Genetic organization of plasmid pXF51 from the plant pathogen Xylella fastidiosa

The sequence of plasmid pXF51 from the plant pathogen Xylella fastidiosa, t he causal agent of citrus variegated chlorosis, has been analyzed. This pla smid codes for 65 open reading frames (ORFs), organized into four main regi ons, containing genes related to replication, mobilization, and conjugative transfer. Twenty-five ORF, have no counterparts in the public sequence dat abases, and 7 are similar to conserved hypothetical proteins from other bac teria. A pXF51 incompatibility group has not been determined, as we could n ot find a typical replication origin. One cluster of conjugation-related ge nes (trb) seems to be incomplete in pXF51, and a copy of this sequence is f ound in the chromosome, suggesting it was generated by a duplication event. A second cluster (tra) contains all genes necessary for conjugation transf er to occur, showing a conserved organization with other conjugative plasmi ds. An identifiable origin of transfer similar to oriT from IncP plasmids i s found adjacent to genes encoding two mobilization proteins. None of die O RFs with putative assigned function could be predicted as having a role in pathogenesis, except for a virulence-associated protein D homolog. These re sults indicate that even though pXF51 appears not to have a direct role in Xylella pathogenesis, it is a conjugative plasmid that could be important f or lateral gene transfer in this bacterium. This property may be of great i mportance for future development of transformation techniques in X. fastidi osa. (C) 2001 Academic Press.