AUTHOR=Drehkopf Sabine , Scheibner Felix , Büttner Daniela 

TITLE=Functional characterization of VirB/VirD4 and Icm/Dot type IV secretion systems from the plant-pathogenic bacterium Xanthomonas euvesicatoria

JOURNAL=Frontiers in Cellular and Infection Microbiology

VOLUME=13

YEAR=2023

URL=https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1203159

DOI=10.3389/fcimb.2023.1203159

ISSN=2235-2988

ABSTRACT=<sec><title>Introduction</title><p>Many Gram-negative plant- and animal-pathogenic bacteria employ type IV secretion (T4S) systems to transport proteins or DNA/protein complexes into eukaryotic or bacterial target cells. T4S systems have been divided into minimized and expanded T4S systems and resemble the VirB/VirD4 T4S system from the plant pathogen <italic>Agrobacterium tumefaciens</italic> and the Icm/Dot T4S system from the human pathogen <italic>Legionella pneumophila</italic>, respectively. The only known plant pathogen with both types of T4S systems is <italic>Xanthomonas euvesicatoria</italic> which is the causal agent of bacterial spot disease on pepper and tomato plants.</p></sec><sec><title>Results and discussion</title><p>In the present study, we show that <italic>virB/virD4</italic> and <italic>icm/dot</italic> T4S genes are expressed and encode components of oligomeric complexes corresponding to known assemblies of VirB/VirD4 and Icm/Dot proteins. Both T4S systems are dispensable for the interaction of <italic>X. euvesicatoria</italic> with its host plants and do not seem to confer contact-dependent lysis of other bacteria, which was previously shown for the chromosomally encoded VirB/VirD4 T4S system from <italic>Xanthomonas axonopodis</italic> pv. <italic>citri.</italic> The corresponding chromosomal T4S gene cluster from <italic>X. euvesicatoria</italic> is incomplete, however, the second plasmid-localized <italic>vir</italic> gene cluster encodes a functional VirB/VirD4 T4S system which contributes to plasmid transfer. In agreement with this finding, we identified the predicted relaxase TraI as substrate of the T4S systems from <italic>X. euvesicatoria</italic>. TraI and additional candidate T4S substrates with homology to T4S effectors from <italic>X. axonopodis</italic> pv. <italic>citri</italic> interact with the T4S coupling protein VirD4. Interestingly, however, the predicted C-terminal VirD4-binding sites are not sufficient for T4S, suggesting the contribution of additional yet unknown mechanisms to the targeting of T4S substrates from <italic>X. euvesicatoria</italic> to both VirB/VirD4 and Icm/Dot T4S systems.</p></sec>