Conjugative Type IV secretion systems in the Xanthomonadaceae family: Investigating structure-function diversity

Abstract

Type IV Secretion systems (T4SS) are macromolecular protein complexes in bacteria which, encompassing the membranes, mediate the exchange of DNA and protein effectors to target cells, playing a key role in conjugation, bacterial competition, eukaryotic cell invasion and immune system evasion in different species. In the last fifteen years, our group have been studying the role of bacteria-killing T4SS in Xanthomonas citri, the causative agent of canker citrus, and in the opportunistic pathogen Stenotrophomonas maltophilia, both belonging to the Xanthomonadaceae family. Our genome searches revealed that these two species contain each an additional T4SS cluster, so far uncharacterized and annotated as participating in bacterial conjugation. These systems have in common the absence of a VirB7 homolog, a so far universal component of the Outer Membrane Core Complex (OMCC), providing a convenient model to explore the minimum requirements for T4SS assembly. Moreover, the genomic location and distance from conjugative components in the S. maltophilia system indicate that its primary function differs from bacterial conjugation, and we hypothesize it may participate in eukaryotic cell invasion in the context of opportunistic pathogenicity. Thus, we aim to evaluate the participation of these two T4SS in conjugation and eukaryotic cell invasion, as well as to define the structure of their OMCC using Single-Particle Analysis Cryo-electronic microscopy (SPA-CryoEM). Minor components of these systems with unknown function will also be studied at their physiological and structural level. The results generated by this project will expand the knowledge on the function and structural diversity of T4SS, as well as on the metabolism of free-living and opportunistic pathogenic bacteria.