Regulation of Capsid Sizes of Large Tailed Bacteriophages

Many bacteriophages and eukaryotic viruses, which share little sequence similarities, have icosahedral protein capsids containing their genetic materials. Generally, an icosahedral viral capsid is assembly of 12 pentamers and a certain number of hexmers of the major capsid protein, following Caspar and Klug¡¯s quasi-equivalence rule. The arrangement of these pentamers and hexmers is characterized by the triangulation (T) number. Questions arise whether viruses have evolved from a few common ancestors, and how the assembly of the icosahedral capsids has been regulated to achieve a defined capsid size and geometry. I present studies of the capsids of several large icosahedral bacteriophages, which broaden our understanding of the regulation of viral capsid assembly. Bacteriophage SPO1 may share common ancestry with herpesvirus, according to the similarities in their T numbers and in the asymmetric molecules slightly off the local three-fold symmetry positions on the outer surface of both capsids. However, the cryo-EM structure of the SPO1 capsid assembled from the uncleaved major capsid protein show that, unlike the herpesvirus asymmetric molecule, the SPO1 asymmetric protein may not be required for the initial procapsid assembly, suggesting that the two asymmetric molecules may have different origins. Phage P1 is excellent for studying size determination in viral capsid since it produces virions of three sizes. The cryo-EM structures of the three capsids and internal capsid proteins identified suggests a control mechanism for P1 capsids, in which the DarA protein functions as a semi-scaffolding protein to assist the assembly of the P1 big capsid. Jumbo phages have been rarely studied. The structural studies on four jumbo phages showed their T numbers. N3, PAU and 121Q are the first T = 19, 25 and 28 viral capsids found. These results suggest that T-numbers larger than 16 may generally be allowed

Capsid Structure and DNA Packing in Jumbo Bacteriophages

Jumbo phages, the phages with genome length larger than 200 Kbp, are extreme examples of how the capsid and genome coordinate in evolution. To learn the mechanism of capsid size change during evolutionary time, the capsid and DNA of several jumbo phages were characterized. The capsid structure and protein of a T=25 Sphingomonas paucimobilis phage PAU and a T=28 Escherichia coli phage 121Q were studied in detail. The high resolution cryo-EM structures show that the major capsid proteins (MCPs) of both phages adopt the HK97 fold which is conserved in all solved tailed phage MCP structures. The capsids contain decoration proteins with unprecedented shape and location. A pentameric protein structure is attached on the inner surface of the pentamer in both capsids. The PAU capsid has arcs of density located on hexamers surrounding the pentons, which may bend the conformation of the subunit it interacts with to improve capsid stability. 121Q capsid contains dimeric density near the local 2-fold symmetry axes and knob-like density at the middle of the hexamer, which may participate in forming the capsid shell because the 121Q MCP leaves holes at the two locations. Both capsids contain a number of internal proteins whose roles are not clear. The study on the jumbo phage DNA started with showing that partial modification of the cytosine in PAU and phage G DNA significantly slowed the phage DNA in electrophoresis. A new technique was developed to quantify the effect of base modifications in large DNA in electrophoresis, with which reliable measurements of the chromosome size of our jumbo phage collection were made. Jumbo phages have larger terminal redundancy and lower DNA packing density compared to small and mid-sized phages. The result on the DNA packing density in different sizes of phages reveals a negative correlation between the capsid size and the DNA packing density. We explain this relationship by a model based on the strength limit between the capsomers of the capsid shell

High efficiency uniform positron beam loading in a hollow channel plasma wakefield accelerator

We propose a novel positron beam loading regime in a hollow plasma channel that can efficiently accelerate $e^+$ beam with high gradient and narrow energy spread. In this regime, the $e^+$ beam coincides with the drive $e^-$ beam in time and space and their net current distribution determines the plasma wakefields. By precisely shaping the beam current profile and loading phase according to explicit expressions, three-dimensional Particle-in-Cell (PIC) simulations show that the acceleration for $e^+$ beam of $\sim$nC charge with $\sim$GV/m gradient, $\lesssim$0.5% induced energy spread and $\sim$50% energy transfer efficiency can be achieved simultaneously. Besides, only tailoring the current profile of the more tunable $e^-$ beam instead of the $e^+$ beam is enough to obtain such favorable results. A theoretical analysis considering both linear and nonlinear plasma responses in hollow plasma channels is proposed to quantify the beam loading effects. This theory agrees very well with the simulation results and verifies the robustness of this beam loading regime over a wide range of parameters

Estimating ammonia emissions from cropland in China based on the establishment of agro-region-specific models

ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China under a grant numbers 41877546 and U1612441, and a BBSRC-Newton Funded project (BB/N013484/1). This work also contributes to the activities of Top-notch Academic Programs Project of Jiangsu Higher Education Institution of China (PPZY2015A061), and Program for Student Innovation through Research and Training (1913A22).Peer reviewedPostprin

Tunable Magnetocaloric Effect in Ni-Mn-Ga Microwires

Abstract Magnetic refrigeration is of great interest due to its high energy efficiency, environmental friendliness and low cost. However, undesired hysteresis losses, concentrated working temperature interval (WTI) and poor mechanical stability are vital drawbacks that hinder its practical application. Off-stoichiometric Ni-Mn-Ga Heusler alloys are capable of giant magnetocaloric effect (MCE) and tunable transformation temperatures. Here, by creating Ni-Mn-Ga microwires with diameter of 35–80 μm using a melt-extraction technique, negligible hysteresis and relatively good mechanical stability are found due to the high specific surface area (SSA) that reduces incompatibility between neighboring grains. The high SSA also favors the element evaporation at high temperatures so that the transformation temperatures can be feasibly adjusted. Tunable magnetocaloric effect owing to different magneto-structural coupling states is realized by (i) composition design and subsequent tuning, which adjusts the temperature difference between the martensite transformation (MT) and the magnetic transition, and (ii) creation of gradient composition distribution state, which manipulates the MT range. Magnetic entropy change ΔS m ~−18.5 J kg−1 K−1 with relatively concentrated WTI and WTI up to ~60 K with net refrigeration capacity ~240 J kg−1 at 50 kOe are demonstrated in the present Ni-Mn-Ga microwires. This criterion is also applicable for other small-sized materials