The asymmetric dimeric polymerase hypothesis: a progress report.

In 1983, my laboratory first proposed that the DNA polymerase III holoenzyme is an asymmetric dimer with distinguishable leading and lagging strand polymerases. Here, I review progress by my laboratory and others in testing this hypothesis. To date, the hypothesis is supported by our demonstration of (i) an asymmetry in function of two populations of holoenzyme in solution in their ability to use the ATP analog, ATP gamma S, to support initiation complex formation, (ii) the stabilization of a dimeric polymerase structure by the tau subunit, (iii) allosteric communication between polymerase halves and (iv) the coexistence of gamma and the tau, subunits which share common sequences, within the same holoenzyme assemblies. This latter observation may provide a structural basis for holoenzyme asymmetry. I discuss the implications of the asymmetric dimer hypothesis to the solution of problems encountered by polymerases at the replication fork and delineate further tests required before the hypothesis can be firmly established.