Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2012
Publication Date: 12/14/2012
Citation: Rogers, E.E., Stenger, D.C. 2012. A conjugative 38kB plasmid is present in multiple subspecies of Xylella fastidiosa. PLoS One. 7:e52131. Interpretive Summary: Xylella fastidiosa is a bacterium that causes economically important leaf scorch diseases in a number of crop species, most notably citrus, grape, and almond. Many bacteria, including X. fastidiosa, contain small genetic elements called plasmids that are easily transferred between individual bacteria, including bacteria of different species. Such plasmids are an important mechanism for transfer of genetic information among bacterial species and often confer disease-promoting attributes or resistance to antibiotics, mercury, or other toxic substances. This work reports the DNA sequence of a plasmid from X. fastidiosa subsp. multiplex strain Riv5 which was originally found in ornamental plum near Riverside, CA. This plasmid is almost identical to a plasmid found in a different subspecies of X. fastidiosa, subsp. fastidiosa isolated from grapevine. Other work indicates that X. fastidiosa subspecies fastidiosa was introduced to the United States relatively recently. Therefore, transfer of the plasmid among subspecies is a recent event. Portions of the plasmid reported here are similar to a plasmid found in the bacterium Yersinia pseudotuberculosis, which causes the human disease Far East Scarlet-like fever. The similar portions are part of the mechanism for plasmid transfer between bacteria; the Y. pseudotuberculosis plasmid contains several antibiotic resistance determinants that are not present on the X. fastidiosa plasmid. In general, mobile genetic material such as this plasmid are important mechanisms for rapid adaptation to changing environmental conditions. X. fastidiosa potentially may acquire the ability to cause disease in new plant hosts based entirely or in part on plasmid transfer of genetic material. Therefore it is important to understand X. fastidiosa plasmids and their movement among different populations.
Technical Abstract: A ~38kB plasmid was present in the Riv5 strain of Xylella fastidiosa subsp. multiplex isolated from ornamental plum in southern California. This plasmid, pXF-RIV5, encodes a complete type IV secretion system necessary for conjugation and DNA transfer. pXF-RIV5 is almost identical to pXFAS01 from X. fastidiosa subsp. fastidiosa strain M23; the two only vary by 6 nucleotides. pXF-RIV5 and pXFAS01 share extensive similarity to chromosomal and plasmid (pXF51) sequences of X. fastidiosa subsp. pauca strain 9a5c and more distant similarity to many other bacterial plasmids. Mating pair formation proteins (Trb) from Yersinia pseudotuberculosis IP31758 are the mostly closely related non-X. fastidiosa proteins to most of the Trb proteins encoded by pXF-RIV5 and pXFAS01. Unlike many bacterial conjugative plasmids, pXF-RIV5 appears not to carry cargo proteins such as virulence factors or antibiotic resistance genes that confer selective advantage on host bacteria. There is a small transposon-associated region encoding only a putative transposase and associated factor. pXF-RIV5 vegetative replication appears to be under control of a RepA protein; it is rather more common for conjugative plasmids to encode TrfA and contain an origin of replication similar to those found in IncP-1 incompatibility group plasmids. The presence of almost identical plasmids in two separate subspecies of X. fastidiosa is indicative of recent horizontal transfer, probably subsequent to the introduction of the fastidiosa subspecies to the United States in the late 19th century.