CRYPTIC SINGLE-STRANDED-DNA BINDING ACTIVITIES OF THE PHAGE-LAMBDA-P AND ESCHERICHIA-COLI DNAC REPLICATION INITIATION PROTEINS FACILITATE THE TRANSFER OF ESCHERICHIA-COLI DNAB HELICASE ONTO DNA

The bacteriophage lambda P and Escherichia coli DnaC proteins are know n to recruit the bacterial DnaB replicative helicase to initiator comp lexes assembled at the phage and bacterial origins, respectively, Thes e specialized nucleoprotein assemblies facilitate the transfer of one or more molecules of DnaB helicase onto the chromosome; the transferre d DnaB, in turn, promotes establishment of a processive replication fo rk apparatus, To learn more about the mechanism of the DnaB transfer r eaction, we investigated the interaction of replication initiation pro teins with single-stranded DNA (ssDNA). These studies indicate that bo th P and DnaC contain a cryptic ssDNA-binding activity that is mobiliz ed when each forms a complex with the DnaB helicase. Concomitantly, th e capacity of DnaB to bind to ssDNA as judged by UV-crosslinking analy sis, is suppressed upon formation of a P . DnaB or a DnaB . DnaC compl ex, This novel switch in ssDNA-binding activity evoked by complex form ation suggests that interactions of P or DnaC with ssDNA may precede t he transfer of DnaB onto DNA during initiation of DNA replication, Fur ther, we find that the lambda O replication initiator enhances interac tion of the P . DnaB complex with ssDNA, Partial disassembly of a ssDN A:O . P . DnaB complex by the DnaK/DnaJ/GrpE molecular chaperone syste m results in the transfer in cis of DnaB to the ssDNA template, On the basis of these findings, we present a general model for the transfer of DnaB onto ssDNA or onto chromosomal origins by replication initiati on proteins.