ABSTRACT
To survive in host cells, intracellular pathogens or symbiotic bacteria require protective mechanisms to overcome the oxidative stress generated by phagocytic activities of the host. By genomic library tagging, we cloned a
dps
(stands for
D
NA-binding
p
rotein from
s
tarved cells) gene of the symbiotic “
Candidatus
Legionella jeonii” organism (called the X bacterium) (
dps
X
) that grows in
Amoeba proteus
. The gene encodes a 17-kDa protein (pI 5.19) with 91% homology to Dps and DNA-binding ferritin-like proteins of other organisms. The cloned gene complemented the
dps
mutant of
Escherichia coli
and conferred resistance to hydrogen peroxide. Dps
X
proteins purified from
E. coli
transformed with the
dps
X
gene were in oligomeric form, formed a complex with pBlueskript SKII DNA, and protected the DNA from DNase I digestion and H
2
O
2
-mediated damage. The expression of the
dps
X
gene in “
Candidatus
Legionella jeonii” was enhanced when the host amoeba was treated with 2 mM H
2
O
2
and by phagocytic activities of the host cell. These results suggested that the Dps protein has a function protective of the bacterial DNA and that its gene expression responds to oxidative stress generated by phagocytic activities of the host cell. With regard to the fact that invasion of
Legionella
sp. into respiratory phagocytic cells causes pneumonia in mammals, further characterization of
dps
X
expression in the
Legionella
sp. that multiplies in a protozoan host in the natural environment may provide valuable information toward understanding the protective mechanisms of intracellular pathogens.