Antibodies raised against an Escherichia coli-produced recombinant protein encoding a 76-kDa section (region C) of malaria transmission-blocking vaccine candidate, Pfs230, have previously been shown to significantly reduce the ability of Plasmodium falciparum parasites to infect mosquitoes (71.2-89.8%). To further define the region of the Pfs230 required for transmission-blocking activity, four recombinant proteins each encoding a section of region C (Pfs230 amino acids 443-1132) were produced using the same E. coli expression system and tested for immunogenicity in mice: (i) r230/MBP.C5' encodes the first half of region C (amino acids 443-791, six cysteines); (ii) r230/MBP.CM1 encodes only cysteine motif (CM) 1 (amino acids 583-913, eight cysteines); (iii) r230/MBP.C1.6 (amino acids 453-913, eight cysteines) also includes all of CM1; and (iv) r230/MBP.C2 encodes only CM2 (amino acids 914-1268, 11 cysteines). All the recombinant proteins induced antibodies that recognized parasite-produced Pfs230, but the titre of the Pfs230 specific-antibodies generated varied, C = C1.6 = C5' > CM1 > CM2. Two recombinants, r230/MBP.C5' and r230/MBP.C1.6, induced antibody titres that were equivalent to or greater than the titre generated by r230/MBP.C. However, in contrast to r230/MBP.C, none of the recombinants induced antibodies that effectively blocked parasite infectivity to mosquitoes. This suggests that the inclusion of amino acids 914-1132 is important for the production of the transmission-blocking epitope present in region C.
Six modes of transmission electron microscopy (TEM) are compared by a numerical simulation of the image formation. The comparison includes five modes of the conventional electron microscope (CEM) (axial bright field, Unwin's phase plate, central stop dark field, tilted beam dark field, conical illumination dark field) and the annular detector mode of the scanning transmission electron microscope (STEM). It is assumed that the illumination is perfectly coherent and that the interaction between electron beam and specimen may be described as an entirely elastic event. Furthermore, the influence of radiation damage and noise is neglected. The restrictions due to these approximations are discussed and shown to be unessential to the conclusions. Recent results from single-atom scattering theory are used to describe the specimen, a one-dimensional model of a thin carbon film carrying six osmium atoms. The calculated through-focus series demonstrates the nonlinearity of most modes of the CEM, which generates interference artifacts and introduces focusing difficulties and the risk of misinterpretation of the micrographs. STEM images, however, are shown not to be subject to these artifacts, which thereby explains the excellent quality of micrographs generated by this new instrument.
Six modes of transmission electron microscopy are compared by a numerical simulation of the image formation assuming perfectly coherent illumination and ignoring the influence of radiation damage and noise. The comparison includes five modes of conventional electron microscopy (CEM): axial bright field, Unwin's phase plate, central stop dark field, tilted-beam dark field and conical illumination dark field, and the annular detector mode of the scanning transmission electron microscope (STEM).
A high resolution scanning transmission electron microscope has been constructed and is operating. The initial task of this instrument is to attempt the sequencing of DNA by heavy-atom specific staining. It is also suitable for many other biological investigations requiring high resolution, low contamination and minimum radiation damage.The basic optical parameters are: 20 to 100 KV acceleration potential, objective lens focal length of 1.0 mm. with Cs = 0.7 mm., and two additional lenses designated as condensor and diffraction lenses. The purpose of the condensor lens is to provide a parallel beam incident to the objective, and the diffraction lens produces an image of the back focal plane of the objective in the plane of an annular detector.
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