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(1/327) Marker effects on reversion of T4rII mutants.

The frequencies of 2-aminopurine- and 5-bromouracil-induced A:T leads to G:C transitions were compared at nonsense sites throughout the rII region of bacteriophage T4. These frequencies are influenced both by adjacent base pairs within the nonsense codons and by extracodonic factors. Following 2AP treatment, they are high in amber (UAG) and lower in opal (UGA) codons than in allelic ochre (UAA) codons. In general, 5BU-induced transitions are more frequent in both amber and opal codons than in the allelic ochre codons. 2AP- and 5BU-induced transition frequencies in the first and third positions of opal codons are correlated with those in the corresponding positions of the allelic ochre codons. Similarly, the frequencies of 2AP-induced transition in the first and second positions of amber codons and their ochre alleles are correlated. However, there is little correlation between the frequencies of 5BU-induced transitions in the first and second positions of allelic amber and ochre codons.  (+info)

(2/327) Base pairing of anhydrohexitol nucleosides with 2,6-diaminopurine, 5-methylcytosine and uracil asbase moiety.

Hexitol nucleic acids (HNAs) with modified bases (5-methylcytosine, 2,6-diaminopurine or uracil) were synthesized. The introduction of the 5-methylcytosine base demonstrates that N -benzoylated 5-methylcytosyl-hexitol occurs as the imino tautomer. The base pairing systems (G:CMe, U:D, T:D and U:A) obey Watson-Crick rules. Substituting hT for hU, hCMefor hC and hD for hA generally leads to increased duplex stability. In a single case, replacement of hC by hCMedid not result in duplex stabilization. This sequence-specific effect could be explained by the geometry of the model duplex used for carrying out the thermal stability study. Generally, polypurine HNA sequences give more stable duplexes with their RNA complement than polypyrimidine HNA sequences. This observation supports the hypothesis that, besides changes in stacking pattern, the difference in conformational stress between purine and pyrimidine nucleosides may contribute to duplex stability. Introduction of hCMeand hD in HNA sequences further increases the potential of HNA to function as a steric blocking agent.  (+info)

(3/327) Direct selection for mutators in Escherichia coli.

We have constructed strains that allow a direct selection for mutators of Escherichia coli on a single plate medium. The plate selection is based on using two different markers whose reversion is enhanced by a given mutator. Plates containing limiting amounts of each respective nutrient allow the growth of ghost colonies or microcolonies that give rise to full-size colonies only if a reversion event occurs. Because two successive mutational events are required, mutator cells are favored to generate full-size colonies. Reversion of a third marker allows direct visualization of the mutator phenotype by the large number of blue papillae in the full-size colonies. We also describe plate selections involving three successive nutrient markers followed by a fourth papillation step. Different frameshift or base substitution mutations are used to select for mismatch-repair-defective strains (mutHLS and uvrD). We can detect and monitor mutator cells arising spontaneously, at frequencies lower than 10(-5) in the population. Also, we can measure a mutator cascade, in which one type of mutator (mutT) generates a second mutator (mutHLS) that then allows stepwise frameshift mutations. We discuss the relevance of mutators arising on a single medium as a result of cells overcoming successive growth barriers to the development and progression of cancerous tumors, some of which are mutator cell lines.  (+info)

(4/327) Purine analogue 6-methylmercaptopurine riboside inhibits early and late phases of the angiogenesis process.

Angiogenesis has been identified as an important target for antineoplastic therapy. The use of purine analogue antimetabolites in combination chemotherapy of solid tumors has been proposed. To assess the possibility that selected purine analogues may affect tumor neovascularization, 6-methylmercaptopurine riboside (6-MMPR), 6-methylmercaptopurine, 2-aminopurine, and adenosine were evaluated for the capacity to inhibit angiogenesis in vitro and in vivo. 6-MMPR inhibited fibroblast growth factor-2 (FGF2)-induced proliferation and delayed the repair of mechanically wounded monolayer in endothelial GM 7373 cell cultures. 6-MMPR also inhibited the formation of solid sprouts within fibrin gel by FGF2-treated murine brain microvascular endothelial cells and the formation of capillary-like structures on Matrigel by murine aortic endothelial cells transfected with FGF2 cDNA. 6-MMPR affected FGF2-induced intracellular signaling in murine aortic endothelial cells by inhibiting the phosphorylation of extracellular signal-regulated kinase-2. The other molecules were ineffective in all of the assays. In vivo, 6-MMPR inhibited vascularization in the chick embryo chorioallantoic membrane and prevented blood vessel formation induced by human endometrial adenocarcinoma specimens grafted onto the chorioallantoic membrane. Also, topical administration of 6-MMPR caused the regression of newly formed blood vessels in the rabbit cornea. Thus, 6-MMPR specifically inhibits both the early and the late phases of the angiogenesis process in vitro and exerts a potent anti-angiogenic activity in vivo. These results provide a new rationale for the use of selected purine analogues in combination therapy of solid cancer.  (+info)

(5/327) The comparative effects of famciclovir and valacyclovir on herpes simplex virus type 1 infection, latency, and reactivation in mice.

