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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.
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
153.
Perception of inert particles by calanoid copepods: behavioral observations and a numerical model 总被引:2,自引:0,他引:2
Bundy Marie H.; Gross Thomas F.; Vanderploeg Henry A.; Rudi Strickler J. 《Journal of plankton research》1998,20(11):2129-2152
High-resolution video showed freely swimming Diaptomus sicilisattacking and capturing inert 5O µm polystyrene beadsthat were outside the influence of the copepod feeding current.The beads were frequently more than half a body length awayand were attacked after the bow wake of the movingcopepod displaced the bead away from the copepod. To investigatethe hypothesis that deformation of streamlines around the copepodand its first antennae stimulated the attack response, a finiteelement numerical model was constructed. The model describedthe fluid interactions between a large object approaching asmaller object in a laminar flow at Reynolds number 5, whichis characteristic of the fluid regime experienced by foragingcopepods. The model revealed that fluid velocity fluctuationsand streamline deformations arose in the region between thetwo objects as separation distance between the objects decreased.The video observations and the model results support the hypothesesthat chemoreception is not required for the detection and captureof large phytoplankton cells [Vanderploeg et ai, in Hughes,R.N. (ed.), Behavioral Mechanisms of Food Selection. NATO ASISeries G20, 1990; DeMott and Watson, /. Plankton Res., 13, 1205-1222,1991], and that swimming behavior plays an integral role inprey detection.
4Present address: Academy of Natural Sciences Estuarine ResearchCenter, 10545 Mackall Road, St Leonard, MD 20685, USA 相似文献
154.
The nature of the cell types capable of mediating mitogen-induced cellular cytotoxicity (MICC) and antibody-dependent cellular cytotoxicity (ADCC) was investigated utilizing effector cells from athymic nude and euthymic heterozygous control littermate mice as well as Sephadex anti-Fab immunoabsorbent column purified spleen cell populations from normal (CS7BL/6) mice. Chicken erythrocytes (CRBC) and the mouse lymphoma, EL-4, were used as target cells in both cytotoxicity assays. MICC utilizing CRBC targets was mediated by several effector cell types whereas MICC utilizing EL-4 lymphoma targets was T-cell dependent. ADCC against both CRBC and EL-4 lymphoma targets occurred independently of the presence of T-cells. In addition, effector cell populations incapable of mediating MICC against EL-4 lymphoma targets were capable of mediating ADCC against the same EL-4 targets. Thus, utilizing the appropriate target cells, EL-4 but not CRBC, a sharp distinction can be made between the effectors for ADCC and MICC: ADCC is T-cell independent while MICC is dependent on the presence of mature thymus-derived cells. Furthermore these studies demonstrate that the nature of the target cell employed in MICC and ADCC reactions plays a critical role in defining the types of effector cells capable of mediating these cytotoxicity reactions. 相似文献
155.
156.
F.A. Kimball G.L. Bundy A. Robert J.R. Weeks 《Prostaglandins & other lipid mediators》1979,17(5):657-666
The in vivo monkey uterine stimulating potency of 9-deoxo-16,16-dimethyl-9-methylene-PGE2 is similar to that of 16,16-dimethyl-PGE2 and approximately 15 times that of PGE2. Low doses of this compound stimulated uterine contractions when administered vaginally. Pregnancy was terminated prematurely following subcutaneous, intramuscular or vaginal suppository treatment. Estimates of potential for gastrointestinal side effects using the rat enteropooling assay and in vivo monkey effects indicate that diarrhea will be substantially reduced with retention of uterine stimulating potency. 相似文献
157.
Richard E. Lenski Michael J. Wiser Noah Ribeck Zachary D. Blount Joshua R. Nahum J. Jeffrey Morris Luis Zaman Caroline B. Turner Brian D. Wade Rohan Maddamsetti Alita R. Burmeister Elizabeth J. Baird Jay Bundy Nkrumah A. Grant Kyle J. Card Maia Rowles Kiyana Weatherspoon Spiridon E. Papoulis Rachel Sullivan Colleen Clark Joseph S. Mulka Neerja Hajela 《Proceedings. Biological sciences / The Royal Society》2015,282(1821)
Many populations live in environments subject to frequent biotic and abiotic changes. Nonetheless, it is interesting to ask whether an evolving population''s mean fitness can increase indefinitely, and potentially without any limit, even in a constant environment. A recent study showed that fitness trajectories of Escherichia coli populations over 50 000 generations were better described by a power-law model than by a hyperbolic model. According to the power-law model, the rate of fitness gain declines over time but fitness has no upper limit, whereas the hyperbolic model implies a hard limit. Here, we examine whether the previously estimated power-law model predicts the fitness trajectory for an additional 10 000 generations. To that end, we conducted more than 1100 new competitive fitness assays. Consistent with the previous study, the power-law model fits the new data better than the hyperbolic model. We also analysed the variability in fitness among populations, finding subtle, but significant, heterogeneity in mean fitness. Some, but not all, of this variation reflects differences in mutation rate that evolved over time. Taken together, our results imply that both adaptation and divergence can continue indefinitely—or at least for a long time—even in a constant environment. 相似文献
158.
Rhona K. Stuart Randelle Bundy Kristen Buck Majid Ghassemain Kathy Barbeau Brian Palenik 《Environmental microbiology》2017,19(2):756-769
Picocyanobacteria from the genus Synechococcus are ubiquitous in ocean waters. Their phylogenetic and genomic diversity suggests ecological niche differentiation, but the selective forces influencing this are not well defined. Marine picocyanobacteria are sensitive to Cu toxicity, so adaptations to this stress could represent a selective force within, and between, ‘species’, also known as clades. Here, we compared Cu stress responses in cultures and natural populations of marine Synechococcus from two co‐occurring major mesotrophic clades (I and IV). Using custom microarrays and proteomics to characterize expression responses to Cu in the lab and field, we found evidence for a general stress regulon in marine Synechococcus. However, the two clades also exhibited distinct responses to copper. The Clade I representative induced expression of genomic island genes in cultures and Southern California Bight populations, while the Clade IV representative downregulated Fe‐limitation proteins. Copper incubation experiments suggest that Clade IV populations may harbour stress‐tolerant subgroups, and thus fitness tradeoffs may govern Cu‐tolerant strain distributions. This work demonstrates that Synechococcus has distinct adaptive strategies to deal with Cu toxicity at both the clade and subclade level, implying that metal toxicity and stress response adaptations represent an important selective force for influencing diversity within marine Synechococcus populations. 相似文献
159.
160.
This study focuses on the cytotoxic effects of fumonisin B1 (FB1) on both immortalised and immortalised and subsequently transfected normal human bronchial epithelial (NHBE) cells of human
origin using four bioassays. While the MTT, Neutral Red and hexosaminidase colorimetric assays showed little difference between
the toxic effects on the two related cell lines, the clonogenic assay, measuring cell survival and proliferation, indicated
that FB1 had a more toxic effect on the nontransfected cells. This kind ofin vitro approach using cells which retain many characteristics of normal cell growth and differentiation can go some way to developing
evaluation models for food safety in the case of mycotoxin contamination without resorting totally to whole animal testing.
Nevertheless, one or two cytotoxicity tests may be inadequate for a complete appraisal of toxic potential: rather, as wide
a range of methodologies as feasible should be employed initially before meaningful conclusions may be drawn. 相似文献