Construction of a shuttle vector and transformation of Xylella fastidiosa with plasmid DNA

We have isolated, cloned, and sequenced a 5823-bp cryptic plasmid from a strain of Xylella fastidiosa. This plasmid encodes five open reading frames (ORF) greater than 400 nucleotides each. ORF 2 encodes a protein with 37% amino acid identity to the replication initiator protein of plasmid pECB2 from Pseudomonas alcaligenes. This RepA protein from X. fastidiosa contains both a leucine zipper and helix turn helix motif characteristic of proteins involved in DNA replication. The sequence 5' of ORF 2 has all of the features characteristic of plasmid origins of replication as well as regulatory elements required for transcription of ORF 2. Open reading frame 2, along with the upstream origin of replication, was cloned as an EcoRI fragment into pUC19 to create a shuttle vector. This construct was introduced into Xylella fastidiosa by electroporation, with selection for carbenicillin resistance. Transformation was verified by both PCR and Southern hybridization experiments. Frequency of transformation was low, but increased ten-fold when the plasmid was grown in X. fastidiosa rather than Escherichia coli prior to transformation. This work represents the first step towards the development of a system for genetic analysis of this important plant pathogen of citrus, grapevines, and other horticultural crops.