This Week in Science

Science  05 Sep 1997:
Vol. 277, Issue 5331, pp. 1413
  1. Cdc25 and the cell cycle

    The cell division cycle is arrested when cells contain damaged DNA, and failure of this “DNA damage checkpoint” may contribute to the generation of cancerous cells. The phosphatase Cdc25 is a critical regulator of the cell cycle that dephosphorylates and activates the cyclin dependent kinase Cdc2. Three reports present evidence that signals initiated in response to DNA damage act to inhibit cell division by causing phosphorylation and inactivation of Cdc25 (see the Perspective by Weinert, p. 1450). Furnari et al. (p. 1495) studied the requirement for Cdc25 for cell cycle arrest in fission yeast exposed to irradiation, and implicated the protein kinase Chk1 as the enzyme that phosphorylates Cdc25. Peng et al. (p. 1501) found that human Cdc25C is regulated by phosphorylation of a specific serine residue. Mutants lacking this serine residue did not bind to 14-3-3 proteins (which may contribute to regulation of Cdc25 activity) and, when overexpressed, allowed cells to escape cell cycle arrest in response to DNA damage. Sanchez et al. (p. 1497) cloned the human Chk1 protein kinase. They show that it may mediate the direct phosphorylation of human Cdc25 and thus inhibit entry of cells into mitosis.

  2. Excess x-rays

    Young stellar objects (YSOs) produce much greater x-ray emission than more evolved, young sunlike stars. Shu et al. (p. 1475) discuss how the time-dependent interaction of the magnetosphere of the YSO with a surrounding accretion disk could produce excess x-rays. They modified their “x-wind” model of this interaction to include fluctuations that will occur in the magnetic fields that connect the YSO and the accretion disk because of their differing rotation rates. Such processes would enhance x-ray flaring and also help explain the formation of chondrules (which requires repeated cycles of melting and cooling of the particles) and their abundances of short-lived radioactive element (created by x-ray bombardment).

  3. Electrons over metals

    Image potential states are excited electronic states above a metal surface; these are weakly bound electrons that have a relatively long lifetime. Höfer et al. (p. 1480; see the Perspective by Plummer, p. 1447) used two-photon photoemission spectroscopy on a femtosecond time scale to show that the periodic motions of these electronic states can be observed in real time. The observed dynamics reflect the isolated quantum states for low quantum states, and the coherent phenomena arise from simultaneous excitation of several states for higher quantum states. For these higher quantum states, oscillations with a period of 800 femtoseconds were observed, and the electrons could travel as far as 200 angstroms from the surface.

  4. Sulfur strands

    Bacteria have recently been recognized to be particularly important in mineralization. Taylor and Wirsen (p. 1483) now describe a bacteria that produces long filaments of pure sulfur. Attachment of these strands may help the bacteria persist in areas where fluids are venting. The filaments may also explain the origin of abundant sulfur particles emitted at hydrothermal vents and sulfur mats.

  5. Big craters on little bodies

    Vesta is one of the brightest and largest Earth-crossing asteroids. Its brightness is related to its distinctive composition for an asteroid: The basaltic surface of Vesta makes it a prime candidate for the source of the basaltic achondrite meteorites (such as howardites, eucrites, and diogenites). Thomas et al. (p. 1492; see the cover) observed Vesta during its 1996 closest approach to Earth with the Hubble Space Telescope and found a huge impact crater near its south pole that has a comparable diameter to the asteroid itself. This huge crater provides a probable source for the basaltic achondrites and a family of smaller basaltic asteroids that may be larger ejecta fragments from the impact event. In addition, these images of Vesta suggest that relatively small celestial bodies in our solar system can survive relatively large impacts.

  6. Conditional mutators in tumor development

    A subset of human cancers have defects in genes encoding mismatch repair (MMR) proteins. These defects have been postulated to generate a mutator phenotype that drives the accumulation of mutations required for tumor development. In studies of MMR-deficient tumor cell lines, Richards et al. (p. 1523; see the Perspective by Loeb, p. 1449) found that mutations accumulated at high frequency when the cells were maintained at high density but not when the culture conditions allowed rapid cell growth. This “conditional mutator” phenotype may be important in intact tumors, where the microenvironment often limits cell growth.

