Breaking a barrier: In trans vlsE recombination and genetic manipulation of the native vlsE gene of the Lyme disease pathogen.

Host-pathogen interactions represent a dynamic evolutionary process, wherein both hosts and pathogens continuously develop complex mechanisms to outmaneuver each other. Borrelia burgdorferi, the Lyme disease pathogen, has evolved an intricate antigenic variation mechanism to evade the host immune response, enabling its dissemination, persistence, and pathogenicity. Despite the discovery of this mechanism over two decades ago, the precise processes, genetic elements, and proteins involved in this system remain largely unknown. The vls locus, which is the site of antigenic variation, has been notoriously challenging to manipulate genetically due to its highly conserved structural features, even with significant advancements in molecular biology and genetic engineering for this highly segmented pathogen. Our study highlights the pivotal role of plasmid topology in facilitating in trans gene recombination. We demonstrate that gene conversion can occur in trans when a copy of vlsE gene is present on a linear plasmid, contrary to previous observations suggesting a cis arrangement is required for vlsE recombination. Significantly, employing this in trans gene conversion strategy with a linear plasmid, we have, for the first time, achieved targeted genetic mutation of putative cis-acting elements in the native vlsE gene. This has unveiled a potentially crucial role for the 17 bp direct repeats that flank the central variable cassette region of vlsE. Furthermore, we validated the reliability and reproducibility of our mutational approach by successfully inserting stop codons at two distinct sites within the central variable cassette of vlsE. Thus, this study presents a significant methodological innovation enabling the direct manipulation of the vls locus and lays the groundwork for systematic exploration of specific mutations affecting the mechanism of antigenic variation. As a result, it creates new avenues for research and raises intriguing questions that could guide the development of novel methods to explore host-pathogen interactions of the agent of Lyme disease.