The eukaryotic leading and lagging strand DNA polymerases are loaded onto primer-ends via separate mechanisms but have comparable processivity in the presence of PCNA

Saccharomyces cerevisiae DNA polymerase δ (Pol δ) and DNA polymerase ε (Pol ε) are replicative DNA polymerases at the replication fork. Both enzymes are stimulated by PCNA, although to different levels. To understand why and to explore the interaction with PCNA, we compared Pol δ and Pol ε in physical interactions with PCNA and nucleic acids (with or without RPA), and in functional assays measuring activity and processivity. Using surface plasmon resonance technique, we show that Pol ε has a high affinity for DNA, but a low affinity for PCNA. In contrast, Pol δ has a low affinity for DNA and a high affinity for PCNA. The true processivity of Pol δ and Pol ε was measured for the first time in the presence of RPA, PCNA and RFC on single-stranded DNA. Remarkably, in the presence of PCNA, the processivity of Pol δ and Pol ε on RPA-coated DNA is comparable. Finally, more PCNA molecules were found on the template after it was replicated by Pol ε when compared to Pol δ. We conclude that Pol ε and Pol δ exhibit comparable processivity, but are loaded on the primer-end via different mechanisms

High-power ECH and fully non-inductive operation with ECCD in the TCV tokamak

Experiments with high-power electron cyclotron heating (ECH) and current drive (ECCD) in the TCV tokamak are discussed. Power up to 2.7 MW from six gyrotrons is delivered to the tokamak at the second-harmonic frequency (82.7 GHz) in X-mode. The power is transmitted to the plasma by six independent launchers, each equipped with steerable mirrors that allow a wide variety of injection angles in both the poloidal and toroidal directions. Fully non-inductive operation of the tokamak has been achieved in steady state, for the full 2 s gyrotron pulse duration, by co-ECCD with a highest current to date of 210 kA at full power. The experimentally measured ECCD efficiency agrees well with predictions obtained from linear modelling. We have observed that the highest global efficiency attainable at a given power is limited by stability constraints. While the efficiency is maximum bn the magnetic axis, a disruptive MHD instability occurs when the width of the deposition profile is lower than a minimum value, which increases with total power. Many ECCD discharges display a high level of electron energy confinement, enhanced by up to a factor of two over the Rebut-Lallia-Watkins (RLW) scaling law, which by contrast is well satisfied in ohmic conditions. The longest confinement times (up to four times RLW) are observed with central counter-ECCD. Central electron heat diffusivities comparable to ohmic levels are obtained in these scenarios, with electron temperatures in excess of 10 keV

Recent results from the electron cyclotron heated plasmas in Tokamak à Configuration Variable (TCV)

In noninductively driven discharges, 0.9 MW second harmonic (X2) off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier (eITB) in steady plasma conditions resulting in a 1.6-fold increase of the confinement time (tau(Ee)) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction (up to 80%) and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q(0)/q(min)) and the reversed shear volume (rho(q-min)(2)) is shown. In other discharges heated with X2 the sawteeth are destabilized (respectively stabilized) when heating just inside (respectively outside) the q=1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the newly completed third harmonic (X3) system and achieving up to 100% absorption are also discussed, along with comparison of experimental results with the TORAY-GA ray tracing code [K. Matsuda, IEEE Trans. Plasma Sci. PS-17, 6 (1989); R. H. Cohen, Phys. Fluids 30, 2442 (1987)]. (C) 2003 American Institute of Physics