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151.
Chromosomal DNA from 23 closely related, pathogenic strains of Escherichia
coli was digested and probed for the insertion sequences IS1, IS2, IS4,
IS5, and IS30. Under the assumption that elements residing in DNA
restriction fragments of the same apparent length are identical by descent,
parsimony analysis of these characters yielded a unique phylogenetic tree.
This analysis not only distinguished among bacterial strains that were
otherwise identical in their biochemical characteristics and enzyme
electrophoretic mobilities, but certain aspects of the topology of the tree
were consistent across several unrelated insertion elements. The
distribution of IS elements was then reexamined in light of the inferred
phylogenetic relationships to investigate the biological properties of the
elements, such as rates of insertion and deletion, and to discover apparent
recombinational events. The analysis shows that the pattern of distribution
of insertion elements in the bacterial genome is sufficiently stable for
epidemiological studies. Although the rate of recombination by conjugation
has been postulated to be low, at least two such events appear to have
taken place.
相似文献
152.
Biosynthesis of a variant surface glycoprotein of Trypanosoma brucei. Processing of the glycolipid membrane anchor and N-linked oligosaccharides 总被引:8,自引:0,他引:8
J D Bangs T L Doering P T Englund G W Hart 《The Journal of biological chemistry》1988,263(33):17697-17705
The variant surface glycoprotein (VSG) of the ILTat 1.3 variant of Trypanosoma brucei has two asparagine-linked glycan moieties, as well as a phosphatidylinositol glycan membrane anchor. We have investigated the structure and processing of each of these oligosaccharides through analysis of the intact protein and of glycopeptides. Processing has been examined by comparing glycan structures purified from an immature intracellular form (58 kDa) of VSG with those of the mature form (59 kDa) found on the parasite surface. We find exclusively high mannose oligosaccharides (Man4-7-GlcNAc2) at Asn-432 in both the immature 58-kDa and mature 59-kDa forms. In contrast, the "core" oligosaccharide of Asn-419 (Man3-GlcNAc2) appears to be nearly quantitatively processed to a complex biantennary structure [Gal-GlcNAc-Man)2-Man-GlcNAc2) during VSG maturation. The asparagine-linked structures at Asn-419, but not those at Asn-432, are resistant to endo-beta-N-acetylglucosaminidase H within 30 s of biosynthesis. This suggests possible novel and selective mechanisms for glycosylation in African trypanosomes. Finally, we show that the carboxyl-terminal glycolipid is galactosylated (3-4 residues) relatively late in VSG biosynthesis. Phosphatidylinositol glycans have been identified on a growing number of eukaryotic membrane proteins. This report provides a direct demonstration of the processing of such a glycolipid anchor following its attachment to protein. 相似文献
153.
Homologues of glucosephosphate isomerase (GPI, EC 5.3.1.9) were purified to
homogeneity and kinetically characterized from Mytilus edulis and Isognomon
alatus, two bivalve molluscs experiencing contrasting thermal environments.
The enzyme isolated from I. alatus functions at warmer temperatures (25-35
C) than GPI from M. edulis, a species that inhabits colder marine littoral
habitats (5-20 C). The former exhibits apparent first-order (with respect
to substrate) catalytic rate constants (Vmax/KM) in vitro that become
progressively greater than the mussel enzyme as the assay temperature is
raised. Apparent zero-order catalytic rate constants (Vmax) are relatively
less differentiated. Catalytic efficiency, defined as the rate at which a
catalytic event occurs in either reaction direction for reference standard
states (substrate concentrations), is greater for the enzyme from the
tropical species (I. alatus) at all realistic combinations of temperature
and substrate concentration except for the lowest temperatures and highest
substrate concentrations, where the GPI from the boreal/temperate M. edulis
is more efficient. This pattern of catalytic divergence appears to be due
primarily to differentiation in Vmax/KM. These results and other published
data are reviewed and shown to be inconsistent with claims that adaptation
of enzymes to higher cell temperatures requires a loss in catalytic
efficiency.
相似文献
154.
