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The relationship between thef-ratio [NO3 uptake/(NO3+ NH4+) uptake] and ambient nitrate concentration was evaluatedfor eight data sets from coastal waters. The f-ratio increasedasymptotically with increase in nitrate concentration in mostdata sets. However, the rate at which f-ratio increased at lownitrate concentration (slope = m) and the maximum attained f-ratio(fmax) varied among regions; the initial slope varied most withvalues ranging in excess of an order of magnitude. The datawere analyzed in relation to environmental factors and methodologicalconsiderations known to influence the f-ratio. Ambient ammoniumconcentration was important in accounting for regional differencesin the f versus NO3 relationship. A further analysisof the data, relating f-ratio to the ratio of NO3/(NO3+ NH4+) concentrations yielded a much more regionally consistentand approximately linear relationship; slopes varied by lessthan a factor of two in the extreme cases. Inclusion of knownalternative (aside from NH4+) sources of reduced-N (e.g. urea)and correction for methodological/computational errors (isotopedilution) systematically reduce f-ratio estimates. Other factors,e.g. reduced-N uptake by microheterotrophs, may systematicallyincrease the f-ratio. 相似文献
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Proteoglycan metabolism associated with mouse metanephric development: morphologic and biochemical effects of beta-D-xyloside 总被引:3,自引:0,他引:3
Morphology and de novo incorporation of [35S]sulfate into proteoglycans were studied in fetal mouse kidneys at the onset of organogenesis. Branching morphogenesis and nephron development in organ culture and in vivo were associated with de novo synthesis of chondroitin-SO4 and heparan-SO4 proteoglycans. The role of proteoglycan metabolism in metanephrogenesis was then studied by analysis of the effects of p-nitrophenyl-beta-D-xylopyranoside (beta-D-xyloside) on renal development and proteoglycan metabolism. Incubation of fetal kidneys in beta-D-xyloside at concentrations of 1.0 and 0.5 mM, but not at 0.1 mM, caused inhibition of ureteric branching and markedly diminished synthesis of a large Mr 2.0 X 10(6) Da chondroitin-SO4 proteoglycan. Incorporation of [35S]sulfate was stimulated at all beta-D-xyloside concentrations, reflecting synthesis of xyloside initiated dermatan-35SO4 chains. In contrast to dramatic effects on chondroitin-SO4 synthesis and ureteric branching, beta-D-xyloside had no effect on heparan-SO4 synthesis or on development of the glomerulus and glomerular basement membrane. We thus characterize the proteoglycans synthesized early in the course of renal organogenesis and describe observations which suggest an association between metabolism of chondroitin-SO4 proteoglycan and development of the ureter. 相似文献
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Summary Previous reports indicate that in laboratory strains of mice, males are distinct from females in possession of repetitive DNA, notably devoid of Eco RI and Hae III sites and rich in the simple tetranucleotides GATA/GACA. We report here that such sequences originated in an ancestor common to laboratory mice,Mus hortulanus, M. spretus, and possibly alsoM. cookii. Interestingly, other male-specific satellite sequences were detected inM. caroli, M. cookii, M. saxicola, andM. minutoides. This novel satellite is also likely to be composed of simple repetitious sequences, but does not contain GATA and GACA. Thus, the Y chromosome appears to contain a disproportionately large amount of simple repetitious DNA. An attractive explanation for these results is that long tandem arrays of simple repeated sequences are generated at high frequency throughout the genome and that they are retained for a longer time on the Y chromosome due to the absence of homologous pairing at meiosis. 相似文献
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The velocity and orientation of T4 and lambda DNA have been measured for the first 20 s during pulsed-field gel electrophoresis in order to clarify the DNA motions that occur. For a square pulse with field strength E = 10 V/cm, the velocity of lambda DNA increases gradually to 10.5 microns/s in 1.0 s, declines to 8.6 microns/s, and then rises to a plateau value of 9.3 microns/s after 4 s. T4 DNA behaves similarly, but more slowly. Parallel measurements of fluorescence-detected linear dichroism show that the DNA becomes substantially aligned with its chain axis parallel to the electrophoretic field E after the pulse is applied. The alignment also shows an overshoot, an undershoot, and a plateau comparable to those seen for velocity. When the field strength increases, both the velocity and the alignment reach their peaks more quickly. For all field strengths and both molecular weights, the velocity peak occurs when the molecular center of mass has moved 0.3 to 0.5 L, where L is the chain contour length. A qualitative model is provided. 相似文献
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The study reported here is an examination of the organization and evolution of three Y chromosomal repeated sequences, designated pBC10-0.6, pBC15-1.1, and pBA33-1.8, in five closely related species of the genus Mus. The species distributions of major restriction fragment length polymorphisms produced with a panel of restriction enzymes is used to develop the phylogenetic relationships between the five species studied. However, the apparent degree of relatedness among these species varied a great deal with each of the three probes and was also highly dependent on the particular restriction enzyme used. The usefulness for phylogenetic studies of closely associated sequences varying in evolutionary stability is discussed. 相似文献
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B Bartlomowicz T Friedberg D Utesch E Molitor K Platt F Oesch 《Biochemical and biophysical research communications》1989,160(1):46-52
The phosphorylation of the two major phenobarbital-inducible cytochrome P450 isoenzymes IIB1 and IIB2 was increased in hepatocytes by the action of the membrane permeating cAMP derivatives N6-dibutyryl-cAMP and 8-thiomethyl-cAMP. Under these conditions the dealkylation of 7-pentoxyresorufin, a selective substrate of cytochrome P450IIB1 and P450IIB2 was markedly reduced. 16 beta-Hydroxylation of testosterone which is catalyzed specifically only by cytochrome P450IIB1 and IIB2 was strongly reduced; for 16 alpha-hydroxylation which is also catalyzed by cytochrome P450IIB1 and IIB2 but additionally by 3 further cytochrome P450 isoenzymes, this reduction was less pronounced; for the oxidation of the 17 beta-hydroxyl group which besides cytochromes P450IIB1 and IIB2 is additionally catalyzed not only by other cytochromes P450 but also by 17 beta-hydroxysteroid dehydrogenase there was a clear tendency of reduction which, however, no longer reached statistical significance. Hydroxylation at other positions of testosterone which are catalyzed by other cytochrome P450 isoenzymes were not significantly changed. Hence isoenzyme-selective phosphorylation of cytochrome P450 leads to a corresponding isoenzyme-selective modulation of monooxygenase activity which holds promise to be especially important as a fast regulation of the control of genotoxic metabolites. 相似文献
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Summary Effects of variation in fire season on flowering of forbs and shrubs were studied experimentally in two longleaf pine forest habitats in northern Florida, USA. Large, replicated plots were burned at different times of the year, and flowering on each plot was measured over the twelve months following fire. While fire season had little effect on the number of species flowering during the year following fire, fires during the growing season decreased average flowering duration per species and increased synchronization of peak flowering times within species relative to fires between growing seasons. Fires during the growing season also increased the dominance of fall flowering forbs and delayed peak fall flowering. Differences in flowering resulting from variation in fire season were related to seasonal changes in the morphology of clonal forbs, especially fall-flowering composites. Community level differences in flowering phenologies indicated that timing of fire relative to environmental cues that induced flowering was important in determining flowering synchrony among species within the ground cover of longleaf pine forests. Differences in fire season produced qualitatively similar effects on flowering phenologies in both habitats, indicating plant responses to variation in the timing of fires were not habitat specific. 相似文献