No abstract

Gal4p activates transcription of the Saccharomyces GAL genes in response to galactose and is phosphorylated during interaction with the RNA polymerase II (Pol II) holoenzyme. One phosphorylation at S699 is necessary for full GAL induction and is mediated by Srb10p/CDK8 of the RNA Pol II holoenzyme mediator subcomplex. Gal4p S699 phosphorylation is necessary for sensitive response to inducer, and its requirement for GAL induction can be abrogated by high concentrations of galactose in strains expressing wild-type GAL2 and GAL3. Gal4p S699 phosphorylation occurs independently of Gal3p and is responsible for the long-term adaptation response observed in gal3 yeast. SRB10 and GAL3 are shown to represent parallel mechanisms for GAL gene induction. These results demonstrate that Gal4p activity is controlled by two independent signals: one that acts through Gal3p-galactose and a second that is mediated by the holoenzyme-associated cyclindependent kinase Srb10p. Since Srb10p is regulated independently of galactose, our results suggest a function for CDK8 in coordinating responses to specific inducers with the environment through the phosphorylation of gene-specific activators.Eukaryotic cells react to their environment by regulating transcription factors bound to promoters of responsive genes (28). Cells grown in culture are typically provided with sufficient essential nutrients and factors to ensure unchecked propagation. However, in their natural environment, cell growth is ordinarily limited by the scarcity of one or more factors or nutrients. In such circumstances, there must be mechanisms to ensure that transcriptional responses to one signal do not surpass what the cell can accommodate with its limited growth potential. This issue has not yet been addressed in eukaryotes. In this report, we demonstrate that the prototypical transactivator Gal4p is regulated by two separate signals represented by the specific inducer galactose and the RNA polymerase II (Pol II) holoenzyme-associated cyclin-dependent kinase Srb10p/ CDK8. These observations suggest a mechanism whereby responses to a specific inducer can be coordinated with the physiological environment.Gal4p regulates expression of the yeast GAL genes in response to galactose. In noninducing conditions, Gal4p is bound to the upstream activating sequences for galactose (UAS G ) but is prevented from activating transcription by the inhibitor Gal80p (32,43). Rapid induction by galactose requires the product of GAL3 (40, 52, 57), which is a regulatory protein with similarity to the galactokinase encoded by GAL1 (3, 9, 50), although Gal3p does not have galactokinase activity (9). Recent experiments demonstrate that Gal3p, when bound to galactose, directly interacts with Gal80p in the presence of ATP (42,50,59,60). Gal3p-galactose is thought to cause induction of the GAL genes by producing a conformational change in the Gal4p-Gal80p complex that allows interaction of the Gal4p activating domains with the general transcription factors (42, 59). The induced conformation may ...

We present the structural and magnetic properties of a new compound family, Mg2RE3Sb3O14 (RE = Gd, Dy, Er), with a hitherto unstudied frustrating lattice, the "tripod kagome" structure. Susceptibility (ac, dc) and specific heat exhibit features that are understood within a simple Luttinger-Tisza type theory. For RE = Gd, we found long ranged order (LRO) at 1.65 K, which is consistent with a 120• structure, demonstrating the importance of diople interactions for this 2D Heisenberg system. For RE = Dy, LRO at 0.37 K is related to the "kagome spin ice (KSI)" physics for a 2D system. This result shows that the tripod kagome structure accelerates the transition to LRO predicted for the related pyrochlore systems. For RE = Er, two transitions, at 80 mK and 2.1 K are observed, suggesting the importance of quantum fluctuations for this putative XY system. Introduction.-The two-dimensional (2D) kagome lattice magnet (KLM) has been a favorite in the theoretical condensed matter community since the experimental work on SCGO [1], due to the strong frustration associated with its network of corner-shared triangles. [15]. From a materials standpoint, however, these two systems are limited by (i) known defect prone structures [14,16], and (ii) the inability to substitute facilely on the magnetic site (e.g with non-Heisenberg spins) to realize states other than the QSL. Clearly then, finding new KLM-containing compounds with spin-type variability is a challenge of the highest order.Intriguingly, a 2D KLM is naturally contained in the frustrated 3D pyrochlore structure. In pyrochlores RE 2 X 2 O 7 (RE = rare earth element, X = transition metal element), both the RE 3+ and X 4+ sublattices form alternating kagome and triangular layers along the [111] axis as a result of corner-shared tetrahedrons ( Fig. 1(a)) [17]. However, the strong inter-layer interaction enforces three-dimensionality. An exception is found in studies of Dy 2 Ti 2 O 7 in a [111] magnetic field, which polarizes the triangular layer spins, effectively decoupling the kagome planes, leading to a KSI state [17].Obviously, if one can remove the magnetic moment of the triangular layers in the pyrochlore lattice, a RE-kagome-

