Copper Hypersensitivity and Uptake in Pseudomonas syringae Containing Cloned Components of the Copper Resistance Operon

Copper resistance in Pseudomonas syringae carrying the copABCD operon is associated with accumulation of copper in the periplasm and outer membrane, apparently as a function of the copper-binding activities of the copABC gene products. However, no specific function for copD has been determined. In this study, P. syringae cells containing copCD or copBCD cloned behind the lac promoter were hypersensitive to copper. An increased accumulation of copper was measured in cells containing several combinations of cop genes that included copC and copD. Our data suggest that CopC, a periplasmic copper-binding protein, and CopD, a probable inner membrane protein, may function together in copper uptake.

Copper resistance gene homologs in pathogenic and saprophytic bacterial species from tomato

Copper-resistant strains of Xanthomonas campestris pv. vesicatoria, Pseudomonas cichorii, Pseudomonas putida, Pseudomonas fluorescens, and a yellow Pseudomonas sp. were isolated from tomato plants or seeds. In Southern hybridizations, DNA from each strain showed homology with the copper resistance (cop) operon previously cloned from Pseudomonas syringae pv. tomato PT23. Homology was associated with plasmid and chromosomal DNA in X. compestris pv. vesicatoria, P. putida, and the yellow Pseudomonas sp. Homology was detected only in the chromosomal DNA of P. cichorii and P. fluorescens. Homology with cop was also detected in chromosomal DNA from copper-sensitive strains of P. cichorii, P. fluorescens, and P. syringae pv. tomato, suggesting that the cop homolog may be indigenous to certain Pseudomonas species and have some function other than copper resistance. No homology was detected in DNA from a copper-sensitive X. campestris pv. vesicatoria strain. Copper-inducible protein products were detected in each copper-resistant bacterium by immunoblot analysis with antibodies raised to the CopB protein from the cop operon. The role of the homologous DNA in copper resistance was confirmed for the X. campestris pv. vesicatoria strain by cloning and transferring the cop homolog to a copper-sensitive strain of X. campestris pv. vesicatoria. The possibility and implications of copper resistance gene exchange between different species and genera of pathogenic and saprophytic bacteria on tomato plants are discussed.

Identification and characterization of a Pantoea citrea gene encoding glucose dehydrogenase that is essential for causing pink disease of pineapple

Pantoea citrea, a member of the family Enterobacteriaceae, causes pink disease of pineapple, whose symptom is characterized by the formation of pink to brown discolorations of the infected portions of the pineapple fruit cylinder upon canning. Molecular genetic approaches were applied to elucidate the mechanism responsible for this fruit discoloration. A P. citrea mutant strain, CMC6, defective in its ability to cause pink disease and fruit discoloration, was generated by nitrosoguanidine mutagenesis. A DNA fragment that restored these activities was isolated by screening a genomic cosmid library of P. citrea. A large open reading frame of 2,361 bp, identified by nucleotide sequencing of a subclone of the complementing DNA, showed high similarities to identified genes encoding glucose dehydrogenase (GDH) in Escherichia coli, Acinetobacter calcoaceticus, and Gluconobacter oxydans. The predicted amino acid sequence of GDH of P. citrea was identical to known GDHs in these bacteria by 54, 44, and 34%, respectively. GDH of P. citrea has a predicted molecular mass of 86.2 kDa, contains a conserved binding domain for the cofactor pyrroloquinoline quinone, and possesses GDH activity as demonstrated by biochemical assay. GDH is the key branch point enzyme leading to the biosynthesis of gluconate, which in turn serves as the substrate leading to the formation of 2-ketogluconate, 2,5-diketogluconate, 6-phosphogluconate, and 2-keto-6-phosphogluconate. Addition of gluconate to CMC6 restores the juice- and fruit-discoloring activity. Although the pigments formed by heating (or canning) have not been identified, it is clear that GDH is one of the enzymes required for pigment formation leading to pink disease.

Copper resistance in Pseudomonas syringae mediated by periplasmic and outer membrane proteins

Copper-resistant strains of Pseudomonas syringae pathovar tomato accumulate copper and develop blue colonies on copper-containing media. Three of the protein products of the copper-resistance operon (cop) were characterized to provide an understanding of the copper-resistance mechanism and its relationship to copper accumulation. The Cop proteins, CopA (72 kDa), CopB (39 kDa), and CopC (12 kDa), were produced only under copper induction. CopA and CopC were periplasmic proteins and CopB was an outer membrane protein. Leader peptide sequences of CopA, CopB, and CopC were confirmed by amino-terminal peptide sequencing. CopA, CopB, and CopC were purified from strain PT23.2, and their copper contents were determined. One molecule of CopA bound 10.9 +/- 1.2 atoms of copper and one molecule of CopC bound 0.6 +/- 0.1 atom of copper. The Cop proteins apparently mediate sequestration of copper outside of the cytoplasm as a copper-resistance mechanism.