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11.
New function of vitamin B12: cobamide-dependent reduction of epoxyqueuosine to queuosine in tRNAs of Escherichia coli and Salmonella typhimurium. 总被引:16,自引:12,他引:4 下载免费PDF全文
Queuosine (Q), 7-[(4,5-cis-dihydroxy-2-cyclopentene-1-yl)-amino)methyl)-7- deazaguanosine, and Q derivatives usually replace guanosine in the anticodon of tRNAs(GUN) of eubacteria and of cytoplasmic and mitochondrial tRNAs of lower and higher eucaryotes except yeasts. Q appears to be synthesized de novo exclusively in eubacteria, and the free-base queuine serves as a nutrient factor for eucaryotes. Recently, a Q derivative, oQ, containing a 2,3-epoxy-4,5-dihydroxycyclopentane ring, has been identified in Escherichia coli tRNA(Tyr). Here we show that oQ is formed when E. coli or Salmonella typhimurium is grown in glucose-salt medium. The formation of oQ was independent of molecular oxygen, and oQ-tRNAs were converted to Q-tRNAs by adding cobalamin to the growth medium. Under strictly anaerobic conditions, considerable amounts of Q were present in E. coli and S. typhimurium tRNAs when the bacteria were grown in the presence of cobalt ions with glycerol as the carbon source and fumarate as the electron acceptor. Under these conditions, the biosynthesis of cobalamin was induced. The results suggest that oQ is derived from ribose and that oQ is finally reduced to Q by a cobamide-dependent enzyme. 相似文献
12.
13.
Distances of tyrosine residues from a spin-label hapten in the combining site of a specific monoclonal antibody 总被引:4,自引:0,他引:4
The nuclear magnetic resonance spectra of an Fab fragment of a monoclonal antibody specifically directed against a nitroxide spin-label hapten have been recorded at different concentrations of the hapten. The hybridoma producing this antibody was grown on deuterated phenylalanine, tryptophan, and 3,5-dideuteriotyrosine or 2,6-dideuteriotyrosine. Difference spectra--without hapten minus with hapten--were calculated for each concentration of hapten. The difference spectra reveal five well-resolved singlet proton resonance signals from tyrosine deuterated in the 3,5-positions (H 2,6 Tyr) and nine from tyrosine deuterated in the 2,6-positions (H 3,5 Tyr). The measured intensities of these signals as a function of combining site occupation have been interpreted in terms of a theory involving intrinsic line widths (T2), the hapten off-rate (k), and distances to the paramagnetic center. Good agreement with theory is found for all of the isolated proton signals. The best estimate of k is 350 s-1; distances in the range 13 to less than 9 A are calculated. Extension of this analysis to other amino acids is discussed. 相似文献
14.
Overinitiation of chromosome and plasmid replication in a dna Acos mutant of Escherichia coli K12. Evidence for dnaA-dnaB interactions 总被引:4,自引:0,他引:4
The dnaAcos mutations are phenotypic suppressors of dnaAts46 that are co-transduced with dnaA, render the cell cold sensitive, and cause an excess of chromosome replication relative to cell mass when the cells are shifted from 42 degrees C to 32 degrees C. We have used pulse labelling and DNA-DNA hybridization to follow the effect of a temperature shift on the replication of the chromosome and of the plasmids pSC101, RTF-Tc, and lambda dv in such strains. After a shift of a dnaAcos strain from 42 degrees C to 32 degrees C (non-permissive temperature), initiation of the chromosome and replication of the plasmid pSC101 are stimulated, while the dnaA-independent plasmid RTF-Tc is not affected. The presence of pSC101 does not affect the level of overinitiation of the chromosome. The presence of lambda dv suppresses the cold sensitivity of dnaAcos mutants and allows the cells to grow at both 32 degrees C and 42 degrees C. The presence of lambda dv suppresses the overinitiation of chromosome and of pSC101 replication at 32 degrees C. Previous reports had shown that these suppressions involve an interaction between the dnaA product and the lambda P protein, which is also known to interact with dnaB. We show here that the mutant prophage P1 bac-crr, which produces high levels of a dnaB analogue, suppresses the dnaAcos phenotype, while wild type P1 does not. These results suggest that initiation involves interactions between the dnaA and dnaB products. 相似文献
15.
Insertion mutations in the dam gene of Escherichia coli K-12 总被引:40,自引:0,他引:40
M G Marinus M Carraway A Z Frey L Brown J A Arraj 《Molecular & general genetics : MGG》1983,192(1-2):288-289
The dam gene of E. coli can be inactivated by insertion of Tn9 or Mud phage. Strains bearing these mutations are viable indicating that the dam gene product is dispensable. 相似文献
16.
