Comparison of the efficiency of synthesis past single bulky DNA adducts in vivo and in vitro by the polymerase III holoenzyme

The response of Escherichia coli to the alkylating agents chloroacetaldehyde and styrene oxide

DNA damage is ubiquitous and can arise from endogenous or exogenous sources. DNA-damaging alkylating agents are present in environmental toxicants as well as in cancer chemotherapy drugs and are a constant threat, which can lead to mutations or cell death. All organisms have multiple DNA repair and DNA damage tolerance pathways to resist the potentially negative effects of exposure to alkylating agents. In bacteria, many of the genes in these pathways are regulated as part of the SOS reponse or the adaptive response. In this work, we probed the cellular responses to the alkylating agents chloroacetaldehyde (CAA), which is a metabolite of 1,2-dichloroethane used to produce polyvinyl chloride, and styrene oxide (SO), a major metabolite of styrene used in the production of polystyrene and other polymers. Vinyl chloride and styrene are produced on an industrial scale of billions of kilograms annually and thus have a high potential for environmental exposure. To identify stress response genes in E. coli that are responsible for tolerance to the reactive metabolites CAA and SO, we used libraries of transcriptional reporters and gene deletion strains. In response to both alkylating agents, genes associated with several different stress pathways were upregulated, including protein, membrane, and oxidative stress, as well as DNA damage. E. coli strains lacking genes involved in base excision repair and nucleotide excision repair were sensitive to SO, whereas strains lacking recA and the SOS gene ybfE were sensitive to both alkylating agents tested. This work indicates the varied systems involved in cellular responses to alkylating agents, and highlights the specific DNA repair genes involved in the responses.

Biological properties of single chemical-DNA adducts: a twenty year perspective

The genome and its nucleotide precursor pool are under sustained attack by radiation, reactive oxygen and nitrogen species, chemical carcinogens, hydrolytic reactions, and certain drugs. As a result, a large and heterogeneous population of damaged nucleotides forms in all cells. Some of the lesions are repaired, but for those that remain, there can be serious biological consequences. For example, lesions that form in DNA can lead to altered gene expression, mutation, and death. This perspective examines systems developed over the past 20 years to study the biological properties of single DNA lesions.

Steric and electrostatic effects in DNA synthesis by the SOS-induced DNA polymerases II and IV of Escherichia coli

The SOS-induced DNA polymerases II and IV (pol II and pol IV, respectively) of Escherichia coli play important roles in processing lesions that occur in genomic DNA. Here we study how electrostatic and steric effects play different roles in influencing the efficiency and fidelity of DNA synthesis by these two enzymes. These effects were probed by the use of nonpolar shape analogues of thymidine, in which substituted toluenes replace the polar thymine base. We compared thymine with nonpolar analogues to evaluate the importance of hydrogen bonding in the polymerase active sites, while we used comparisons among a set of variably sized thymine analogues to measure the role of steric effects in the two enzymes. Steady-state kinetics measurements were carried out to evaluate activities for nucleotide insertion and extension. The results showed that both enzymes inserted nucleotides opposite nonpolar template bases with moderate to low efficiency, suggesting that both polymerases benefit from hydrogen bonding or other electrostatic effects involving the template base. Surprisingly, however, pol II inserted nonpolar nucleotide (dNTP) analogues into a primer strand with high (wild-type) efficiency, while pol IV handled them with an extremely low efficiency. Base pair extension studies showed that both enzymes bypass non-hydrogen-bonding template bases with moderately low efficiency, suggesting a possible beneficial role of minor groove hydrogen bonding interactions at the N-1 position. Measurement of the two polymerases' sensitivity to steric size changes showed that both enzymes were relatively flexible, yielding only small kinetic differences with increases or decreases in nucleotide size. Comparisons are made to recent data for DNA pol I (Klenow fragment), the archaeal polymerase Dpo4, and human pol kappa.

Kinetics of nucleotide incorporation opposite DNA bulky guanine N2 adducts by processive bacteriophage T7 DNA polymerase (exonuclease-) and HIV-1 reverse transcriptase

Six oligonucleotides with carcinogen derivatives bound at the N2 atom of deoxyguanosine were prepared, including adducts derived from butadiene, acrolein, crotonaldehyde, and styrene, and examined for effects on the replicative enzymes bacteriophage DNA polymerase T7- (T7-) and HIV-1 reverse transcriptase for comparison with previous work on smaller DNA adducts. All of these adducts strongly blocked dCTP incorporation opposite the adducts. dATP was preferentially incorporated opposite the acrolein and crotonaldehyde adducts, and dTTP incorporation was preferred at the butadiene- and styrene-derived adducts. Steady-state kinetic analysis indicated that the reduced catalytic efficiency with adducted DNA involved both an increased Km and attenuated kcat. Fluorescence estimates of Kd and pre-steady-state kinetic measurements of koff showed no significantly decreased affinity of T7- with the adducted oligonucleotides or the dNTP. Pre-steady-state kinetics showed no burst phase kinetics for dNTP incorporation with any of the modified oligonucleotides. These results indicate that phosphodiester bond formation or a conformational change of the enzyme.DNA complex is rate-limiting instead of the step involving release of the oligonucleotide. Thio elemental effects for dNTP incorporation were generally relatively small but variable, indicating that the presence of adducts may sometimes make phosphodiester bond formation rate-limiting but not always.