Primary and secondary structure of U2 snRNA

With the improved rapid sequencing techniques, the earlier sequence of U2 RNA of Novikoff hepatoma (Shibata et al, J. Biol. Chem. 250, 3909-3920, 1975) was reanalyzed and modified. The improved sequence of U2 RNA is 188 (or 189) nucleotides long and is in register with a characterized U2 RNA pseudogene (Denison et al, PNAS 78, 810-814, 1981) except for an 11 nucleotide sequence (nucleotides 147-157) which is absent from the pseudogene. From these results, a secondary structure of U2 RNA is proposed which is supported by the preferred cleavage sites with T1-RNase, RNase A and S1 nuclease. Isolated U2 RNA was cleaved by T1-RNase preferentially at positions 64 and 164, whereas U2 RNA in U2-snRNP was cleaved only at position 64, indicating that position 164 is protected in U2-snRNP. As with U1 RNA (Epstein et al, PNAS 78, 1562-1566, 1981) the 5'-end of isolated U2 RNA was not preferentially cleaved by T1-RNase.     

H-2 antigens as genetic markers: A two-dimensional gel electrophoretic study

Direct immunoprecipitation and two-dimensional (2D) gel electrophoresis have been used to identify and characterize genetic variation of theH-2K andH-2D regions. Using inbred strains of mice and alloantisera, haplotype-specific polypeptides were defined for five differentH-2 haplotypes. Specific immunoprecipitates prepared from strains of different haplotypes were applied to 2D gels in pairwise combinations to determine whether peptides specific to one haplotype can be distinguished from peptides specific to another. Those haplotype-specific peptides that migrate to unique positions on 2D gels with respect to the positions occupied by haplotype-specific peptides of another haplotype are useful as biochemical genetic markers. Cross-reactivity amongK- andD-region antigens of different haplotypes was identified on 2D gels and found to correlate well with existing data based on serological cross-reactivity. An anti-mouse 2-microglobulin serum was found to be a useful general reagent for immunoprecipitating haplotype-specific H-2 antigens to permit their visualization on 2D gels.Abbrevations used in this paper NP-40 nonidet P-40 - 2D two-dimensional - SDS sodium dodecyl sulfate - IEF isoelectric focusing     

A K+ transport ATPase in Escherichia coli.

A K+ -stimulated ATPase in membranes of Escherichia coli has been identified as an activity of the Kdp system, and ATP-driven K+ transport system. Three characteristics support association of the ATPase with the Kdp system: (i) ATPase and Kdp transport are both repressed by growth in media containing high concentrations of K+; (ii) the ATPase and Kdp system accept only K+ as substrate, neither requires Na+ nor accepts Rb+ as a substrate; (iii) the affinity of the ATPase and that of th Kdp system for K+ is similar and is altered by mutations in the structural genes of the Kdp system. Discovery of an ATPase associated with a bacterial transport system suggests functional similarities with the ATP-driven transport systems of animal cells.     

An Inborn Error of Potassium Metabolism in the Tomato, Lycopersicon esculentum

A mutant of the tomato (Lycopersicon esculentum Mill.) was obtained through treatment of the seed with ethyl methane sulfonate. Its chief distinguishing characteristic is the requirement for an extraordinarily high K concentration in the medium (20 mm) if it is to develop without pronounced K deficiency symptoms, while the wild type grows without any such symptoms at 0.1 to 0.2 mm K. The lesion of the K-inefficient mutant does not lie in its inability to absorb and translocate K.     

Catalytic and kinetic properties of purified high-affinity cyclic AMP phosphodiesterase from dog kidney

High-affinity cyclic AMP phosphodiesterase purified to homogeneity from dog kidney was studied with respect to its stability, its catalytic and kinetic properties, and its sensitivity to pharmacological agents. The enzyme was shown to rapidly lose activity upon dilution to low protein concentrations in aqueous media, but this activity loss was largely prevented by the presence of bovine serum albumin or ethylene glycol. Similarly, maximum activity required bovine serum albumin to be present during incubation for activity analysis. Enzyme activity required a divalent cation; Mg²⁺, Mn²⁺, and Co²⁺ each supported activity, but highest activity was obtained with Mg². The temperature optimum ranged from 30 to 45 °C and depended on substrate concentration; the E_a = 10,600 cal/mol. The pH optimum of the enzyme was broad, with a maximum from pH 8.0 to 9.5. The enzyme exhibits linear Michaelis-Menton kinetics for hydrolysis of cyclic AMP at all substrate concentrations tested and for hydrolysis of cyclic GMP at > 20 μm. The K_m for cyclic AMP hydrolysis was 2 μm, and that for cyclic GMP hydrolysis was 312 μm. The K_i values for the competitive inhibition of hydrolysis of each substrate by the other were similar to their K_m values suggesting a single active site. Cyclic AMP hydrolysis was weakly inhibited by cyclic GMP, cyclic IMP, adenine, and adenosine, but was not inhibited by the mono-, di, or trinucleotides of adenosine, guanosine, or inosine. Activity was competitively inhibited with K_i values in the micromolar range by drugs representative of methylxanthines, isoquinolines, pyrazolopyridines, imidazolidinones, triazolopyrimidines, pyridylethylenediamines, phenothiazines, and calcium antagonists. The results are discussed with reference to the similarities and differences between high- and low-affinity phosphodiesterase forms.     

