Bacterial Resistance to Nanosilver: Molecular Mechanisms and Possible Ways to Overcome them

This review contains the results of experimental studies of recent years dedicated to the resistance of bacteria to the action of nanosized silver and describes the putative molecular mechanisms of its development. Emphasis is placed on the study of works devoted to the investigation of the mechanisms of the resistance of bacteria to silver ions, which are the main factor of the bactericidal action of nanoparticles. The review also contains suggestions for further research aimed at developing of ways to overcome the problem of resistance of individual bacterial strains to the action of nanosilver and methods preventing its further spread.

     

2',3'-Dideoxy-3'-aminonucleoside 5'-triphosphates inhibit the repair of DNA in rat liver chromatin

2',3'-Dideoxy-3'-aminonucleoside 5'-triphosphates are shown to be strong inhibitors of repair DNA synthesis in gamma-irradiated rat liver chromatin. The activity of these compounds is comparable with that of the most effective inhibitor of the DNA polymerase beta-catalyzed repair DNA synthesis.     

DNA Polymerase-Associated and Autonomous 3"→5" Exonucleases in Invertebrates, Unicellulars, and Bacteria

Recently we have revealed a high content of autonomous 3"5" exonucleases (AE), i.e., those not bound covalently with DNA polymerases, in cells of vertebrates, from fish to human [1]. In the present work, using gel filtration method, cell-free extracts were studied from 15 objects located at different positions on the phylogenetic tree, such as archaebacteria, eubacteria, fungi, infusorians, coelenterates, annelids, and arthropods. It is shown that enzymatic activity of AE accounts for from 30 to 88% of the total 3"5" exonuclease activity of the extracts. A part of AE is revealed in zone of high-molecular DNA polymerases and can be separated by change of the chromatography conditions. It indicates a probable formation of complexes of AE with DNA polymerases. The high AE activity in cells of different organisms, from archae- and eubacteria to human, allows suggesting these enzymes to play a significant role in correction of polymerase errors in the processes of DNA replication and reparation, as well as in postreplicative correction of heteroduplexes in DNA.     

Unusual metastasis of papillary thyroid carcinoma to larynx and hypopharynx a case report

Background  

Although direct infiltration of papillary carcinoma of thyroid to larynx, trachea and esophagus is well recognized, lymphatic and vascular metastases to larynx and hypopharynx have rarely been reported.     

Properties of autonomous 3′→5′ exonucleases

Autonomous 3′→5′ exonucleases (AE) are not bound covalently to DNA polymerases, but they are often included into the replicative complexes. Intracellular AE overproduction in bacteria results in sharp suppression of mutagenesis, whereas inactivation of these enzymes in bacteria and fungi leads to an increase in mutagenesis frequency by 2–3 orders of magnitude. Correction of DNA polymerase errors in vitro occurs after addition of AE to the incubation medium. This correction is clearly manifested under conditions of mutational stress (during induced but not spontaneous mutagenesis), for instance, with an imbalance of dNTPs — error-prone conditions. At equimolar dNTP (error-free conditions), the correction is relatively weak. The gene knockout of both alleles of the major AE gene in mice does not influence spontaneous mutagenesis though a substantial increase could be expected. The frequency of induced mutagenesis has not been yet measured, though the inactivation of AE could increase the frequency of mutagenesis. Complete inactivation of the major AE leads to inflammatory myocarditis and a 5-fold reduction of life span of mice. Dominant heterozygous mutations were found in various loci of the AE gene, which caused the development of Aicardi-Goutieres (autosomal recessive encephalopathy) syndrome, familial chilblain lupus, systemic lupus erythematosus, retinal vasculopathy, and cerebral leukodystrophy. In the nucleus, AE have a corrective function, but after transition into cytoplasm these enzymes destroy aberrant DNA that appears during replication and thereby save the cells from autoimmune diseases. Depending on their intracellular localization, AE carry out various biological functions but employ the same mechanism of the catalyzed reactions.     

Eukaryotic error-prone DNA polymerases: The presumed roles in replication, repair, and mutagenesis

A number of error-prone DNA polymerases have been found in various eukaryotes, ranging from yeasts to mammals, including humans. According to partial homology of the primary structure, they are grouped into families B, X, and Y. These enzymes display a high infidelity on an intact DNA template, but they are accurate on a damaged template. Error-prone DNA polymerases are characterized by probabilities of base substitution or frameshift mutations ranging from 10^?3 to 7.5 · 10^?1 in an intact DNA, whereas the spontaneous mutagenesis rate per replicated nucleotide varies between 10^?10 and 10^?12. Low-fidelity polymerases are terminal deoxynucleotidyl transferase (TdT) and DNA polymerases β, ζ, κ, η, ι, λ, μ, and Rev1. The main characteristics of these enzymes are reviewed. None of them exhibits proofreading 3′ → 5′ exonuclease (PE) activity. The specialization of these polymerases consists in their capacity for synthesizing opposite DNA lesions (not eliminated by the numerous repair systems), which is explained by the flexibility of their active centers or a limited ability to express TdT activity. Classic DNA polymerases α, δ, ε, and γ cannot elongate primers with mismatched nucleotides at the 3′-end (which leads to replication block), whereas some specialized polymerases can catalyze this elongation. This is accompanied by overcoming the replication block, often at the expense of an increased mutagenesis rate. How can a cell exist under the conditions of this high infidelity of many DNA polymerase activities? Not all tissues of the body contain a complete set of low-fidelity DNA polymerases, although some of these enzymes are vitally important. In addition, cells “should not allow” error-prone DNA polymerases to work on undamaged DNA. After a lesion on the DNA template is bypassed, the cell should switch over from DNA synthesis catalyzed by specialized polymerases to the synthesis catalyzed by relatively high-fidelity DNA polymerases δ and ? (with an error frequency of 10^?5 to 10^?6) as soon as possible. This is done by forming complexes of polymerase δ or ? with proliferating cell nuclear antigen (PCNA) and replication factors RP-A and RF-C. These highly processive complexes show a greater affinity to correct primers than specialized DNA polymerases do. The fact that specialized DNA polymerases are distributive or weakly processive favors the switching. The fidelity of these polymerases is increased by the PE function of DNA polymerases δ and ε, as well as autonomous 3′ → 5′ exonucleases, which are widespread over the entire phylogenetic tree of eukaryotes. The exonuclease correction decelerates replication in the presence of lesions in the DNA template but increases its fidelity, which decreases the probability of mutagenesis and carcinogenesis.