Characterization of the unique C terminus of the Escherichia coli tau DnaXprotein - Monomeric C-tau binds alpha and DnaB and can partially replace tau in reconstituted replication forks

A contact between the dimeric tau subunit within the DNA polymerase III hol oenzyme and the DnaB helicase is required for replication fork propagation at physiologically-relevant rates (Kim, S., Dallmann, H. G., ;cHenry, C. S. , and Marians, K. J. (1996) Cell 84, 643-650). In this report, we exploit t he OmpT protease to generate C-tau a protein containing only the unique C-t erminal sequences of tau, free of the sequences shared with the alternative gamma frameshifting product of dnaX. We have established that C-tau is a m onomer by sedimentation equilibrium and sedimentation velocity ultracentrif ugation, Monomeric C-tau. binds the oc catalytic subunit of DNA polymerase III with a 1:1 stoichiometry, C-tau also binds DnaB, revealed by a coupled immunoblotting method. C-tau restores the rapid replication rate of ineffic ient forks reconstituted with only the gamma dnaX gene product. The acceler ation of the DnaB helicase can be observed in the absence of primase, when only leading-strand replication occurs. This indicates that C-tau bound onl y to the leading-strand polymerase, can trigger the conformational change n ecessary for DnaB to assume the fast, physiologically relevant form.