Conjugative Plasmid Transfer in Xylella fastidiosa Is Dependent on <i>tra</i> and <i>trb</i> Operon Functions

Abstract

The insect-transmitted plant pathogen <i>Xylella fastidiosa</i> is capable of efficient horizontal gene transfer (HGT) and recombination. Natural transformation occurs at high rates in <i>X. fastidiosa</i>, but there also is evidence that certain strains of <i>X. fastidiosa</i> carry native plasmids equipped with transfer and mobilization genes, suggesting conjugation as an additional mechanism of HGT in some instances. Two operons, <i>tra</i> and <i>trb</i>, putatively encoding a conjugative type IV secretion system, are found in some but not all <i>X. fastidiosa</i> isolates, often on native plasmids. <i>X. fastidiosa</i> strains that carry the conjugative transfer genes can belong to different subspecies and frequently differ in host ranges. Using <i>X. fastidiosa</i> strain M23 (<i>X. fastidiosa</i> subsp. <i>fastidiosa</i>) or Dixon (<i>X. fastidiosa</i> subsp. <i>multiplex</i>) as the donor strain and Temecula (<i>X. fastidiosa</i> subsp. <i>fastidiosa</i>) as the recipient strain, plasmid transfer was characterized using the mobilizable broad-host-range vector pBBR5pemIK. Transfer of plasmid pBBR5pemIK was observed under <i>in vitro</i> conditions with both donor strains and was dependent on both <i>tra</i> and <i>trb</i> operon functions. A conjugative mechanism likely contributes to gene transfer between diverse strains of <i>X. fastidiosa</i>, possibly facilitating adaptation to new environments or different hosts.<b>IMPORTANCE</b><i>Xylella fastidiosa</i> is an important plant pathogen worldwide, infecting a wide range of different plant species. The emergence of new diseases caused by <i>X. fastidiosa</i>, or host switching of existing strains, is thought to be primarily due to the high frequency of HGT 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 <i>X. fastidiosa</i> contributes to a better understanding of HGT, adaptation, and disease emergence in this diverse pathogen.