Functional and phylogenetic analysis of a plant-inducible oligoribonuclease (orn) gene from an indigenous Pseudomonas plasmid

Application of a promoter-trapping strategy to identify plant-inducible genes carried on an indigenous Pseudomonas plasmid, pQBR103, revealed the presence of a putative oligoribonuclease (orn) gene that encodes a highly conserved 3′ to 5′ exoribonuclease specific for small oligoribonucleotides. The deduced amino acid sequence of the plasmid-derived orn (orn pl) showed three conserved motifs characteristic of Orn from both prokaryotes and eukaryotes. Deletion of orn pl generated no observable phenotype, but inactivation of the chromosomal copy caused slow growth in Pseudomonas putida KT2440. This defect was fully restored by complementation with orn from Escherichia coli (orn E.coli). Plasmid-derived orn pl was capable of partially complementing the P. putida orn mutant, demonstrating functionality of orn pl. Phylogenetic analysis showed that plasmid-encoded Orn was distinct from Orn encoded by the chromosome of proteobacteria. A survey of orn pl from related Pseudomonas plasmids showed a sporadic distribution but no sequence diversity. These data suggest that the orn pl was acquired by pQBR103 in a single gene-transfer event: the donor is unknown, but is unlikely to be a member of the Proteobacteria.