Coalescence of magnetic islands in the low-resistivity, Hall-MHD regime.

The coalescence of magnetic islands in the low-resistivity eta, Hall-MHD regime is studied. The interaction between the ion inertial length d(i) and the dynamically evolving current sheet scale length deltaJ is established. Initially, d(i) << deltaJ. If eta is such that deltaJ dynamically thins down to d(i) prior to the well-known sloshing phenomena, then sloshing is avoided. This results in eta independent peak reconnection rates. However, if d(i) is small enough that deltaJ cannot be thinned down to this scale prior to sloshing, then sloshing proceeds as in the resistive MHD model.

Magnetic reconnection in the two-dimensional Kelvin-Helmholtz instability.

Magnetic reconnection in the two-dimensional Kelvin-Helmholtz instability is studied. The flow is modeled by the reduced MHD equations with constant resistivity and viscosity. For super-Alfvénic flow, localized transient reconnection is observed on the Kelvin-Helmholtz time scale (this is not new). We study this transient reconnection and consider the peak reconnection rate which occurs with the initial vortex formation. Over the range of resistivities considered, it is shown that this peak reconnection rate is not a function of resistivity, and is a function of the initial flow shear. Additionally, it is demonstrated that there is a fundamental difference between the evolution of a problem at S = 200 and S = 10,000.

Transient magnetic reconnection and unstable shear layers.

We study three-dimensional magnetic reconnection caused by the Kelvin-Helmholtz (KH) instability and differential rotation in subsonic and sub-Alfvenic flows. The flows, which are modeled by the resistive magnetohydrodynamic equations with constant resistivity, are stable in the direction of the magnetic field but unstable perpendicular to the magnetic field. Localized transient reconnection is observed on the KH time scale, and kinetic energy increases with decreasing resistivity. As in flux-transfer events in the Earth's magnetopause boundary layer, bipolar structures in the normal flux and bidirectional jetting away from reconnection zones are observed.

Mapping of the active site of T7 RNA polymerase with 8-azidoATP.

The photoaffinity analog of ATP, 8-azidoATP, labels T7 RNA polymerase. Photoincorporation exhibits saturation behavior and is protected against by the substrate ATP. 8-AzidoATP is a competitive inhibitor of ATP incorporation with Ki approximately 40 microM. The photolabeled T7 RNA polymerase, following cyanogen bromide digestion, was analyzed by phenylboronate agarose column chromatography followed by reverse-phase high pressure liquid chromatography. Sequencing of the peptides labeled with radioactive photoprobe allowed the identification of three peptides, P314-M362 (I), L550-M666 (II), and F751-M861 (III). These peptides are in the proximity of the photoprobe 8-azidoATP and, therefore, expected to contain functionally significant residues and define an active site domain. These peptides (I and II) contain residues previously implicated in T7 RNA polymerase activity or show homology to active site regions of the Klenow fragment of DNA polymerase I (II and III).

Separation of bile vesicles and micelles by gel filtration chromatography: the importance of the intermicellar bile salt concentration.

Micelles and vesicles coexist in native bile. Mixed micelles are composed of bile salt, phospholipid, and cholesterol. Micellar bile salt is in equilibrium with the aqueous phase bile salt (intermicellar bile salt), and mixed micelles can be converted to cholesterol-phospholipid vesicles by depletion of bile salt. To determine the amount of cholesterol carried in vesicles and micelles, these two populations must be separated without altering the relative proportion of each. Based on the size difference between micelles and vesicles, gel filtration chromatography has been used to accomplish this separation. We reasoned that to maintain the proportion of micelles and vesicles in bile, the column must be equilibrated and eluted with buffer containing the intermicellar bile salt concentration (IMBC) and species. To test this hypothesis we created a model bile composed exclusively of micelles, a solution containing micelles and vesicles, and a model bile containing all vesicles, as demonstrated by quasielastic light scattering. Gel filtration on Sepharose 4B demonstrated that model vesicles and micelles could be separated on a column eluted with buffer containing bile salt at the IMBC. However, a modest decrease in the buffer bile salt concentration (less than 1 mmol/L) resulted in complete conversion of micelles to vesicles. A comparable increase in the buffer bile salt concentration converted vesicles to micelles. Using only taurocholate in the eluting buffer at the IMBC caused a complete shift of micelles to vesicles, whereas using only taurochenodeoxycholate resulted in conversion of vesicles to micelles. An initial collection of rat bile separated on a column equilibrated with the measured IMBC demonstrated that 94% of the cholesterol was in the micellar fractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Cation radius effects on the helix-coil transition of DNA. Cryptates and other large cations.

Most polyelectrolyte theories of the effect of ions on the thermal melting of DNA assume that the predominant influence of the cations comes through their charge. Ion size and structure are treated, for analytic convenience, as negligible variables. We have examined the validity of this assumption by measuring the melting temperature of calf thymus DNA as a function of salt concentration with four univalent cations of different hydrated radii. These are K+ (3.3 A), (n-Pr)4N+ (4.5 A), (EtOH)4N+ (4.5 A), and C222-K+ (5 A). C222-K+ is a complex of cryptand C222 with K+. With K+ as the sole cation, Tm varies linearly with the log of ionic strength over the range 0.001-0.1 M. With all the K+ sequestered by an equimolar amount of C222, Tm is depressed by 10-20 degrees C and the slope of Tm vs. ionic strength is lower. At low ionic strength, an even greater reduction in Tm is achieved with (n-Pr)4N+; but the similar-sized (EtOH)4N+ gives a curve more similar to K+. Theoretical modeling, taking into account cation size through the Poisson-Boltzmann equation for cylindrical polyelectrolytes, predicts that larger cations should be less effective in stabilizing the double helix; but the calculated effect is less than observed experimentally. These results show that valence, cation size, and specific solvation effects are all important in determining the stability of the double-helical form of DNA.

In vitro evidence that eukaryotic ribosomal RNA transcription is regulated by modification of RNA polymerase I.

We have utilized a cell-free transcription system from Acanthamoeba castellanii to test the functional activity of RNA polymerase I and transcription initiation factor I (TIF-I) during developmental down regulation of rRNA transcription. The results strongly suggest that rRNA transcription is regulated by modification, probably covalent, of RNA polymerase I: (1) The level of activity of TIF-I in extracts from transcriptionally active and inactive cells is constant. (2) The number of RNA polymerase I molecules in transcriptionally active and inactive cells is also constant. (3) In contrast, though the specific activity of polymerase I on damaged templates remains constant, both crude and purified polymerase I from inactive cells have lost the ability to participate in faithful initiation of rRNA transcription. (4) Polymerase I purified from transcriptionally active cells has the same subunit architecture as enzyme from inactive cells. However, the latter is heat denatured 5 times faster than the active polymerase.