Infections by herpes simplex virus (HSV) cannot yet be eliminated, but the severity of the disease can be reduced. Two newer drugs with established efficacy for such infections, famciclovir and valacyclovir, were tested in a mouse eye model of HSV infection. Both drugs significantly reduced mortality and titers of virus shed from the eyes of mice infected with an otherwise lethal dose of HSV type 1 (HSV-1). Similar titers of HSV-1 were found in the eyes, ganglia, and brains of treated animals. Although valacyclovir reduced the latent viral DNA load better in these studies than did famciclovir, rates of reactivation by explantation and UV exposure were the same. Thus, in this study, famciclovir and valacyclovir were equally effective in limiting the virulence and spread of HSV-1, despite their biochemical and pharmacologic differences.  (+info)

(6/327) Characterization of the interaction of lambda exonuclease with the ends of DNA.

Lambda exonuclease processively degrades one strand of double-stranded DNA (dsDNA) in the 5"-3" direction. To understand the mechanism through which this enzyme generates high processivity we are analyzing the first step in the reaction, namely the interaction of lambda exonuclease with the ends of substrate DNA. Endonuclease mapping of lambda exonuclease bound to DNA has shown that the enzyme protects approximately 13-14 bp on dsDNA, and no nucleo-tides on the single-stranded tail of the DNA product. We have developed a rapid fluorescence-based assay using 2-aminopurine and measured the steady-state rate constants for different end-structures of DNA. The relative k(cat)for 5" ends decreases in the order 5" recessed > blunt >> 5" overhang. However, k(cat)/K(m)remains relatively constant for these different structures suggesting they are all used equally efficiently as substrates. From these data we propose that a single-stranded 5" overhang end can bind non-productively to the enzyme and the non-hydrolyzed strand is required to aid in the proper alignment of the 5" end. We have also measured the length-dependence of the steady-state rate para-meters and find that they are consistent with a high degree of processivity.  (+info)

(7/327) Mechanism of action and in vitro activity of 1',3'-dioxolanylpurine nucleoside analogues against sensitive and drug-resistant human immunodeficiency virus type 1 variants.

(-)-Beta-D-1',3'-Dioxolane guanosine (DXG) and 2,6-diaminopurine (DAPD) dioxolanyl nucleoside analogues have been reported to be potent inhibitors of human immunodeficiency virus type 1 (HIV-1). We have recently conducted experiments to more fully characterize their in vitro anti-HIV-1 profiles. Antiviral assays performed in cell culture systems determined that DXG had 50% effective concentrations of 0.046 and 0.085 microM when evaluated against HIV-1(IIIB) in cord blood mononuclear cells and MT-2 cells, respectively. These values indicate that DXG is approximately equipotent to 2', 3'-dideoxy-3'-thiacytidine (3TC) but 5- to 10-fold less potent than 3'-azido-2',3'-dideoxythymidine (AZT) in the two cell systems tested. At the same time, DAPD was approximately 5- to 20-fold less active than DXG in the anti-HIV-1 assays. When recombinant or clinical variants of HIV-1 were used to assess the efficacy of the purine nucleoside analogues against drug-resistant HIV-1, it was observed that AZT-resistant virus remained sensitive to DXG and DAPD. Virus harboring a mutation(s) which conferred decreased sensitivity to 3TC, 2',3'-dideoxyinosine, and 2',3'-dideoxycytidine, such as a 65R, 74V, or 184V mutation in the viral reverse transcriptase (RT), exhibited a two- to fivefold-decreased susceptibility to DXG or DAPD. When nonnucleoside RT inhibitor-resistant and protease inhibitor-resistant viruses were tested, no change in virus sensitivity to DXG or DAPD was observed. In vitro drug combination assays indicated that DXG had synergistic antiviral effects when used in combination with AZT, 3TC, or nevirapine. In cellular toxicity analyses, DXG and DAPD had 50% cytotoxic concentrations of greater than 500 microM when tested in peripheral blood mononuclear cells and a variety of human tumor and normal cell lines. The triphosphate form of DXG competed with the natural nucleotide substrates and acted as a chain terminator of the nascent DNA. These data suggest that DXG triphosphate may be the active intracellular metabolite, consistent with the mechanism by which other nucleoside analogues inhibit HIV-1 replication. Our results suggest that the use of DXG and DAPD as therapeutic agents for HIV-1 infection should be explored.  (+info)

(8/327) In vitro induction of human immunodeficiency virus type 1 variants resistant to phosphoralaninate prodrugs of Z-methylenecyclopropane nucleoside analogues.

Two methylenecyclopropane nucleoside analogues with a phenylphosphoralaninate moiety, QYL-685 and QYL-609, exert potent and specific activities against human immunodeficiency virus type 1 strain LAI (HIV-1(LAI)) and HIV-2 in vitro. In this study, we induced HIV-1 variants resistant to QYL-685 by exposing HIV-1(LAI) to increasing concentrations of QYL-685. After 16 passages, the virus (HIV-1(P16)) was less sensitive to QYL-685 (104-fold), QYL-609 (>41-fold), and (-)-beta-2',3'-dideoxy-3'-thiacytidine (3TC) (>1, 100-fold) than was HIV-1(LAI) and contained an M184I mutation. Two infectious clones, HIV-1(M184I) and HIV-1(M184V), were resistant to QYL-685, QYL-609, and 3TC, confirming that the M184I mutation was responsible for the observed resistance. Viral-fitness analyses (competitive HIV-1 replication assays) revealed that in the absence of drugs, M184I and M184V conferred a replication disadvantage on the virus compared to the replication efficiency of the wild-type infectious clone (HIV-1(wt)). However, in the presence of QYL-685 (4 microM), HIV-1(M184I) and HIV-1(M184V) showed greater fitness than HIV-1(wt). These data may provide structural and virological relevance with regard to the emergence of M184I and M184V substitutions in HIV-1.  (+info)