Glycoproteins E and I facilitate neuron-to-neuron spread of herpes simplex virus. 总被引:21,自引:16,他引:5
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Two herpes simplex virus (HSV) glycoproteins E and I (gE and gI) form a heterooligomer which acts as an Fc receptor and also facilitates cell-to-cell spread of virus in epithelial tissues and between certain cultured cells. By contrast, gE-gI is not required for infection of cells by extracellular virus. HSV glycoproteins gD and gJ are encoded by neighboring genes, and gD is required for both virus entry into cells and cell-to-cell spread, whereas gJ has not been shown to influence these processes. Since HSV infects neurons and apparently spreads across synaptic junctions, it was of interest to determine whether gD, gE, gI and gJ are also important for interneuronal transfer of virus. We tested the roles of these glycoproteins in neuron-to-neuron transmission of HSV type 1 (HSV-1) by injecting mutant viruses unable to express these glycoproteins into the vitreous body of the rat eye. The spread of virus infection was measured in neuron-rich layers of the retina and in the major retinorecipient areas of the brain. Wild-type HSV-1 and a gJ- mutant spread rapidly between synaptically linked retinal neurons and efficiently infected major retinorecipient areas of the brain. gD mutants, derived from complementing cells, infected only a few neurons and did not spread in the retina or brain. Mutants unable to express gE or gI were markedly restricted in their ability to spread within the retina, produced 10-fold-less virus in the retina, and spread inefficiently to the brain. Furthermore, when compared with wild-type HSV-1, gE- and gI- mutants spread inefficiently from cell to cell in cultures of neurons derived from rat trigeminal ganglia. Together, our results suggest that the gE-gI heterooligomer is required for efficient neuron-to-neuron transmission through synaptically linked neuronal pathways. 相似文献
155.
Many molecular motors move unidirectionally along a DNA strand powered by nucleotide hydrolysis. These motors are multimeric ATPases with more than one hydrolysis site. We present here a model for how these motors generate the requisite force to process along their DNA track. This novel mechanism for force generation is based on a fluctuating electrostatic field driven by nucleotide hydrolysis. We apply the principle to explain the motion of certain DNA helicases and the portal protein, the motor that bacteriophages use to pump the genome into their capsids. The motor can reverse its direction without reversing the polarity of its electrostatic field, that is, without major structural modifications of the protein. We also show that the motor can be driven by an ion gradient; thus the mechanism may apply as well to the bacterial flagellar motor and to ATP synthase. 相似文献
156.
157.
Glucose 6-phosphate dehydrogenase from exists in two interconvertible species having molecular weights of 110,000 and 220,000, respectively. By use of sodium dodecyl-sulfate the enzyme can be dissociated into 2 electrophoretically separable subunits. The native enzyme was shown to be strongly inhibited by low concentrations of erythrose 4-phosphate and glyceraldehyde 3-phosphate, respectively. Since both these sugar phosphates are metabolites in the later course of the oxidative pathway this regulation may be considered as a negative feedback control of the pentose phosphate cycle. 相似文献
158.
159.
The amino acid requirements for sporulation were studied by use of auxotrophic mutants of Bacillus subtilis 168. Cells were grown to T(0) in medium containing the test amino acid and were then transferred to a minimal medium lacking that amino acid. Omission of leucine caused no reduction in sporulation. Omission of methionine, lysine, and phenylalanine appeared to cause reduced levels of sporulation, and sporulation was completely inhibited when isoleucine, tryptophan, and threonine were omitted. The amino acids in this third class showed a sequence of requirements, with tryptophan required earlier than isoleucine, which in turn was required earlier in the sporulation process than threonine. Isoleucine omission did not affect the early sporulation functions of extracellular protease formation or septum formation, but prevented the increased levels of protein synthesis and oxygen consumption that normally accompany early sporulation stages. Isoleucine did not appear to be metabolized to other compounds in significant amounts during sporulation. The role of isoleucine in the sporulation process remains unclear. 相似文献
160.
Quantitative relationships between biological resources and hydrological variables are often needed to fulfill legislative requirements for management of estuaries and coastal waters. We used a 28-year record of blue crab landings to support development of a minimum flow (defined in Florida Statutes as the point at which further withdrawals of freshwater will cause “significant harm” to the resources or ecology) for the Caloosahatchee Estuary in Florida, USA. Annual catch per unit effort (CPUE) was computed from monthly landings of crabs and measures of fishing effort. Both unadjusted and de-trended annual CPUE were significantly correlated with rainfall, freshwater inflow and the Multivariate ENSO Index during the previous year’s dry season. Increases in CPUE from 1 year to the next were positively related to dry season rainfall in the first of the 2 years. The Minimum Flow was intended to protect the resources from an impact taking 2 or more years to recover. Geometric mean functional regressions and Monte Carlo simulations were used to identify the dry season rainfall associated with this level of impact. Both a spectral analysis and the Monte Carlo simulations indicated that this level of rainfall occurred about once every 6 years. The corresponding Minimum Flow ranged from 8.6 to 12.3 m3/s (averaged over the dry season and measured at the headwater control structure of the estuary). This study demonstrated the relationship between hydrologic variables (rainfall, freshwater inflow) and blue crab landings and the usefulness of fisheries catch data in providing the technical basis for developing environmental flow targets. 相似文献