Multienzyme systems of DNA replication.

Replication is accomplished by multienzyme systems whose operations are usefully considered in respect to three stages of the process: initiation, elongation, anid termination. 1) Initiation entails synthesis of a short RNA fragment that serves as primer for the elongation step of DNA synthesis. This stage, probed by SS phage DNA templates, reveals three distinctive and highly specific systems in E. coli. The Ml3 DNA utilizes RNA polymerase in a manner that may reflect how plasmid elements are replicated in the cell. The ØX174 DNA does not rely on RNA-polymerase, but requires instead five distinctive proteins which may belong to an apparatus for initiating a host chromosome replication cycle at the origin. The G4 DNA, also independent of RNA polymerase, needs simply the dnaG protein for its distinctive initiation and may thus resemble the system that initiates the replication fragments at the nascent growing fork. In each case it is essential that in vitro the DNA-unwinding protein coat the viral DNA and influence its structure. 2) Elongation is achieved in every case by the multisubunit, holoenzyme form of DNA polymerase III. Copolymerase III, which is an enzyme subunit, and adenosine triphosphate are required to form a proper complex with the primer template but appear dispensable for the ensuing chain growth by DNA polymerase (33). 3) Termination requires excision of the RNA priming fragment, filling of gaps and sealing of interruptions to produce a covalently intact phosphodiester backbone. DNA polymerase I has the capacity for excision and gapfilling and DNA ligase is required for sealing. What once appeared to be a simple DNA polymerase-mediated conversion of a single-strand to a duplex circle (34) is now seen as a complex series of events in which diverse multienzyme systems function. Annoyance with the difficulties in resolving and reconstituting these systems is tempered by the conviction that these are the very systems used ,by the cell in replicating its chromosome and extrachromosomal elements. Thus, understanding of the regulation of replication events in the cell, their localization at membrane surfaces and integration with cell division, and their coordination with phage DNA maturation and particle assembly will all be advanced by knowledge of the components of the replicative machinery.