Michelle C. Silva

Selective disruption of the DNA polymerase III α–β complex by the umuD gene products

DNA polymerase III (DNA pol III) efficiently replicates the Escherichia coli genome, but it cannot bypass DNA damage. Instead, translesion synthesis (TLS) DNA polymerases are employed to replicate past damaged DNA; however, the exchange of replicative for TLS polymerases is not understood. The umuD gene products, which are up-regulated during the SOS response, were previously shown to bind to the α, β and ε subunits of DNA pol III. Full-length UmuD inhibits DNA replication and prevents mutagenic TLS, while the cleaved form UmuD′ facilitates mutagenesis. We show that α possesses two UmuD binding sites: at the N-terminus (residues 1–280) and the C-terminus (residues 956–975). The C-terminal site favors UmuD over UmuD′. We also find that UmuD, but not UmuD′, disrupts the α–β complex. We propose that the interaction between α and UmuD contributes to the transition between replicative and TLS polymerases by removing α from the β clamp.

Polymerase manager protein UmuD directly regulates Escherichia coli DNA polymerase III α binding to ssDNA

Replication by Escherichia coli DNA polymerase III is disrupted on encountering DNA damage. Consequently, specialized Y-family DNA polymerases are used to bypass DNA damage. The protein UmuD is extensively involved in modulating cellular responses to DNA damage and may play a role in DNA polymerase exchange for damage tolerance. In the absence of DNA, UmuD interacts with the α subunit of DNA polymerase III at two distinct binding sites, one of which is adjacent to the single-stranded DNA-binding site of α. Here, we use single molecule DNA stretching experiments to demonstrate that UmuD specifically inhibits binding of α to ssDNA. We predict using molecular modeling that UmuD residues D91 and G92 are involved in this interaction and demonstrate that mutation of these residues disrupts the interaction. Our results suggest that competition between UmuD and ssDNA for α binding is a new mechanism for polymerase exchange.

Polymerase Switching in Response to DNA Damage

DNA polymerases are highly efficient and accurate macromolecular machines. They are capable of replicating DNA at up to 1,000 nucleotides per second while making less than one error in 100,000 additions. However, DNA is constantly subjected to damage from myriad sources. DNA damage disrupts normal cellular DNA replication by interfering with the accuracy and efficiency of replicative DNA polymerases. Specialized Y family DNA polymerases exist that can copy damaged DNA, although that ability often has a mutagenic cost. Therefore, Y family DNA polymerase activity is highly regulated in the cell. This chapter presents the functions of both replicative and Y family DNA polymerases and the cellular mechanisms of polymerase management. The focus is on Escherichia coli systems but also briefly discusses eukaryotic Y family polymerases. We first present DNA replication carried out by prokaryotic DNA polymerase III and describe its subunits and the coordination of leading and lagging strand replication. We then discuss DNA damage and specialized Y family DNA polymerases. Different models for the management of replicative and Y family DNA polymerases are presented. Finally, we briefly compare the eukaryotic systems with their prokaryotic counterparts.