Isolation and renaturation of alpha 2-macroglobulin receptor from diploid human fibroblasts. 总被引:1,自引:0,他引:1 下载免费PDF全文
alpha 2-Macroglobulin receptor was extracted from human diploid fibroblasts and purified by affinity chromatography in a single step. The receptor had mol.wt. 125 000 after sodium dodecyl sulphate (SDS)/polyacrylamide-gel electrophoresis. The isolated receptor was separated by SDS/polyacrylamide-gel electrophoresis, transferred on to nitrocellulose sheets and subsequently renatured, as shown by a specific binding test, by incubation with Nonidet P40. 相似文献
17.
The polarization of fluorescence of a chromophore chemically bound on the NH2 end of a poly(L -benzyl glutamate) molecule has been studied as a function of temperature, viscosity, and solvent. The relaxation times depend on both the overall rotation of the helix and the local rotation of the endgroup. In m–cresol the endgroup is rigidly bound and the rotational diffusion constant of the molecule is in good agreement with the values obtained by Kerr effect and dielectric relaxation. In other helicogenic solvents (DMF, DCE, etc.) the local rotation is nearly free. In m-cresol-DMF mixtures a sharp decrease of the polarization around a composition of 40% DMF can be interpreted as a change in the freedom of rotation of the endgroup. No discontinuity in the optical rotation is observed in the solvent mixture. The question of how a rapidly rotating endgroup could show an extrinsic Cotton effect as observed by Bloutand Yamaoka for the system Acridine Orange–PBLG in chloroform is then raised. Polarization of fluorescence measurements on this system show a nearly complete freedom of rotation of the dye and OH D measurements show no detectable Cotton effect in the dye absorption band. 相似文献
18.
S. Liechti-Gallati M. Koenig L. M. Kunkel D. Frey E. Boltshauser V. Schneider S. Braga H. Moser 《Human genetics》1989,82(4):343-348
Summary DNA from 80 Duchenne (DMD) and 15 Becker (BMD) index patients was analyzed with 12 genomic probes and the total cDNA. Deletions
were detected in 24 DMD (30%) and 10 BMD patients (67%) by genomic probes alone, mostly p20, pXJ, and/or pERT87. All deletions
were confirmed by cDNA probes, and an additional 29 DMD deletions were detected, resulting in a total of 63/95 deletions (66%).
The majority of the deletions are localized between kb 6.7 and 9.7 of the cDNA; a smaller group, between kb 0.5 and 3.5. Of
the deletions, 90% are detected by the three cDNA probes 1–2a, 7, and 8. This can be applied to strategies for carrier detection
and prenatal diagnosis. The order of 13 exon-containing HindIII fragments in the region between probes 7 and 9–10, where most of the deletions are found, could be defined. The deletion
patterns in DMD and BMD patients are different and well in accordance with the “reading frame theory” of Monaco and coworkers.
Thus our findings indicate that a DMD or BMD phenotype may be predicted according to the breakpoint position and the number
of deleted exons. 相似文献
19.
Excision of the En/Spm transposable element of Zea mays requires two element-encoded proteins. 总被引:9,自引:1,他引:8 下载免费PDF全文
An excision assay system for En/Spm was developed in transgenic tobacco. The characteristics of excision and integration are similar to the natural system of Zea mays. In this transgenic model system two En/Spm encoded trans-acting functions, TNPA and TNPD, are required for excision. A biochemical model for transposition is proposed that might also be applicable to other transposable elements. 相似文献
20.
To elucidate the role of interleukin 2 (IL-2) activation in CD3- lymphocytes, we examined the ability of monoclonal antibody (MAb) TU27, developed against the IL-2 receptor (IL-2R) p75 protein (IL-2R beta), to block lymphocyte activation with exogenous IL-2, as well as its innate ability to activate lymphocytes as a result of its surface ligand interaction. The binding of the TU27 MAb and the results of 125I-IL-2 cross-linking experiments suggest that the IL-2R beta chain is expressed primarily on CD3-, CD56+ lymphocytes; although the protein was also detected in a small portion of CD3+ cells, its expression appeared to be donor dependent. In the present study, we found that TU27 totally blocked natural killer (NK) cell activation in a 4-h assay but had no effect on basal levels of NK activity. When treatment was extended to 24 to 72 h, the MAb was able to block the induction of both NK and lymphokine-activated killer (LAK) activity. Of interest was the observation that MAb treatment alone augmented NK activity and subsequent interferon gamma (IFN gamma) production in CD3- lymphocytes but did not activate LAK activity or induce cell growth. Collectively, these results indicate that TU27 not only reacts with p70-75 IL-2R beta but can abrogate IL-2 binding and subsequent activation events. In addition, some CD3- lymphocyte functions (e.g., NK activity and IFN gamma secretion) are directly induced by the binding of MAb to p70-75 through signals that only partially mimic IL-2. 相似文献