This Week in Science

Science  14 Mar 1997:
Vol. 275, Issue 5306, pp. 1541
  1. Off-site transcriptional activation

    Activation of transcription requires proteins that bind DNA near the promoter sequence and recruit RNA polymerase (RNAP) and others that function at distant sites. Studies of bacterial transcription have begun to reveal the importance of protein-protein interactions. Wyman et al. (p.1658) show that the transcriptional activator NtrC (nitrogen regulatory protein C) forms large oligomers at a distant DNA enhancer site. The non-DNA-binding components of these NtrC oligomers are essential for activation, possibly because they interact with RNAP at the promoter site. Miller et al. (p.1655 ) show that bacteriophage N4SSB (single-stranded DNA binding protein) activates transcription not through its DNA binding domain but rather one that binds to RNAP b'. In a Perspective, Geiduschek (p. 1614) speculates that NtrC and N4SSB may both reconfigure the RNAP complex into a transcriptionally competent form.

  2. Phonon squeezing

    Heisenberg's Uncertainty Principle states that we cannot know simultaneously the values of conjugate variables (such as position and momentum) to arbitrary accuracy; the product of their errors must always exceed a certain value. However, experiments can be designed that reduce the error in one variable below the quantum limit—“squeezed states” of photons have been produced for some time. Garrett et al. (p.1638 ; see the news story by Cleary, p. 1566) have created a nearly periodic squeezed state of acoustic-mode phonons (lattice vibrations) in crystals of KTaO3. Femtosecond laser pulses induced counterpropagating phonons that select for a high density of phonon states. The variance in atomic displacements as the atoms vibrate dips below the quantum limit for half of each cycle.

  3. Borehole baseline

    One problem in interpreting currently observed global warming is that earlier temperature records for comparison are scarce. Harris and Chapman (p. 1618) show that temperature estimates from boreholes can provide a baseline that is also tied to the current observational record. Data from a borehole and meteorological network in Utah show that temperatures in the 19th century there were about 0.6oC below recent mean temperatures.

  4. Sulfur in the core

    Determination of the melting temperature of Earth's iron outer core requires a knowledge of how other elements such as sulfur can form stable phases with iron at high temperatures and pressures. Fei et al. (p. 1621) conducted multianvil experiments and found an additional phase, Fe3S2, near 14 gigapascals. The presence of Fe3S2 suggests that the temperature of the core may be overestimated by extrapolation from a simple Fe-FeS binary eutectic.

  5. Cooler carbonates

    The temperature of formation of carbonate minerals precipitated on fracture surfaces in the martian meteorite ALH84001 has been controversial and is key for understanding the early evolution of Mars as well as whether the meteorite might contain traces of early life. Two reports provide evidence that the carbonates formed at low temperatures. Kirschvink et al. (p. 1629) show that the magnetic directions of pyroxene grains across a fracture are rotated. Magnetic directions should have been reset if the rock was heated above a few hundred degrees Celsius. The data also imply that early Mars had a substantial magnetic field. Valley et al. (p. 1633) show that the oxygen isotope compositions of the carbonate minerals are high and variable, suggesting that the minerals precipitated at low temperatures and in disequilibrium.

  6. Under continents

    The upper mantle beneath continents differs from that beneath ocean basins for reasons that have not been clear. Thybo and Perchuc (p. 1626) examined long-range seismic reflections profiles from several continents where the source and receiver were separated by at least 700 kilometers (8o). The sections imply that the mantle beneath the continents may be a region of partial melting; it is stratified to at least 100 kilometers above a zone of low seismic velocity.

  7. Reactive oxygen and cell proliferation

    Reactive oxygen species (ROS) are produced by some cancer cells, and antioxidants appear to protect against some forms of cancer. However, the role of ROS in controlling cell proliferation is largely unknown. Irani et al. (p. 1649; see the news story by Pennisi, p. 1567) found that cells transformed by a constitutively active form of the Ras guanosine triphosphatase produced increased amounts of superoxide. Proliferation of Ras-transformed cells was inhibited by a chemical antioxidant or transfection of cells with catalase. The findings indicate that production of ROS is necessary for enhanced cell proliferation induced by an oncogenic form of Ras and also provide a hypothetical mechanism for some of the observed anti-proliferative actions of antioxidants.

  8. Calcium control

    In many cells, release of calcium from intracellular stores is a critical step in regulating various processes. Golovina and Blaustein (p. 1643) used high-resolution imaging and calcium-sensitive fluorescent dyes to visualize calcium storage and release of calcium in stimulated cells. Calcium exists in small spatially and functionally distinct compartments that can respond in distinct ways to physiological stimuli—the concentration of calcium in some stores even increased while others released calcium. Such local control of the concentration of intracellular calcium apparently allows calcium-dependent regulation of multiple processes within a single cell.

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