Conjugative transfer of the IncN plasmid pKM101 is mediated by dynamic interactions between the TraK accessory factor and TraI relaxase.

Abstract

Conjugative dissemination of mobile genetic elements (MGEs) among bacteria is initiated by assembly of the relaxosome at the MGE's origin-of-transfer (oriT) sequence. A critical but poorly defined step of relaxosome assembly involves recruitment of the catalytic relaxase to its DNA strand-specific nicking site within oriT. Here, we present evidence by AlphaFold modeling, affinity pulldowns, and in vivo site-directed photocrosslinking that the TraK Ribbon-Helix-Helix DNA-binding protein recruits TraI to oriT through a dynamic interaction in which TraI's C-terminal unstructured domain (TraI_CTD) wraps around TraK's C-proximal tetramerization domain. Upon relaxosome assembly, conformational changes disrupt this contact, and TraI_CTD instead self-associates as a prerequisite for relaxase catalytic functions or substrate engagement with the transfer channel. These findings delineate key early-stage processing reactions required for conjugative dissemination of a model MGE.