Enzyme interactions involving T4 phage-coded thymidylate synthase and deoxycytidylate hydroxymethylase.

Abstract

The enzymatic machinery for DNA replication functions at a high rate and with extremely high accuracy. Typically, an Escherichia coli cell replicates its 4000-kilobase pair genome in 40 minutes, using two replication forks, each of which extends two chains. Thus, the rate of chain growth is about 800 nucleotides per second at 37°. The four deoxyribonucleoside triphosphates (dNTPs) must be provided efficiently at these few sites, in order to meet the demand. At the same time the four dNTPs must be maintained at balanced concentrations. An excess or deficiency of any one of the four leads to inaccurate DNA replication, thereby stimulating spontaneous mutagenesis. Evidence suggests that steady-state dNTP concentrations at replication sites are severalfold higher than intracellular concentrations, as estimated from dNTP pool measurements1. Thus, dNTP concentration gradients must be maintained in the face of enormous rates of dNTP pool turnover.