  7. Responses to estrogen

    Estrogen therapy has been widely used to reduce the effects of menopause; however, hormone replacement has been implicated in increasing the chance of breast cancer. Some antiestrogens are used for the treatment of breast cancer, but these agents can increase the chance of other cancers. In an effort to better understand estrogen signaling, researchers have examined the transactivation properties of the two estrogen receptors, ERα and ERβ. Paech et al. (p. 1508; see the news story by Pennisi, p. 1439) report that the two receptors signal in different ways, depending on the ligand and the promoter response element. This differential action suggests that the receptors participate in different regulation pathways and presents possibilities for future pharmacological research.

  8. Asymmetry across the synapse

    The protein PSD-95 is found on the postsynaptic (receiving) side of synapses, the cell-to-cell communication ports between neurons. Three regions of the protein, the PDZ domains, hold other molecules at the synapse. Potassium channels, the N-methyl-d-aspartate (NMDA) receptor, and nitric oxide synthase bind to two of the PDZ domains; Irie et al. (p. 1511) now show that the third PDZ domain binds to neuroligins, a transmembrane protein whose extracellular portion binds to another transmembrane protein on other cells, neurexin. This interaction could provide the basis for the asymmetrical junction of the synapse.

  9. The complete sequence of E. coli

    The bacterium Escherichia coli was first isolated in 1922 and became a major model system for biochemical genetics, molecular biology, and biotechnology. Blattner et al. report that they have sequenced the entire 4.6-megabase genome of a strain of E. coli and provide a foldout in this issue with the arrangement of putative and known genes, operons, promoters, and protein binding sites. Comparisons with other genomes indicate the existence of widely distributed gene families. More than one-third of the identified genes have no known function. The availability of the complete sequence will facilitate studies of biological evolution and function. [See the news story by Pennisi.]

  10. The glowing night sky

    Light traveling from outer space through the atmosphere that reaches the ground (resulting in the so-called terrestrial nightglow) is affected by the chemical compounds in the atmosphere, which both absorb and emit light in response to the incoming light. The exact effect of the atmosphere on the incoming light has been difficult to determine because of insufficient spectral resolution and sensitivity of previous studies. Slanger et al. show that high-resolution data from the Keck telescope can provide insights into the hydroxyl radical and molecular oxygen emission and absorption in the nightglow spectra.

  11. Helium-rich Hale-Bopp

    Spectroscopic study of comet Hale-Bopp with the Extreme Ultraviolet Explorer satellite by Krasnopolsky et al. shows that the comet contains helium, but not much neon, and confirms that the comet emits soft x-rays. The lack of abundant neon implies that the comet formed at or was heated to temperatures greater than 25 kelvin. Both the helium and the soft x-ray emissions may have their origins in interactions of the comet with the solar wind.

  12. A sense of light

    Phytochromes serve as photoreceptors for a great variety of plants. Yeh et al. have shown that a phytochrome from cyanobacteria responds to changes in light conditions by phosphorylating a protein encoded by a gene adjacent to the phytochrome gene. Together the two proteins form a system with interesting similarities to bacterial two-component signal transduction systems. Aspects of this system may account for the light-responsive signaling mechanisms from higher plant phytochromes as well.

  13. Cleanup squad

    Neurons communicate with one another by releasing neurotransmitters into the specialized space between cells, the synaptic cleft. After the cell on the other side of the cleft responds, transporters remove the excess neurotransmitter from the cleft. By detailed electrophysiological analysis of synapses in cerebellar neurons, Otis et al. have now found that transporters for the neurotransmitter glutamate are concentrated in the postsynaptic cell near the receptors for glutamate, at a ratio of at least 15 transporters to each AMPA glutamate receptor. This ensures rapid removal of glutamate from the synaptic cleft into the postsynaptic cell by the binding of at least 850 transporters per exocytotic event.

  14. Combing through genes

    Many diseases are associated with small deletions in critical genes. The search for such microdeletions can be arduous and time-consuming. Michalet et al. present a technique for “combing” or uniformly stretching out large pieces of DNA. Fluorescent labeling allowed the easy detection of gaps only a few thousand bases in size. This approach should also facilitate map preparation and evaluation that are critical for the human genome sequencing effort.

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