Bacterial Conjugation: Cell-Cell Contact-Dependent Horizontal Gene Spread

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For many years, the F factor was the only conjugative plasmid that had been identified and conjugation was thought to be an unusual biological phenomenon. This chapter focuses on conjugation in gram-negative bacteria. In the gram-negative conjugation systems, DNA-transfer competence can be reached in the absence of recipients. Next, potential recipients must be present and these recipients must be recognized by the donor cell and distinguished from other donors. Recipient cells are very likely identified by pilus contacts involving the pilus tip and surface components on the recipient cell. The chapter states how the replication machinery is coupled to the transporter complex. The conjugative plasmids of gram-negative bacteria are grouped into incompatibility classes. In bacterial conjugation the substrate that is transported is single-stranded plasmid DNA. The conjugation systems described in the chapter are dedicated to DNA transfer, the type IV secretion systems of bacterial pathogens transport proteins. Systems for regulating tra gene expression and gene transfer by conjugation have evolved to transduce signals to the conjugative plasmid residing in the cytoplasm. The signal that triggers DNA replication and transfer in donors is unknown. Bacterial conjugation mediated by conjugative plasmids is now in a transition phase from the classical view on this important variant of horizontal gene transfer to a modern view. The modern view is that cell-cell contact-dependent gene-transfer systems are related to type IV secretion systems of gram-negative bacteria. The phenomenon of bacterial conjugation, despite its discovery more than fifty years ago, still carries its mysteries.