Urea transport in Saccharomyces cerevisiae.

Urea transport in Saccharomyces cerevisiae occurs by two pathways. The first mode of uptake is via an active transport system which: (i) has an apparent Km value of 14 muM, (ii) is absolutely dependent upon energy metabolism, (iii) requires pre-growth of the cultures in the presence of oxaluric acid, gratuitous inducer of the allantoin degradative enzymes, and (iv) is sensitive to nitrogen repression. The second mode of uptake which occurs at external urea concentrations in excess of 0.5 mM is via either passive or facilitated diffusion.     

Urea transport-defective strains of Saccharomyces cerevisiae.

Experiments characterizing the urea active transport system in Saccharomyces cerevisiae indicate that (i) formamide and acetamide are strong competitive inhibitors of urea accumulation, (ii) uptake is maximal at pH 3.3 and is 80% inhibited at pH 6.0, and (iii) adenosine 5'-triphosphate generated by glycolysis in conjunction with formation of an ion gradient is likely the driving force behind urea transport. Mutant strains were isolated that are unable to accumulate urea at external concentrations of 0.25 mM. These strains also exhibit a depressed growth rate on 10 mM urea, indicating existence of a relationship between the active transport and facilitated diffusion modes of urea uptake.     

Urea denaturation of chromatin periodic structure.

Isolated chicken erythrocyte nuclei dispersed in urea solutions (0-5.0 M) have been examined in terms of their low-angle X-ray diffraction and electron microscopic properties. At high urea concentrations, the characteristic low-angle X-ray reflections of chromatin are absent, and the spheroid chromatin particles (v bodies) are markedly perturbed. This lability of chromatin periodic structure to high concentrations of urea is consistent with previous hydrodynamic and spectroscopic studies.     

尿素动力学模型及尿素反弹的实验研究

综述了尿素动力学模型的研究进展,并分析了几种模型的优缺点。建立了串联双室模型实验系统,研究了在透析间期尿素反弹的情况。该系统通过控制泵流量（F）来模拟两室室间溶质清除率（KIE）;用KCl代替尿素,通过实验曲线拟合得到KIE、R^2;通过与临床数据比较,获得了适合于尿素反弹系统的KIE,并研究了非灌注室浓度（A）对KIE的影响。结果显示,F=580ml／min较好地匹配了患者透析的尿素动力学特征;基于F=580ml／min进行的A对KIE影响的实验中,A与KIE没有必然联系,KIE平均值为512ml／min,标准差为10．25。且室间溶质浓度达到平衡所需的时间均为35-5min。尿素反弹的串联双室模型实验系统能够对患者透析后的尿素动力学进行较好的模拟,并具有较强的稳定性。     

Urea reduces the thermal stability of polyanion-treated chromatin.

The polyanions heparin and polyglutamic acid result in a thermal destabilization of the 78,000g fraction of rat liver chromatin. Urea potentiates this destabilization, especially in combination with polyglutamic acid.     

Denaturing Urea Polyacrylamide Gel Electrophoresis (Urea PAGE)

Urea PAGE or denaturing urea polyacrylamide gel electrophoresis employs 6-8 M urea, which denatures secondary DNA or RNA structures and is used for their separation in a polyacrylamide gel matrix based on the molecular weight. Fragments between 2 to 500 bases, with length differences as small as a single nucleotide, can be separated using this method¹. The migration of the sample is dependent on the chosen acrylamide concentration. A higher percentage of polyacrylamide resolves lower molecular weight fragments. The combination of urea and temperatures of 45-55 °C during the gel run allows for the separation of unstructured DNA or RNA molecules.In general this method is required to analyze or purify single stranded DNA or RNA fragments, such as synthesized or labeled oligonucleotides or products from enzymatic cleavage reactions.In this video article we show how to prepare and run the denaturing urea polyacrylamide gels. Technical tips are included, in addition to the original protocol ^1,2.     

Urea, a specific inhibitor of catabolite sensitive operons.

Low concentrations of urea inhibit specifically the expression of operons sensitive to catabolite repression. This inhibition is not relieved by cyclic AMP. It is suggested that the promoter of catabolite sensitive operons may be the target of the urea effect.     

Urea amidolyase of Candida utilis. Characterization of the urea cleavage reactions.

Evidence is presented that the enzymes catalyzing the three reactions involved in urea cleavage in Candida utilis, biotin carboxylation, urea carboxylation, and allophanate hydrolysis occur as a complex of enzymes. The allophanate-hydrolyzing activity could not be separated from the urea-cleaving activity using common methods of protein purification. Further, urea cleavage and allophanate hydrolysis activities are induced coordinately in cells grown on various nitrogen sources. The reactions involved in urea cleavage can be distinguished from one another on the basis of their sensitivities to (a) heat, (b) pH, and (c) chemical inhibitors. Evidence is presented for the product of the first reaction in urea cleavage, biotin carboxylation. Production of carboxylated enzyme is ATP dependent and avidin sensitive. Carboxylated enzyme is not observed in the presence of 1 mM urea.     

Urea and Biuret as Nitrogen Sources for Rhizobium Spp.

Ability to utilize urea as nitrogen source proved to be universal in 80 fast and 40 slow growing strains of Rhizobium spp. from 23 genera of host plants, and also in 10 strains of Agrobacterium radiobacter. Rhizobium meliloti (32 strains) as well as A. radiobacter constantly failed to utilize biuret. Most rhizobia from other host plants (with 8 exceptions among 88) were able to use biuret as a somewhat suboptimal source of nitrogen, which was generally assimilated at a slower rate than urea and rarely resulted in the same amount of growth, particularly in the fast growing strains; in exceptional cases and mostly among the slow growing strains biuret appeared slightly superior to urea. Adaptation experiments showed that urease occurred as a constitutive enzyme in Rh. leguminosarum , while the biuret decomposing enzyme appeared to be inducible.