Estrogen receptor can distinguish among various halodeoxyuridine-substituted DNAs

Mutational and pseudomutational effects of 5-bromodeoxyuridine in human lymphoblasts

We have studied the effects of 5-bromodeoxyuridine (BrdUrd) at two genetic loci in diploid human lymphoblast cells. In thymidine kinase heterozygotes (tk +/-), a 2-h dose of BrdUrd induced a transient, non-heritable resistance to the thymidine analogue, trifluorothymidine (F3TdR). We have called this phenomenon pseudomutation and have shown that affected cells acquire the ability to survive in the presence of F3TdR and then, after degradation of F3TdR in the medium, return to an apparently normal wild-type state. Our data suggest that BrdUrd incorporation into DNA as a thymidine analogue is responsible for the effect, which we interpret as a temporary loss of thymidine kinase activity. This effect is not seen in tk +/+ homozygotes. In contrast, at the hypoxanthine-guanine phosphoribosyl transferase locus in tk +/- heterozygotes, BrdUrd did not induce a permanent, heritable resistance to 6-thioguanine (gene locus mutation). We detected such mutations only in the tk +/+ homozygote and only at external BrdUrd concentrations considerably higher than those which saturate the uptake of BrdUrd into DNA as a thymidine analogue. We postulate that the reduced TK enzyme levels (30%) in the heterozygote prevent the build-up of a sufficiently high intracellular BrdUrd triphosphate pool to promote the misincorporations as deoxycytidine triphosphate which may be responsible for gene locus mutation.

Cytosol induces apparent selectivity of glucocorticoid receptor binding to nucleic acids of different secondary structure

Unpurified rat liver glucocorticoid-receptor complexes within cytosol show a distinct binding preference for double-stranded DNA over single-stranded DNA; the binding to Escherichia coli rRNA is negligible. Extensive purification of the receptor abolishes its ability to distinguish among DNAs of different secondary structure and the affinity of the purified receptor toward RNA is greatly enhanced, reaching 30-50% of that of DNA. The purification effect is reversible: after cytosol addition to purified receptor preparation the binding preference restores. NaCl does not mimic the effect of cytosol. The flow-through fraction of a phosphocellulose column retains the ability of crude cytosol to produce selective decrease in the receptor binding to single-stranded DNA. This effect may also be observed by using two types of DNA-cellulose bearing double-stranded or denatured DNA, pretreated with crude cytosol. Additionally, pretreatment of immobilized DNA with even low cytosol concentrations has been shown to markedly enhance receptor binding, although this enhancement was lacking specificity with respect to DNA secondary structure. The nature of cytosolic active principle and some possible regulatory implications are discussed.

Enzymatic synthesis, ligation, and restriction of DNA containing deoxy-4-thiothymidine

Phage fd RF I DNA1 about 90% substituted by deoxy-4-thiothymidine (s4Td) in the codogenic strand was synthesized by the simultaneous actions of DNA polymerase I and DNA ligase. While the rate of DNA synthesis was considerably reduced, the yield the rate of DNA synthesis was considerably reduced, the yield was not affected in the presence of s4TdTP. The conversion of RF II to RF I DNA by DNA ligase was even improved. This effect seems to be related with an altered ratio of affinity of polymerase and ligase for the s4Td-containing substrate. The presence of the base analogue in the DNA was verified independently by chromatographic and spectroscopic methods. The modified genome could be cleaved by restriction endonucleases Hpa II (C/CGG)d and Taq I (T/CGA)d. A number of the fragments produced showed altered mobilities under the conditions of polyacrylamide gel electrophoresis.

On the influence of thymidine analogues on the activity of phage fd promoters in vitro

RF I DNA of phage fd containing 5-bromo-deoxyuridine (br5Ud) or deoxyuridine (Ud) instead of deoxythymidine (Td) inthe codogenic strand was synthesized in vitro. The modified genomes could be cleaved by restriction endonuclease Hpa II. Although the recognition site of Hpa II is CCGG, the cleavage rate was significantly reduced with Ud-containing DNA. Both base substitutions altered the mobilities of several DNA fragments under the conditions of polyacrylamide gel electrophoresis. The fragments containing binding sites for RNA polymerase were assayed for the rates of stable complex formation. The substitution of Td for both, Ud and br5Ud, strongly influenced this parameter. Thus the methyl group of Td has to be regarded as one of the sites in DNA which determine the rate of stable RNA polymerase binding and thereby possibly mediate promoter activity in vitro (24,25,26). In most cases the rate of complex formation was decreased by Ud, but increased by br5Ud.