Studies of recombination-dependent replication (RDR) in the T4system have revealed the critical roles played by mediator proteins in the timely and productive loading of specific enzymes onto single-stranded DNA (ssDNA) during phage RDR processes. The T4 recombination mediator protein, uvsY, is necessary for the proper assembly of the T4 presynaptic filament (uvsX recombinase cooperatively bound to ssDNA), leading to the recombination-primed initiation of leading strand DNA synthesis. In the lagging strand synthesis component of RDR, replication mediator protein gp59 is required for the assembly of gp41, the DNA helicase component of the T4 primosome, onto lagging strand ssDNA. Together, uvsY and gp59 mediate the productive coupling of homologous recombination events to the initiation of T4 RDR. UvsY promotes presynaptic filament formation on 3 ssDNA-tailed chromosomes, the physiological primers for T4 RDR, and recent results suggest that uvsY also may serve as a coupling factor between presynapsis and the nucleolytic resection of double-stranded DNA ends. Other results indicate that uvsY stabilizes uvsX bound to the invading strand, effectively preventing primosome assembly there. Instead, gp59 directs primosome assembly to the displaced strand of the D loop͞replication fork. This partitioning mechanism enforced by the T4 recombination͞replication mediator proteins guards against antirecombination activity of the helicase component and ensures that recombination intermediates formed by uvsX͞uvsY will efficiently be converted into semiconservative DNA replication forks. Although the major mode of T4 RDR is semiconservative, we present biochemical evidence that a conservative ''bubble migration'' mode of RDR could play a role in lesion bypass by the T4 replication machinery. Bacteriophage T4 provides an excellent model system for biochemical and genetic studies of recombinationdependent replication (RDR), because DNA replication and recombination are closely coupled throughout much of the phage life cycle. After infecting a host Escherichia coli cell, T4 first replicates its genome via an origin-dependent replication initiation pathway. This pathway is shut off after a few rounds of replication, after expression of the T4 uvsW RNA͞DNA helicase, which resolves R loops required for origin function (1). T4 then relies on a recombination-dependent mechanism to initiate DNA synthesis, and this pathway accounts for a large fraction of the total DNA synthesis observed during T4 infection. In the T4 RDR pathway (reviewed in refs. 2 and 3), branched recombination intermediates generated by the phage homologous recombination machinery are captured and converted into semiconservative DNA replication forks. T4 RDR requires all of the major phage-encoded DNA replication and recombination enzymes including: gp43 (DNA polymerase), gp45 (sliding clamp), gp44͞62 (clamp loader), gp32 [single-stranded DNA (ssDNA) binding protein or ssb], gp61 (primase), gp41 (DNA helicase), gp59 (helicase loader; replication mediator protein or RMP...

In this work, a new kind of N, Cu codoped carbon dots (N/Cu-CDs) was prepared via a facile one-pot hydrothermal method by using citric acid monohydrate, copper acetate monohydrate and diethylenetriamine. The prepared N/Cu-CDs with a high quantum yield (50.1%) showed excitation-independent emission at 460 nm. The structure and fluorescence properties of N/Cu-CDs were characterized by high-resolution transmission electron microscopy, fluorescence spectrofluorometer, FT-IR spectrometer, UV-visible spectrophotometer and X-ray photoelectron spectroscopy. N/Cu-CDs were applied to establishing a ratiometric fluorescence probe toward HO based on the inner filter effect (IFE) between N/Cu-CDs and DAP (2,3-diaminophenazine, the oxidative product of o-phenylenediamine (OPD)), and provided a ratiometric fluorescence universal platform for detection of the metabolites participating in HO-generation reactions (cholesterol and xanthine). The proposed method was demonstrated to be ultrasensitive and highly selective for cholesterol and xanthine assay with detection limits of 0.03 and 0.10 μM, respectively. The fluorescence probe built was applied to the determination of cholesterol and xanthine in human serum with satisfactory results.

Metal–organic framework (MOF) materials provide an excellent platform to fabricate single-atom catalysts due to their structural diversity, intrinsic porosity, and designable functionality. However, the unambiguous identification of atomically dispersed metal sites and the elucidation of their role in catalysis are challenging due to limited methods of characterization and lack of direct structural information. Here, we report a comprehensive investigation of the structure and the role of atomically dispersed copper sites in UiO-66 for the catalytic reduction of NO 2 at ambient temperature. The atomic dispersion of copper sites on UiO-66 is confirmed by high-angle annular dark-field scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, and inelastic neutron scattering, and their location is identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for the reduction of NO 2 at 25 °C without the use of reductants. A selectivity of 88% for the formation of N 2 at a 97% conversion of NO 2 with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h –1 is achieved under nonthermal plasma activation. In situ and operando infrared, solid-state NMR, and EPR spectroscopy reveal the critical role of copper sites in the adsorption and activation of NO 2 molecules, with the formation of {Cu(I)···NO} and {Cu···NO 2 } adducts promoting the conversion of NO 2 to N 2 . This study will inspire the further design and study of new efficient single-atom catalysts for NO 2 abatement via detailed unravelling of their role in catalysis.

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