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The universally conserved J-domain proteins (JDPs) are obligate cochaperone partners of the Hsp70 (DnaK) chaperone. They stimulate Hsp70's ATPase activity, facilitate substrate delivery, and confer specific cellular localization to Hsp70. In this work, we have identified and characterized the first functional JDP protein encoded by a bacteriophage. Specifically, we show that the ORFan gene  057w  of the T4-related enterobacteriophage RB43 encodes a  bona fide  JDP protein, named Rki, which specifically interacts with the  Escherichia coli  host multifunctional DnaK chaperone. However, in sharp contrast with the three known host JDP cochaperones of DnaK encoded by  E. coli , Rki does not act as a generic cochaperone  in vivo  or  in vitro . Expression of Rki alone is highly toxic for wild-type  E. coli , but toxicity is abolished in the absence of endogenous DnaK or when the conserved J-domain of Rki is mutated. Further  in vivo  analyses revealed that Rki is expressed early after infection by RB43 and that deletion of the  rki  gene significantly impairs RB43 proliferation. Furthermore, we show that mutations in the host  dnaK  gene efficiently suppress the growth phenotype of the RB43  rki  deletion mutant, thus indicating that Rki specifically interferes with DnaK cellular function. Finally, we show that the interaction of Rki with the host DnaK chaperone rapidly results in the stabilization of the heat-shock factor σ 32 , which is normally targeted for degradation by DnaK. The mechanism by which the Rki-dependent stabilization of σ 32  facilitates RB43 bacteriophage proliferation is discussed.

A Bacteriophage-Encoded J-Domain Protein Interacts with the DnaK/Hsp70 Chaperone and Stabilizes the Heat-Shock Factor σ32 of Escherichia coli