Pseudomonas putida

Integrative conjugative elements (ICEs) are mobile DNA that remain integrated within the host bacterial genome until they activate their excision and transfer to recipient cells via conjugation. ICE transfer is initiated in a small subpopulation of cells that undergo a hierarchical gene expression cascade leading to transfer competence formation. In this study, we demonstrate that transfer competence formation of the ICEclc element in stationary phase Pseudomonas putida cells is regulated by the coordinated activity of a two-component transcriptional activator, BisC and BisD. Chromatin immunoprecipitation of tagged BisC or BisD followed by high throughput sequencing showed that both proteins accumulate at similar sites around ICEclc transfer competence promoters in P. putida cells. Genetic dissection and single cell microscopy showed that BisD is a dual domain protein, with a C-terminal gene activator domain and an N-terminal ParB-like domain, forming dimeric DNA clamps that self-load at distant sites and reach target promoters after extensive one-dimensional DNA sliding. Expressed mCherry-BisD in P. putida ICEclc cells form discrete fluorescent foci, dependent on parS-like sequences on the ICE. This focus formation is similar as what is seen with canonical ParB proteins accumulating on chromosomal DNA, but in case of BisD corresponds to both chromosomal and excised ICE-molecules. Sliding of BisD over ~50 kb of ICE-DNA is asymmetric and follows the direction of ICE gene transcription. This may help to establish a temporal order of activation of transfer competence formation, optimizing ICE transfer. Given that ICEclc is activated in stationary phase cells, we hypothesize that BisD is not involved in segregating excised ICE-DNA among daughter cells, but rather in actively directing ICE-DNA molecules towards the (multiple) conjugative systems that are produced in transfer competent cells. BisD thus serves as a twin function protein, integrating gene activation and DNA segregation functions.