Conjugative plasmid transfer in Xylella fastidiosa is dependent on tra and trb operon functions

Authors: Burbank, Lindsey; Van Horn, Christopher

Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2017
Publication Date: 8/14/2017
Citation: Burbank, L.P., Van Horn, C.R. 2017. Conjugative plasmid transfer in Xylella fastidiosa is dependent on tra and trb operon functions. Journal of Bacteriology. 199:e00388-17. https://doi.org/10.1128/JB.00388-17.
DOI: https://doi.org/10.1128/JB.00388-17

Interpretive Summary: The bacterium Xylella fastidiosa is an important plant pathogen world-wide, infecting a wide range of different plant species. Although X. fastidiosa strains show specificity for certain host plants, pathogen evolution can lead to emergence of new diseases caused by X. fastidiosa as strains adapt to new host plants. Pathogen adaptation can be facilitated by transfer of genetic material between different X. fastidiosa strains present in the same environment. Conjugation, one mechanism of bacterial gene transfer, involves movement of intact, circular plasmids between bacterial cells via direct cell-cell contact. In this study, X. fastidiosa is shown to transfer a broad host-range plasmid between different strains belonging to different subspecies (strains infecting different hosts). Prevalence of this type of gene transfer in X. fastidiosa has implications for pathogen adaptation, and emergence of new diseases in alternative host plants.

Technical Abstract: The insect-transmitted plant pathogen Xylella fastidiosa is capable of efficient horizontal gene transfer and recombination, leading to diversity between strains and the categorization of X. fastidiosa into multiple subspecies. Although natural transformation is shown to occur at high rates in X. fastidiosa, there also is evidence that certain strains of X. fastidiosa carry native plasmids equipped with transfer and mobilization genes, suggesting conjugation as a mechanism of horizontal gene transfer in some instances. Two operons, tra and trb, putatively encoding a conjugative Type IV secretion system are found in some but not all X. fastidiosa isolates, often on native plasmids. X. fastidiosa strains which carry the conjugative transfer genes belong to different subspecies, and frequently have different host ranges. Using X. fastidiosa strains M23 (subspecies fastidiosa) or Dixon (subspecies multiplex) as the donor strain and Temecula (subspecies fastidiosa) as the recipient strain, plasmid transfer was characterized using the mobilizable broad host range vector pBBR5pemIK. Transfer of plasmid pBBR5pemIK was observed under in vitro conditions with both donor strains, and was dependent on both tra and trb operon functions. A conjugative mechanism likely contributes to gene transfer between diverse strains of X. fastidiosa, facilitating adaptation to new environments or different hosts. IMPORTANCE: Xylella fastidiosa is an important plant pathogen world-wide, infecting a wide range of different plant species. Emergence of new diseases caused by X. fastidiosa, or host-switching of existing strains is thought to be primarily due to the high frequency of horizontal gene transfer and recombination in this pathogen. Transfer of plasmids by a conjugative mechanism enables movement of larger amounts of genetic material at one time compared with other routes of gene transfer such as natural transformation. Establishing the prevalence and functionality of this mechanism in X. fastidiosa contributes to a better understanding of horizontal gene transfer and adaptation, and disease emergence in this diverse pathogen.