Caught in the act: the dialogue between bacteriophage R17 and the type IV secretion machine of plasmid R1.

Bacteria communicate with each other through contact-independent and -dependent signalling mechanisms. Sensory perception of both types of signals is needed for conjugative transfer of mobile DNA elements via type IV secretion systems (T4SSs) to bacterial or eukaryotic target cells. While the regulatory circuitries coupling extracellular quorum and environmental signals to transcription of T4SS genes are increasingly understood, it remains fundamentally unknown how a potential recipient cell stimulates donor conjugative DNA transfer upon contact. In this issue, Lang et al. (2011) report use of the male-specific bacteriophage R17, a phage that binds conjugative pili elaborated by IncF plasmid R1, to define requirements for phage-contact-mediated T4SS activation and phage penetration. They report that R17 penetrates only through T4SS channels engaged for delivery of their plasmid cargo to recipient cells. Engagement requires docking of catalytically active relaxase TraI bound at oriT with the TraD substrate receptor (also termed the T4CP). The data, together with recent ultrastructural and biochemical findings, support an intriguing new model that the T4CP cumulatively senses an intracellular signal (substrate docking) and an extracellular signal (pilus bound by phage or a recipient cell) to co-ordinate a late stage morphogenetic or gating reaction that enables bidirectional transmission of nucleoprotein substrates through the T4SS.