- Studies of the DNA polymerase III holoenzyme of Escherichia coli support a model in which both the leading and lagging strand polymerases are held together in a complex with the replicative helicase and priming activities, allowing two identical alpha catalytic subunits to assume different functions on the two strands of the replication fork. Creation of distinct functions for each of the two polymerases within the holoenzyme depends on the asymmetric character of the entire complex. The asymmetry of the holoenzyme is created by the DnaX complex, a heptamer that includes tau and gamma products of the dnaX gene. tau and gamma perform unique functions in the DnaX complex, and the interaction between alpha and tau appears to dictate the catalytic subunit's role in the replicative reaction. This review considers the properties of the DnaX complex including both tau and gamma, with the goal of understanding the properties of the replicase and its function in vivo. Recent studies in eukaryotic and other prokaryotic systems suggest that an asymmetric dimeric replicase may be universal. The leading and lagging strand polymerases may be distinct in some systems. For example, Pol e and Pol delta may function as distinct leading and lagging strand polymerases in eukaryotes, and PolC and DnaE may function as distinct leading and lagging strand polymerases in low GC content Gram-positive bacteria.