[Ratio of 3' --> 5'-exonuclease and DNA-polymerase activities in normal and cancer cells of rodents and humans].

Mutations in the genes of corrective 3' --> 5'-exonucleases as well as in DNA polymerases lead to decrease in DNA biosynthesis accuracy all over genome. This is accompanied by the increase in mutagenesis and carcinogenesis probabilities. In this work, the activities of 3' --> 5'-exonucleases and DNA polymerases were studied in the extracts from normal and cancer cells of rodents and humans, and we are the first to measure their integral ratios. As example, in cultivated dermal fibroblasts of an adult human, the value of the ratio of activities of 3' --> 5'-exonucleases to DNA polymerase activity (3'-exo/pol) surpassed several folds the such a value for HeLa cells. Similar picture was observed during the comparison of normal fibroblasts of rat embryos and transformed fibroblasts of Chinese hamster A238. Experiments with cell-free extracts of some organs from healthy rats of various ages have shown that normal proliferating cells demonstrate higher 3' --> 5'-exonuclease activity and higher values of 3'-exo/pol that quiescent cells. Comparison of these data suggests a violation of the function of corrective 3' --> 5'-exonucleases in abnormally growing cancer cells.

[Complex of repair DNA polymerase beta with autonomous 3'-->5'-exonuclease shows increased accuracy of DNA synthesis].

The complexes of repair DNA polymerase beta with 3'-exonuclease and some other proteins were isolated from the chromatin of hepatocytes of normal rats for the first time. Biopolymers were extracted from the chromatin by the solution of NaCl and Triton X-100. The extract was fractionated by gel-filtration on Sephacryl S-300 columns successively in low and high ionic strength solutions, on hydroxyapatite, and on Sephadex G-100 columns. The complexes have molecular weights of 100 and 300 kDa. They dissociate to DNA polymerase and exonuclease in the course of chromatography on a DNA-cellulose column or after gel-filtration in the presence of 1 M NaCl. The co-purification of the polymerase and exonuclease is reconstituted in 0.1 M NaCl. The fidelity of monomeric and composite DNA polymerase beta was measured using phage phiX174 amber 3 as a primer/template. The products of the synthesis were transfected into Escherichia coli spheroplasts, and the frequency of reverse mutations was determined. The complex of DNA polymerase beta with 3'-exonuclease was shown to be 30 times more accurate than the monomeric polymerase, which can decrease the probability of repair mutagenesis and carcinogenesis.

[The correcting role of autonomous 3'-->5' exonucleases in mammalian multienzyme DNA polymerase complexes]. / Korrektorskaia rol' avtonomnykh 3'-->5'-ékzonukleaz v sostave mul'tifermentnykh DNK-polimeraznykh kompleksov mlekopitaiushchikh.

A study was made of the correcting role of autonomous 3'-->5' exonucleases (AE) contained in multienzyme DNA polymerase complexes of rat hepatocytes or calf thymocytes. DNA was synthesized on phage psi X174 amber3 or M13mp2 primer-templates, and used to transfect Escherichia coli spheroplasts. Frequencies were estimated for direct and reverse mutations resulting from mistakes made in the course of in vitro DNA synthesis. The mistake rate of the hepatocytic complex was estimated at 3 x 10(-6) with equimolar dNTP, and increased tenfold when proteins accounting for 70% of the total 3'-->5' exonuclease activity of the complex were removed. The fidelity of DNA synthesis was completely restored in the presence of exogenous AE (epsilon subunit of E. coli DNA polymerase III). Nuclear (Pol delta n) and cytosolic (Pol delta c) forms of DNA polymerase delta were isolated from calf thymocytes. The former was shown to contain an AE (TREX2) absent from the latter. As compared with Pol delta c, Pol delta n had a 20-fold higher exo/pol ratio and allowed 4-5 times higher fidelity of DNA synthesis. The mistake rate of DNA polymerase complexes changed when dNTP were used in nonequimolar amounts.

[Contribution of 3'---5'-exonuclease from rat liver nuclei in precision of DNA synthesis, catalyzed by mammalian DNA polymerase alpha]. / Vklad 3'---5'-ekzonukleaz iz iader pecheni krysy v tochnost' sinteza DNK, kataliziruemogo DNK-polimerazami alpha mlekopitaiushchikh.

Mammalian nuclear DNA polymerases alpha and beta are known to be devoid of editing 3'-->5'exonucleolytic activity. Presumably this activity could be effected by the exonucleases non-associated covalently with DNA polymerases. Two 3'-->5'exonucleases with molecular masses of 40 and 50 kDa have been isolated from rat liver nuclei and purified to near homogeneity. They are shown to excise mismatched nucleotides from a poly [d(A-T)] template respectively 10- and 2-fold faster than the matched ones. Upon addition of any of these exonucleases to DNA polymerase alpha from rat liver or calf thymus, the fidelity of in vitro reproduction of primed DNA from bacteriophage phi X174 amber 3 is increased 5-10-fold, the levels of exonuclease and polymerase activities being approximately the same. The extrapolation of replication fidelity to cellular activities of the exonucleases and alpha-polymerase suggests that exonuclease proofreading augments the accuracy of DNA synthesis at least by three orders of magnitude.

