Investigation of the Genes Involved in Antigenic Switching at the vlsE Locus in Borrelia burgdorferi: An Essential Role for the RuvAB Branch Migrase

Persistent infection by pathogenic organisms requires effective strategies for the defense of these organisms against the host immune response. A common strategy employed by many pathogens to escape immune recognition and clearance is to continually vary surface epitopes through recombinational shuffling of genetic information. Borrelia burgdorferi, a causative agent of Lyme borreliosis, encodes a surface-bound lipoprotein, VlsE. This protein is encoded by the vlsE locus carried at the right end of the linear plasmid lp28-1. Adjacent to the expression locus are 15 silent cassettes carrying information that is moved into the vlsE locus through segmental gene conversion events. The protein players and molecular mechanism of recombinational switching at vlsE have not been characterized. In this study, we analyzed the effect of the independent disruption of 17 genes that encode factors involved in DNA recombination, repair or replication on recombinational switching at the vlsE locus during murine infection. In Neisseria gonorrhoeae, 10 such genes have been implicated in recombinational switching at the pilE locus. Eight of these genes, including recA, are either absent from B. burgdorferi, or do not show an obvious requirement for switching at vlsE. The only genes that are required in both organisms are ruvA and ruvB, which encode subunits of a Holliday junction branch migrase. Disruption of these genes results in a dramatic decrease in vlsE recombination with a phenotype similar to that observed for lp28-1 or vls-minus spirochetes: productive infection at week 1 with clearance by day 21. In SCID mice, the persistence defect observed with ruvA and ruvB mutants was fully rescued as previously observed for vlsE-deficient B. burgdorferi. We report the requirement of the RuvAB branch migrase in recombinational switching at vlsE, the first essential factor to be identified in this process. These findings are supported by the independent work of Lin et al. in the accompanying article, who also found a requirement for the RuvAB branch migrase. Our results also indicate that the mechanism of switching at vlsE in B. burgdorferi is distinct from switching at pilE in N. gonorrhoeae, which is the only other organism analyzed genetically in detail. Finally, our findings suggest a unique mechanism for switching at vlsE and a role for currently unidentified B. burgdorferi proteins in this process

The role of modeling in the development of advanced processes for metallic aerospace alloys

The application of various modeling techniques in the design and control of a number of emerging processes for aerospace alloys is summarized. These techniques include those that are based on melting and solidification (electron-beam cold-hearth melting, laser deposition), deformation (severe-plastic deformation), rapid heat treatment (dual-microstructure processing), and metal removal (distortion-free machining, high-speed machining). The models that have been developed and applied to these processes include those that are largely phenomenological (e.g., continuum FEM codes) or mechanism based. The key elements of models for various processes, important analytical/numerical results, and how these results are or can be used for manufacturing design are summarized. Challenges for the further development and application of the models for industrial processes are also described. These include refinement of the physics-based understanding of the processes and measurement of various material properties that are needed to apply the models in a real-world manufacturing environment.X114sciescopuskciothe

Laser Metal Deposition of Titanium Parts with Increased Productivity

Laser Metal Deposition (LMD), an additive manufacturing technique, is described here as an alternate for conventional manufacturing process to build aerospace components. Traditional milling of thin-walled, ribbed-, lightweight, high-valued Titanium structures generate machining wastes as high as 95%. This paper presents an LMD system setup along with an adapted manufacturing process chain for fabrication of near-net shaped Ti–6Al–4V components. Demonstrator parts built using the system setup are then shown