Reconstitution of a nine-protein system that initiates bacteriophage lambda DNA replication.

We have established an in vitro system, composed of highly purified bacteriophage lambda and Escherichia coli proteins, that specifically replicates supercoiled templates bearing the lambda replication origin (ori lambda). The complete system is composed of three groups of proteins: the virus-encoded initiator proteins (the lambda O and P proteins), the E. coli replication fork propagation machinery (single-stranded DNA-binding protein, dnaB helicase, dnaG primase, DNA polymerase III holoenzyme, and DNA gyrase), and two bacterial heat shock proteins (dnaJ and dnaK proteins). DNA replication in this system is initiated at or near ori lambda and proceeds unidirectionally rightwards through theta-structure intermediates, ultimately yielding a pair of intertwined daughter circles as the final product. In striking contrast to the situation in vivo and in crude in vitro systems, initiation of lambda DNA replication in the purified protein system does not require "transcriptional activation" of the origin region by E. coli RNA polymerase. We conclude that E. coli primase generates the primers for all leading and lagging strand DNA chains synthesized in this reconstituted lambda replication system.