[3'-->5'-exonucleases of the rat liver and the correction of replication errors]. / 3'-->5'-ékzonukleazy pecheny krysy i korrektsiia oshibok replikatsii.

Mammalian nuclear DNA polymerases alpha and beta are lack of the proofreading 3'-->5' exonucleolytic activity. 40 and 50 kDa 3'-->5' exonucleases were isolated from rat liver. The exonucleases were shown to excise mismatched nucleotides from poly[d(A--T)] template 10 and 2 fold faster than matched ones. The addition of either exonuclease to DNA polymerase alpha from rat liver or calf thymus 5-10 times increased the accuracy of reproduction of primed DNA from bacteriophage phi X174 amber 3, values of exonuclease and DNA polymerase activities being approximately equal. The exonuclease activity surpasses the DNA polymerase one by an order of magnitude in chromatin and nuclear membrane. These data, taken together, are indicative of potent proofreading into hepatocytes.

[Complexes of nuclear DNA-polymerases with 3'----5'-exonucleases from the rat liver]. / Kompleksy iadernykh DNK-polimeraz c 3'----5'-ékzonukleazami iz pecheni krysy.

"Editing" 3'----5' exonuclease activity of DNA polymerases corrects replication errors. This activity associated with procaryotic DNA polymerases is not intrinsic to purified mammalian DNA polymerases. By means of extraction and subsequent gel filtration, several subspecies of complexes of 3'----5' exonuclease (E.C. 3.1.4.26) with DNA polymerases alpha, beta (E.C. 2.7.7.7) and some other proteins were isolated from chromatin, nucleoplasm, nuclear membrane, and cytosol. Complexes containing 3'----5' exonuclease manifest from 40 to 70% of total DNA polymerase activity revealed in different compartments of a hepatocyte. Molecular masses of the complexes amount from 250 to 1500 kDa They dissociate as a result of solution hydrophobization. DNA polymerase alpha activity enhances 5--8 folds during cell transition from G0 to S-period. The value of the ratio of 3'----5' exonuclease activity of different complexes to their DNA polymerase activity varies from 0.5 to 12. Other cases of discovery of the complexes of DNA polymerases with 3'----5' exonucleases are discussed. It is suggested that the absence of 3'----5' exonuclease active site in the DNA polymerase polypeptide is compensated by the complex formation of the corresponding enzymes.

[Endogenous DNA synthesis in isolated chromatin]. / Endogennyi sintez DNK v vydelennom khromatine.

It was shown that chromatin isolated from the liver of adult rats is an effective cell-free system for studying DNA synthesis without using exogenous enzymes or DNA-template. Both replication synthesis initiated in vivo and unscheduled synthesis activated several fold after gamma-irradiation of isolated chromatin proceed in chromatin preparations. Unscheduled synthesis consists of template-dependent and template-independent synthesis. Template-dependent synthesis proceeds with a maximum rate in the presence of all four deoxynucleoside triphosphates (dNTP). Template-independent synthesis proceeds with an appearable maximum rate in the presence of one dNTP whose incorporation is inhibited by the addition of the rest dNTP. All three DNA synthesis in chromatin are ATP-dependent. Replication synthesis but not the unscheduled one is inhibited by actinomycin D and N-ethylmaleinimide. Repair inhibitor--0.01 M caffeine--suppresses the initiation of unscheduled synthesis, but does not influence its elongation. The incubation conditions of chromatin for unscheduled synthesis are optimalized as for temperature, pH, time of incubation, qualitative ionic composition of the medium, concentration of chromatin, ATP, dNTP, MgCl2, NaCl. Michaelis constants for TTP are equal to 1 mM for template-independent synthesis and 3 mM for template-dependent synthesis. At optimal conditions DNA of chromatin is lengthened by 8 X 10--3% as the result of template-dependent synthesis and by 1 X 10--3% as the result of template-independent. The transition from nuclei to chromatin as well as the purification of chromatin from nuclear membranes enhance the rate of unscheduled synthesis. On the other hand, addition of nucleoplasm or cell extract to the chromatin does not considerably influence the synthesis. So it is suggested that the enzymes of initiation and elongation of unscheduled DNA synthesis are concentrated in the chromatin. The plausible role of unscheduled synthesis in excision and postreplication repair of eukaryotic DNA is discussed.