DNA polymerase V (pol V) is a specialized, error‑prone DNA polymerase found in the bacterium Escherichia coli and related prokaryotes. It functions primarily in translesion DNA synthesis (TLS), allowing replication to continue past DNA lesions that stall the replicative polymerases.
Genetic composition
Pol V is a heterotrimeric complex composed of two identical UmuC subunits and one UmuD′ subunit, the latter being a proteolytically activated form of UmuD. The genes encoding these proteins are organized in the umuDC operon, which is transcriptionally induced as part of the SOS response to extensive DNA damage.
Regulation and activation
Expression of umuDC is tightly controlled. Under normal conditions, the LexA repressor binds the SOS box upstream of the operon, suppressing transcription. DNA damage triggers RecA‑mediated autocleavage of LexA, derepressing umuDC. Additionally, the UmuD protein undergoes limited proteolysis by the Lon protease, generating the active UmuD′ subunit required for polymerase activity. The active pol V complex further requires the RecA/ATP complex (RecA*), which acts as a co‑factor to stimulate its polymerase function.
Catalytic activity
Pol V exhibits low fidelity, frequently incorporating incorrect nucleotides opposite both undamaged and damaged templates. Its catalytic core resides in UmuC, which harbors the polymerase active site characteristic of the Y‑family DNA polymerases. Pol V can insert nucleotides opposite a variety of lesions, such as UV‑induced cyclobutane pyrimidine dimers and N^2‑alkylguanine adducts, albeit with a high propensity for mutagenesis.
Biological role
The principal biological function of pol V is to provide a survival mechanism under conditions of severe DNA damage where high‑fidelity replicative polymerases (Pol III) cannot progress. By bypassing lesions, pol V prevents replication fork collapse and consequent cell death, at the cost of increased mutational load—a trade‑off that can contribute to adaptive mutagenesis.
Structural insights
Crystallographic and cryo‑electron microscopy studies have revealed that the UmuC subunit adopts the characteristic “right‑hand” fold of Y‑family polymerases, with a relatively open active site that accommodates distorted DNA. The UmuD′ subunit interacts with both UmuC and the RecA* filament, positioning the complex for DNA synthesis.
Historical context
Pol V was first identified in the early 1990s through genetic screens for SOS‑induced mutagenesis. The discovery linked the umuDC operon to error‑prone DNA synthesis, expanding understanding of bacterial DNA damage tolerance pathways.
Relevance to research and biotechnology
Because of its mutagenic properties, pol V is employed as a model system for studying TLS mechanisms, mutagenesis, and the regulation of DNA repair pathways. It also serves as a comparative reference for eukaryotic TLS polymerases (e.g., human Pol η, Pol ι).
Key references
- Goodman, M. F., & Woodgate, R. (1999). Translesion synthesis: bypassing the DNA damage checkpoint. Annual Review of Biochemistry, 68, 547‑592.
- Sandigursky, S., et al. (1996). Structural basis for the mutagenic activity of DNA polymerase V. Molecular Cell, 1(4), 617‑624.
Note: Information is based on established peer‑reviewed literature and standard molecular biology textbooks.
