Protection of coliphage lambda O initiator protein from proteolysis in the assembly of the replication complex in vivo.

We have shown previously that, in contrast to the free coliphage lambda O initiator protein rapidly degraded by ClpP/ClpX protease, the lambda present in the replication complex (RC) is protected from proteolysis. Now we asked at which step of the pathway of RC assembly in vivo does the stabilization of lambda O occur. In accordance with the in vitro established order we found that lambda P and DnaB helicase functions are, but those of DnaJ and GrpE chaperones are not, required for the protection of lambda O from proteolysis. Therefore, our results suggest that the first lambda O protecting structure of the pathway of RC assembly is the lambda O-lambda P-DnaB preprimosome. The next step of the pathway, the chaperone-mediated rearrangement of the preprimosome, is not essential for lambda O stabilization. However, in contrast to other chaperones, the DnaK function was required for the protection of lambda O from proteolysis, suggesting an earlier access of DnaK to the pathway of RC assembly in vivo, in accordance with current models by which molecular chaperones facilitate protein assembly.