DNA replication caught in the act

Abstract

By freezing a DNA polymerase enzyme at several points along its reaction pathway, a sequence of X-ray crystal structures has been obtained, showing how the enzyme replicates DNA and revealing surprising mechanistic details. See Article p.196 Chemists would like to be able to determine the structures of true transition states in chemical reactions, but the high energy and unstable nature of transition states had made this goal unattainable. Using a repair reaction catalysed by DNA polymerase η (Pol η) as their model, Wei Yang and colleagues have extended the use of flash–freeze technology to observe DNA synthesis in real time and at atomic resolution using X-ray crystallography to analyse the trapped covalent intermediates. Pol η is particularly well suited to this approach because it has a slow rate of reaction and a relatively rigid catalytic centre. The observed reaction intermediates reveal several unanticipated transient states, and implicate an unexpected third magnesium ion in the reaction mechanism.