Genetic analysis of potassium transport loci in Escherichia coli: evidence for three constitutive systems mediating uptake potassium.

The analysis of mutants of Escherichia coli that require elevated concentrations of K+ for growth has revealed two new genes, trkG, near minute 30 within the cryptic rac prophage, and trkH, near minute 87, the products of which affect constitutive K+ transport. The analysis of these and other trk mutations suggests that high rates of transport, previously considered to represent the activity of a single system, named TrkA, appear to be the sum of two systems, here named TrkG and TrkH. Each of these two is absolutely dependent on the product of the trkA gene, a cytoplasmic protein associated with the inner membrane (D. Bossemeyer, A. Borchard, D. C. Dosch, G. C. Helmer, W. Epstein, I. R. Booth, and E. P. Bakker, J. Biol. Chem. 264:16403-16410, 1989). The TrkH system is also dependent on the products of the trkH and trkE genes, while the TrkG system is also dependent on the product of the trkG gene and partially dependent on the product of the trkE gene. It is suggested that the trkH and trkG products are membrane proteins that form the transmembrane path for the K+ movement of the respective systems. Two mutations altering the trkA product reduce the affinity for K+ of both TrkG and TrkH, indicating that changes in peripheral protein can alter the conformation of the sites at which K+ is bound prior to transport. The TrkD system has a relatively modest rate of transport, is dependent solely on the product of the trkD gene, and is the sole saturable system for Cs+ uptake in this species (D. Bossemeyer, A. Schl?sser, and E. P. Bakker, J. Bacteriol. 171:2219-2221, 1989).     

Ouabain-resistant transfectants of the murine ouabain resistance gene contain mutations in the alpha-subunit of the Na,K-ATPase.

A 6.5-kilobase murine genomic DNA fragment isolated by Levenson et al. (Levenson, R., Racaniello, V., Albritton, L., and Housman, D. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 1489-1493) (called the ouabain resistance gene) has been shown to produce ouabain resistance in primate cells. Preliminary sequence information has revealed no homology with the coding sequence of the Na,K-ATPase. We have introduced this murine sequence into monkey and murine cells in an attempt to characterize its mechanism of action. In our experiments, transfection of this DNA fragment is associated with the low frequency (1 in 8 x 10(5) cells) appearance of ouabain-resistant clones of CV1, COS, and NIH 3T3 cells, an event not seen in control transfections. Characterization of a new clone of ouabain-resistant CV1 cells (called OR8 cells) revealed a 5-fold increase in the IC50 for ouabain inhibition of rubidium uptake and a 10-fold increase in cell survival on ouabain. Although the murine sequence was detectable in Southern blots of ouabain-resistant cells soon after transfection, this exogenous DNA was rapidly lost despite continued exposure to ouabain. Furthermore, we were unable to detect message expression by this genomic sequence in any of the three cell types tested. Instead, we found that all three ouabain-resistant cell lines exhibited point mutations in a domain of the alpha-subunit that has been implicated in ouabain sensitivity (H1-H2). One of these mutations (Asp121-Asn121 in OR8 cells) has been previously reported to cause ouabain resistance (Price, E.M., Rice, D.A., and Lingrel, J.B. (1989) J. Biol. Chem. 264, 21902-21906). Other novel mutations in the H2 transmembrane domain were also detected. We postulate that the "ouabain resistance gene" is important in the early selection process on ouabain but that the permanent ouabain-resistant phenotype is due to a stable mutation in one allele of the alpha-subunit of the Na,K-ATPase.     

Glucose uptake by adipocytes of obese rats: effect of one bout of acute exercise.

The effect of one bout of acute exercise on impaired glucose metabolism was studied in obese (480 +/- 20 g), untrained rats, at rest (n = 10) and after 60 min of swimming (n = 5). Using the euglycemic, hyperinsulinemic (10 mU.kg-1 x min-1) clamp, glucose clearance rate increased from 7.6 +/- 0.9 at rest to 9.7 +/- 0.5 mL.kg-1 x min-1 after exercise (p < 0.05). Glucose (3-O-[14C]methylglucose) transport (GT) into epididymal adipocytes were incubated with or without insulin. In the absence of insulin, GT was 0.13 +/- 0.02 and 0.26 +/- 0.07 fmol.cell-1 x min-1 at rest and after exercise, respectively. In the presence of insulin (25-1000 microU.mL-1) GT increased at rest from 0.97 +/- 0.08 to 1.13 +/- 0.07 fmol.cell-1 x min-1, and after exercise from 1.35 +/- 0.05 to 1.87 +/- 0.11 fmol.cell-1 x min-1. GT was significantly higher after exercise compared with rest (p < 0.004). At rest, maximal insulin effect was achieved at 100 microU.mL-1, whereas with exercise, GT increased gradually with the insulin dosage. The following may be concluded: (i) the biological effect of insulin is amplified in obese rats by one bout of exercise and (ii) exercise affects GT into enlarged adipocytes by enhancing tissue responsiveness to insulin and by a cellular mechanism unrelated to the insulin action.