Sharp kinking of a coiled-coil in MutS allows DNA binding and release

DNA mismatch repair (MMR) corrects mismatches, small insertions and deletions in DNA during DNA replication. While scanning for mismatches, dimers of MutS embrace the DNA helix with their lever and clamp domains. Previous studies indicated generic flexibility of the lever and clamp domains of MutS prior to DNA binding, but whether this was important for MutS function was unknown. Here, we present a novel crystal structure of DNA-free Escherichia coli MutS. In this apo-structure, the clamp domains are repositioned due to kinking at specific sites in the coiled-coil region in the lever domains, suggesting a defined hinge point. We made mutations at the coiled-coil hinge point. The mutants made to disrupt the helical fold at the kink site diminish DNA binding, whereas those made to increase stability of coiled-coil result in stronger DNA binding. These data suggest that the site-specific kinking of the coiled-coil in the lever domain is important for loading of this ABC-ATPase on DNA.

ALTERNATE FIELD EVAPORATION AND KINK RELAXATION ON (001) AND (112) SURFACES OF TUNGSTEN NANOTIPS

Fine-scale field evaporation of stepped (001) and (112) surfaces of tungsten nanotips was studied by field ion microscopy. It was shown that some atoms at kinks and steps are anomalously stable against field evaporation. This effect is responsible for the observed alternate field evaporation near the kink and step sites. The phenomenon of alternate field evaporation could be used to determine an atomic relaxation at kinks on nanotip surface. Using the geometric method of analysis of field ion images, the normal to surface differential displacements of the kink-site atoms were estimated.

Electroluminescence from Single 3D GaN Nanowire Grown by Self-Catalytic Molecular Beam Epitaxy

ABSTRACT3-D branching GaN nanowires have been grown using the intermediate and Ga-rich growth regimes of plasma assisted molecular beam epitaxy. Evidence that the growth is due to an auto-catalytic VLS process is obtained through SEM images showing droplet termination heads, the composition of which is essentially pure Ga. TEM analysis revealed a defect free crystal structure, even in the trunk to branch junction. Cathodoluminescence from the trunk of the branching nanowires produced a strong luminescence feature at 3.44 eV, while a slight decrease in energy to 3.1 eV was observed at the interface between the nanowire and epilayer or kink site. No yellow luminescence was detected, further suggesting a defect free growth. Preliminary I-V measurements give mixed results, suggesting intrinsic n-type nanowires.

Differential REE uptake by sector growth of monazite

Monazite-(Ce) from a dolomite carbonatite at Kangankunde, Malawi, is sector-zoned with variation in La2O3 of up to 6.0 wt.% and in Nd2O3 of up to 3.9 wt.% between sectors. Single crystal X-ray diffraction, backscattered electron imaging and microprobe analysis have been used to establish the relationship between the morphology and sector chemistry of this low-Th monazite, (Ce,La,Nd)PO4. Uptake of La by {011} sector surfaces is enhanced relative to that of and {100} sectors; Ce shows no partitioning differences; and uptake of Nd is more easily facilitated on and {100} surfaces relative to {011}. There appears to be a distinct relationship between the size of the REE ion and the probability of uptake via the different growth surfaces. Interpretation of this uptake behaviour, based on theories involving ‘protosites’, involves an investigation of the possible kink site geometries at edge-steps during growth. Part-formed kink sites with small entrance sizes are calculated to occur with higher frequency on relative to {011}, and this correlates with an increase in the smaller-sized REE (Nd) uptake by growth surfaces. The overall morphology and sector growth is suggested to be a function of uptake chemistry.

Metal deposition and dissolution

On a liquid metal electrode all surface sites are equivalent, and the deposition of a metal ion from the solution is conceptually simple: The ion loses a part of its solvation sheath, is transferred to the metal surface, and is discharged simultaneously; after a slight rearrangement of the surface atoms it is incorporated into the electrode. The details of the process are little understood, but it seems that the discharge step is generally rate determining, and the Butler-Volmer equation is obeyed if the concentration of the supporting electrolyte is sufficiently high. For example, the formation of lithium and sodium amalgams [1] in nonaqueous solvents according to: . . .Li + + e- ⇌ Li(Hg) Na+ = e- ⇌ Cd(Hg) . . . (10.1) obey the Butler-Volrner equation with transfer coefficients that depend on the solvent. On the other hand, the deposition of multivalent ions may involve several steps. Thus, the formation of zinc amalgam from aqueous solutions, with the overall reaction: . . . zn2+ + 2e- ⇌ Zn (Hg) . . . (10.2) occurs in two steps: First, Zn2+ is reduced to an intermediate Zn+ in an electron transfer step, and then the univalent ion is deposited [2]. In contrast, the surface of a solid metal offers various sites for metal deposition. Figure 10.1 shows a schematic diagram for a crystal surface with a quadratic lattice structure. A single atom sitting on a flat surface plane is denoted as an adatom; several such atoms can form an adatom cluster. A vacancy is formed by a single missing atom; several vacancies can be grouped to vacancy clusters. Steps are particularly important for crystal growth, with kink atoms, or atoms in the halfcrystal position, playing a special role. When a metal is deposited onto such a surface, the vacancies are soon filled. However, the addition of an atom in the kink position creates a new kink site; so at least on an infinite plane the number of kink sites does not change, and the current is maintained by incorporation into these sites. Similarly metal dissolution takes place predominantly at half-crystal positions, since the removal of a kink atom creates a new kink site.