We present a systematic study of the structural and magnetic properties of two branches of the rare earth Tripod Kagome Lattice (TKL) family A2RE3Sb3O14 (A = Mg, Zn; RE = Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb; here, we use abbreviation A-RE, as in MgPr for Mg2Pr3Sb3O14), which complements our previously reported work on MgDy, MgGd, and MgEr 15 . The present susceptibility (χ dc , χac) and specific heat measurements reveal various magnetic ground states, including the non-magnetic singlet state for MgPr, ZnPr; long range orderings (LROs) for MgGd, ZnGd, MgNd, ZnNd, and MgYb; a long range magnetic charge ordered state for MgDy, ZnDy, and potentially for MgHo; possible spin glass states for ZnEr, ZnHo; the absence of spin ordering down to 80 mK for MgEr, MgTb, ZnTb, and ZnYb compounds. The ground states observed here bear both similarities as well as striking differences from the states found in the parent pyrochlore systems. In particular, while the TKLs display a greater tendency towards LRO, the lack of LRO in MgHo, MgTb and ZnTb can be viewed from the standpoint of a balance among spin-spin interactions, anisotropies and non-Kramers nature of single ion state. While substituting Zn for Mg changes the chemical pressure, and subtly modifies the interaction energies for compounds with larger RE ions, this substitution introduces structural disorder and modifies the ground states for compounds with smaller RE ions (Ho, Er, Yb).

The heavy fermion compound URu_{2}Si_{2} continues to attract great interest due to the unidentified hidden order it develops below 17.5 K. The unique Ising character of the spin fluctuations and low-temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a cos^{4}θ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter.

An important challenge in magnetism is the unambiguous identification of a quantum spin liquid 1,2 , of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems 3,4 wherein classical order is suppressed by a frustrating lattice 5 , an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point-a region of highly fluctuating spins-by an applied magnetic field. Such systems suffer from a weak specific-heat peak at the quantum critical point, with little entropy available for quantum fluctuations 6 . Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K 2 PbCu(NO 2 ) 6 . Across the temperature-magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point.Previous work on field-tuning of quantum fluctuations has focused on the transverse-field Ising model where the magnetic field (H) couples to a sector of the Hamiltonian not directly modifying the order parameter. While this paradigm has been explored in the three-dimensional (3D) dipolar ferromagnet LiHoF 4 (refs 7,8 ), and the 1D system CoNb 2 O 6 (ref. ), the need for a unique Ising-axis normal restricts the number of potential quantum-spin-liquid host materials. For the much broader class of 3D antiferromagnets (AFs), H can indeed tune the Néel temperature (T N ) into the quantum regime. Within the ordered state, however, on increasing H from zero one first encounters a spin-flop transition (for finite spin anisotropy), followed by a gradual reorientation of those spins (Fig. 1a). Hence, in destroying Néel order, H decreases the transverse components of the staggered moment to a value that is vanishingly small near the transition to the field-polarized state, leaving little entropy to be lost near T = 0, and thus low spectral weight available for quantum entanglement 6 . Here we demonstrate a different approach to tuning through a quantum critical point (QCP). The quasi-1D spin-1/2 AF K 2 PbCu(NO 2 ) 6 orders classically at 0.68 K (ref. 10; Fig. 1b). At lower temperatures within the Néel state, applied H values less than the intra-chain mean field cause little change in the specific heat C(T, H). At the phase boundary, however, a large amount of entropy is released, leading to a peak in C/T (Fig. 1c), the value of which (~ 2 J mol -1 K -2 at the lowest temperatures studied) is suggestive of the spinon ...

The 1/8 fractional plateau phase (1/8-FPP) in Shastry-Sutherland Lattice (SSL) spin systems has been viewed an exemplar of emergence on an Archimedean lattice. Here we explore this phase in the Ising magnet TmB 4 using high-resolution specific heat (C) and magnetization (M) in the field-temperature plane. We show that the 1/8-FPP is smoothly connected to the antiferromagnetic (AF) phase on ramping the field from H = 0. Thus, the 1/8-FPP is not a distinct ground state of TmB 4 . The implication of these results for Heisenberg spins on the SSL is discussed.

Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature ∝ in a single particle model. Here we show that, in closedenvironment-grown polycrystalline samples of Y2Ir2O7, = over four orders of magnitude in . While the measured prefactor, , is obtained from the model using reasonable materials parameters, the behavior extends far beyond the model's range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, « 2 . Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in Y2Ir2O7, are the possible origin of such "bad" Weyl semimetal behavior.