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Crossing experiments with inbred stocks of the snail (Biomphalaria glabrata) demonstrated that variants at two loci determining pigmentation and seven enzyme-determining loci exhibited normal Mendelian segregation ratios in F2 progeny. Among 39 pairwise comparisons for joint segregation, there was evidence of genetic linkage between a locus controlling mantle pigmentation (S) and 6-phosphogluconate dehydrogenase (Pgd) and confirmation of a previously described linkage between esterase-2 (Est-2) and catalase (Cat). Recombination fractions were estimated to be 17 +/- 4 for S-Pgd and 33 +/- 5 for Est-2-Cat. The remaining five loci--Acon-1, Pgm-1, Lap-1, Lap-2, and Pgd--assorted independently. This brings to 17 the number of loci examined for segregation and assortment in this medically important species. As Biomphalaria has a chromosome number n = 18, markers should soon be available for most or all of the linkage groups. 相似文献
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Since its introduction in the early 1970s Biomphalaria straminea (Gastropoda: Planorbidae) has spread rapidly and is locally the most abundant fresh water snail in Hong Kong. Studies of 19 electrophoretically detected loci in four populations show that the colonists retain high levels of variability (P = 0.26, H = 0.056 - 0.097), comparable with those found in autochthonous samples of related species. Genotype frequencies at the five polymorphic loci, and a comparison of maternal and progeny genotypes of individual field-collected snails, revealed no evidence for self-fertilization in these functional hermaphrodites. F statistics indicated minimal genetic structuring, presumably because of outcrossing and recency of origin of the populations. Geographic distribution of various alleles and their frequencies suggest that two southern populations were derived from the original colonists by dispersal but that a northern population represents a second introduction in about 1982. This interpretation (based on genetics) is consistent with the known history of the various populations. The Asian populations of this South American snail are interpreted as being in the "flush" phase of the colonization process. Finally, the probability of the secondary spread of this snail from Hong Kong, and the probability of its parasite, the human blood fluke Schistosoma mansoni, being introduced to Asia are discussed. 相似文献
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Heterodera zeae, the corn cyst nematode, is redescribed and illustrated with comparative details and measurements of females, cysts, and larvae from Maryland, USA; and India. Scanning electron micrographs o f specimens from the United States are also presented. Revised measurements for the larval stylet and new diadnostic characters, especially in the cyst cone, for H. zeae are given. The relationship of H. zeae to close species is discussed. 相似文献
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Involvement of the molecular chaperone BiP in maturation of Sindbis virus envelope glycoproteins. 总被引:11,自引:10,他引:1
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Sindbis virus codes for two membrane glycoproteins, E1 and PE2, which assemble into heterodimers within the endoplasmic reticulum. We have examined the role of the molecular chaperone BiP (grp78) in the maturation of these two proteins. E1, which folds into its mature conformation via at least three intermediates differing in the configurations of their disulfide bonds, was found to interact strongly and transiently with BiP after synthesis. ATP depletion mediated by carbonyl cyanide m-chlorophenylhydrazone treatment results in the stabilization of complexes between BiP and E1. The depletion of intracellular ATP levels also greatly inhibits conversions between the E1 folding intermediates and results in the slow incorporation of E1 into disulfide-stabilized aggregates. These results suggest that the ATP-regulated binding and release of BiP have a role in modulating disulfide bond formation during E1 folding. In comparison with E1, very little PE2 is normally recovered in association with BiP. However, under conditions in which E1 folding is aberrant, increased amounts of PE2 become directly associated with BiP. The formation of these BiP-PE2 interactions occurs after E1 begins to misfold or fails to fold efficiently. We propose that nascent PE2 is stable prior to pairing with E1 for only a limited period of time, after which unpaired PE2 becomes recognized by BiP. This implies that the productive association of PE2 and E1 must occur within a restricted time frame and only after E1 has accomplished certain folding steps mediated by BiP binding and release. Kinetic studies which show that the pairing of E1 with PE2 is delayed after translocation support this conclusion. 相似文献
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Role of glycoprotein PE2 in formation and maturation of the Sindbis virus spike. 总被引:5,自引:5,他引:0
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Sindbis virus envelope assembly is a multistep process resulting in the maturation of a rigid, highly ordered T=4 icosahedral protein lattice containing 80 spikes composed of trimers of E1-E2 heterodimers. Intramolecular disulfide bonds within E1 stabilize E1-E1 associations required for envelope formation and maintenance of the envelope's structural integrity. The structural integrity of the envelope protein lattice is resistant to reduction by dithiothreitol (DTT), indicating that E1 disulfides which stabilize structural domains become inaccessible to DTT at some point during virus maturation. The development of E1 resistance to DTT occurs prior to the completion of E1 folding and is temporally correlated with spike assembly in the endoplasmic reticulum. From these data we have predicted that in the final stages of spike assembly, E1 intramolecular disulfides, which stabilize the structural integrity of the envelope protein lattice, are buried within the spike and become inaccessible to the reductive activity of DTT. The spike is formed prior to the completion of E1 folding, and we have suggested that PE2 (the precursor to E2) may play a critical role in E1 folding after PE2-E1 oligomer formation has occurred. In this study we have investigated the role of PE2 in E1 folding, oligomer formation, and development of E1 resistance to both protease digestion and reduction by DTT by using a Sindbis virus replicon (SINrep/E1) which allows for the expression of E1 in the presence of truncated PE2. Through pulse-chase analysis of both Sindbis virus- and SINrep/E1-infected cells, we have determined that the folding of E1 into a trypsin-resistant conformation and into its most compact and stable form is not dependent upon association of E1 with PE2. However, E1 association with PE2 is required for oligomer formation, the export of E1 from the endoplasmic reticulum, and E1 acquisition of resistance to DTT. 相似文献
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The methionyl-tRNA synthetase from Bacillus stearothermophilus is shown to be a dimer of 2 x 82,000 with identical subunits. It exhibits negative cooperativity in substrate binding and "virtual" halt-of-the-sites reactivity. The enzyme binds only 1 mol of methionine in the absence of other ligands, but several methods show that 2 mol of methionyl adenylate are bound per enzyme dimer. However, one of these adenylates is formed 480 times faster than the other (k1 = 29 sec-1 and k2 = 0.06 sec-1). While the rapid phase of the reaction follows normal saturation kinetics with respect to substrate concentration, the rate of the slow phase is independent of substrate concentrations down to 1 muM. It is suggested that the very slow rate of formation of the second adenylate reflects a rate limiting conformational change which precedes a more rapid chemical step on the second subunit. 相似文献