Bradyrhizobium japonicum porphobilinogen synthase uses two Mg(II) and monovalent cations

Bradyrhizobium japonicum porphobilinogen synthase (B. japonicum PBGS) has been purified and characterized from an overexpression system in an Escherichia coli host (Chauhan, S., and O'Brian, M. R. (1995) J. Biol. Chem. 270, 19823-19827). B. japonicum PBGS defines a new class of PBGS protein, type IV (classified by metal ion content), which utilizes a catalytic MgA present at a stoichiometry of 4/octamer, an allosteric MgC present at a stoichiometry of 8/octamer, and a monovalent metal ion, K+. However, the divalent MgB or ZnB present in some other PBGS is not present in B. japonicum PBGS. Under optimal conditions, the Kd for MgA is <0.2 microM, and the Kd for MgC is about 40 microM. The response of B. japonicum PBGS activity to monovalent and divalent cations is mutually dependent and varies dramatically with pH. B. japonicum PBGS is also found to undergo a dynamic equilibrium between active multimeric species and inactive monomers under assay conditions, a kinetic characteristic not reported for other PBGSs. B. japonicum PBGS is the first PBGS that has been rigorously demonstrated to lack a catalytic ZnA. However, consistent with prior predictions, B. japonicum PBGS can bind Zn(II) (presumably as ZnA) at a stoichiometry of 4/octamer with a Kd of 200 microM; but this high concentration is outside a physiologically significant range.     

Association between the Rfp-Y haplotype and the incidence of Marek's disease in chickens

Certain haplotypes at the major histocompatibility (B) complex (Mhc) of the chicken provide an easily demonstrated influence on tumor formation following infections with Marek's disease virus (MDV). Recognition that there is a second histocompatibility complex of genes in the chicken, Rfp-Y, comprised of Mhc class I and class II genes, some of which are at least transcribed, evokes the question of whether this gene complex might also influence the outcome of MDV infections. To test this hypothesis, pedigree-hatched chicks in families from the original Rfp-Y-defining stock in which three Rfp-Y and two B system haplotypes are segregating were challenged with the RB1B strain of MDV. Birds with the Y3/Y3 genotype were found to have 2.3 times the risk of developing a tumor compared with birds with other Rfp-Y genotypes combined (P <0.02). Additionally, birds carrying the BR9/B11 genotype had 2.3 times the risk of tumor formation, relative to birds with the B11/B11 genotype (P <0.02). We found no evidence for an interaction between genotypes within the B and Rfp-Y systems. These data provide evidence that Rfp-Y haplotypes, as well as B haplotypes, can significantly influence the outcome of infection with MDV.     

Mottle on computed radiographs of the chest in pediatric patients

PURPOSE: To investigate the relationship between radiation exposure and perceived mottle at bedside pediatric chest examinations performed with screen-film and computed radiographic techniques. MATERIALS AND METHODS: In a pediatric intensive care unit, chest radiographs were obtained with both computed radiography (60 radiographs) and a 600-speed screen-film system (14 radiographs). The relative radiation exposure was estimated by using the sensitivity value obtained in the processing of each computed radiograph. Five radiologists assessed the mottle present in the computed radiographs and screen-film images. RESULTS: For computed radiographs, the perceived level of mottle was inversely related to radiation exposure. For the same radiation exposure, the perceived mottle on computed radiographs was significantly higher than that on screen-film images (P < .001 for small cassettes; P < .01 for large cassettes). CONCLUSION: Pediatric computed radiography of the chest requires approximately twice the exposure of a 600-speed screen-film system to attain the same level of mottle.     

The role of perceptual and structural similarity in cross-adaptation

Cross-adaptation, the decrease in sensitivity to one odorant following exposure to a different odorant, is affected by odorant similarity, both perceptual and structural, but the precise relationship is obscure. The present series of studies was designed to explore various aspects of perceptual and structural similarity as they relate to cross-adaptation. In Experiment 1, cross-adaptation was assessed between androstenone and five odorants that share a common urinous note with androstenone, but retain unique perceptual characteristics; only the compound judged most perceptually similar to androstenone cross-adapted it. In Experiment 2, odorants both perceptually and structurally similar (androstenone and androstanone) displayed significant, mutual cross-adaptation. Furthermore, magnitude estimates for androstanone were significantly reduced following exposure to 3-methylidene-5 alpha-androstane (3M5A), a structurally similar, perceptually odorless compound. This finding appears to be the first demonstration that an odorless compound can affect, via cross-adaptation, the perception of an odorous compound. Finally, in Experiment 3, significant, asymmetric cross-adaptation was observed between compounds that are perceptually and structurally dissimilar (4-cyclohexylcyclohexanone [4-CHCH] and androstenone). These findings indicate that the role of similarity in cross-adaptation is difficult to quantify and emphasize the numerous odorant characteristics that can affect cross-adaptation.     

Bilateral lateral geniculitis associated with severe diarrhea

PURPOSE: We examined a 28-year-old woman who developed incongruous binasal and bitemporal visual field defects one week after having a febrile gastroenteritis characterized by severe diarrhea while traveling in Mexico. METHODS: The patient was treated with intravenous methylprednisolone sodium succinate, 1 g daily for three days, followed by an eight-week course of oral prednisone. RESULTS: Neuroradiologic examination demonstrated bilaterally increased signal intensity within the lateral geniculate bodies. Cerebrospinal fluid analysis disclosed isolated protein increase. CONCLUSION: Severe diarrhea may be associated with an aseptic bilateral lateral geniculitis resulting in hourglass-shaped visual fields. Corticosteroid therapy may provide mild improvement.     

Requirement for p56lck tyrosine kinase activation in T cell receptor-mediated thymic selection

The nonreceptor protein tyrosine kinase p56lck (Lck) serves as a fundamental regulator of thymocyte development by delivering signals from the pre-T cell receptor (pre-TCR) that permit subsequent maturation. However, considerable evidence supports the view that Lck also participates in signal transduction from the mature TCR. We have tested this conjecture by expressing a dominant-negative form of Lck under the control of a promoter element (the distal lck promoter) that directs high expression in CD4+CD8+ thymocytes, mature thymocytes, and peripheral T cells, thereby avoiding, complications that result from the well-documented ability of dominant-negative Lck to block very early events in thymocyte maturation. Here we report that expression of the catalytically inactive Lck protein at twice normal concentrations inhibits thymocyte positive selection by as much as 80%, while leaving other aspects of cell maturation intact. This effect was studied in more detail in mice simultaneously bearing the male-specific H-Y alpha/beta TCR transgene and ovalbumin-specific DO10 alpha/beta TCR transgene, where even equimolar expression of the dominant-negative Lck protein substantially vitiated the positive selection process. Although deletion of H-Y alpha/beta thymocytes proceeded normally in male mice despite the presence of catalytically inactive Lck, modest inhibition of superantigen-mediated deletion was in some cases observed. These data further implicate Lck in the propagation of all TCR-derived signals, and indicate that even very modest deficiencies in the representation of functional Lck molecules could in humans, profoundly alter the character of the peripheral TCR repertoire.