Abstract
Trigger factor (TF) is a ribosome-bound protein that combines catalysis of peptidyl-prolyl isomerization and chaperone-like activities in Escherichia coli. TF was shown to cooperate with the DnaK (Hsp70) chaperone machinery in the folding of newly synthesized proteins, and the double deletion of the corresponding genes (tig and dnaK) exhibited synthetic lethality. We used a detailed genetic approach to characterize various aspects of this functional cooperation in vivo. Surprisingly, we showed that under specific growth conditions, one can delete both dnaK and tig, indicating that bacterial survival can be maintained in the absence of these two major cytosolic chaperones. The strain lacking both DnaK and TF exhibits a very narrow temperature range of growth and a high level of aggregated proteins when compared to either of the single mutants. We found that, in the absence of DnaK, both the N-terminal ribosome-binding domain and the C-terminal domain of unknown function are essential for TF chaperone activity. In contrast, the central PPIase domain is dispensable. Taken together, our data indicate that under certain conditions, folding of newly synthesized proteins in E. coli is not totally dependent on an interaction with either TF and/or DnaK, and suggest that additional chaperones may be involved in this essential process.