Editors' Choice

Science  14 May 2004:
Vol. 304, Issue 5673, pp. 929

    At the End of One's Tether

    1. Stella M. Hurtley

    Some DNA viruses, like papillomavirus, permanently reside within a host by establishing a latent infection within host cells. In order to maintain the viral genome inside the nuclei of dividing host cells, the virus must devise a strategem for ensuring faithful partitioning to daughter cell nuclei during disassembly and reassembly of the nuclear envelope. You et al. examined how bovine papillomavirus manages to accomplish this feat and find that one of the papillomavirus-encoded proteins, E2, tethers the viral genome to host chromosomes during mitosis. On the host chromosome, a protein termed Brd4 acts as the anchor for E2; Brd4 and E2 colocalize on mitotic chromosomes. Expression of the C-terminal domain of Brd4, which contains the E2 binding site but which cannot interact with chromosomes, inhibits association of the viral genome with host chromosomes. In mouse cells, blocking the Brd4-E2 interaction inhibits papillomavirus DNA-induced transformation. This interaction appears key to the maintenance of papillomavirus in latently infected cells and is thus a potential therapeutic target. — SMH

    Cell 117, 349 (2004).


    It's All in the Mind

    1. Gilbert J. Chin

    How do physiological and molecular variation combine to create a range of motivational drives that becomes manifest as a range of behaviors? Champagne et al. report measurements of dopamine in the nucleus accumbens of female rats caring for (licking and grooming) their pups. This region of the brain is known to contribute to motivating behaviors, particularly those that are followed by reward, such as the consumption of food. The increase in dopamine preceded the initiation of licking and grooming acts by the mothers, and the size and duration of the dopamine signal correlated with the duration of the act. Furthermore, mothers previously scored as high or low in licking/grooming (greater than 1 standard deviation from the mean of a normally distributed group) exhibited consistently high or low scores in the dopamine signal and in the density of dopamine receptors in the shell of the nucleus accumbens. Taken together with earlier studies in this series, these results begin to establish a neurochemical basis for individual differences in maternal behavior. — GJC

    J. Neurosci. 24, 4113 (2004).


    A Planetary System Is Born?

    1. Linda Rowan

    Herbig Ae/Be stars are intermediate-mass, pre-main sequence stars, and some have circumstellar disks in which extrasolar planets and planetesimal precursors may lurk. The very young Herbig Be star LkHα234 is embedded in a nebula within the NGC 7129 star formation region. Five spectra taken by Chakraborty et al. over 33 days show large and abrupt variations in the sodium absorption lines of LkHα234. The authors infer that an asteroid-sized solid object (about 100 km in diameter) was in the process of falling from the disk onto the stellar surface and disintegrated about 0.1 to 2 AU above the star. This infall may be just one among many in a very young planetary system where planetesimals either accrete to form larger planets, fall into the star, get ejected from the system, or get pushed to the edge of the system, like comets in the solar system. — LR

    Astrophys. J. 606, L69 (2004).


    Reviving Chirality in Microporous Materials

    1. Phil D. Szuromi

    Two routes are being pursued for creating chiral microporous materials from metal-organic frameworks for isomer-selective separations or catalysis. One is to use ligands that are already chiral. The other is to use less expensive achiral ligands and to induce chirality, typically as helicity within the structure, with chiral templates that can be desorbed and reused. However, the latter approach is often hampered by “collapse” of the material into a nonporous material after template removal.

    Bradshaw et al. show that some “collapsed” structures can be revived by making a small change in the framework ligands. Frameworks based on Ni2+ complexes with btc (1,3,5-benzenetricarboxylate) and pyridine ligands formed interpenetrating networks of chiral helices under the direction of chiral propan-1,2-diol guests, but an amorphous nonporous material resulted after template removal. Nevertheless, reaction of this material with 3-methylpyridine created a thermally stable chiral microporous material; the same material could also be made directly by substituting 3-methylpyridine for pyridine in the original synthesis. Structural studies indicate that the change imparted by the methyl group led to small but significant stabilizing interactions; notably, the use of 4-methylpyridine did not stabilize the network. — PDS

    J. Am. Chem. Soc. 10.1021/ja316420 (2004).


    A Guide to Silence

    1. Guy Riddihough

    RNA-directed DNA methylation (RdDM) is an RNA silencing mechanism that so far appears to be specific to plants. RdDM requires a double-stranded RNA that is cut into short 21- to 26- nucleotide fragments, much like the small interfering RNAs seen in RNA interference (RNAi); DNA sequences homologous to these RNAs are then methylated and silenced. As well as being implicated in the methylation of protein-coding regions of genes subject to post-transcription gene silencing, RdDM is also involved in transcriptional gene silencing (TGS) through the RNA-directed methylation of promoter sequences.

    Although several DNA methylases have been implicated in RdDM, it is still not known how RNA targets specific DNA sequences (as in RNAi). In order to delve deeper into the mechanism of RdDM, Kanno et al. have used a TGS-based screen to identify three complementation groups in Arabidopsis, one of which is the gene drd1. This gene is required for non-CpG methylation-based silencing in a target promoter and appears to act locally. It also has homology with SWI/SNF and Drosophila RAD54 chromatin remodeling proteins. Mutations in the various alleles of drd1 all map to conserved functionally important regions of DRD1's putative SWI/SNF ATPase domain, which in other SWI/SNF proteins is essential for remodeling. RAD54 is required for homologous DNA repair, and thus the authors speculate that DRD1 could aid in homology searching, clearing of chromatin, and heteroduplex formation that may guide DNA methylases. — GR

    Curr. Biol. 14, 801 (2004).


    Better Writing and Reading

    1. Ian S. Osborne

    Reducing the operating wavelength of laser diodes is desirable for a number of applications. Shorter wavelength enables the writing and reading of optical information at higher storage densities, as seen in the development of CD and DVD players over the past decade, in which the wavelength has decreased from infrared to blue. In addition, ultraviolet (UV) radiation at wavelengths below 390 nm can be used in fluorescence-based chemical and biological detection schemes.

    The development of suitable materials that can be electrically driven and operated at ambient temperatures has, however, presented a significant challenge. Providing an addition to the small selection of wide-bandgap materials, such as diamond, that emit in the UV, and one that the wafer growers may find more attractive to work with, Fischer et al. present results on UV-emitting AlGaN-based light-emitting diodes. Their devices can operate at room temperature and provide around a milliwatt of 290-nm UV light when biased at around 10 V and with an injection current of 300 mA. — ISO

    Appl. Phys. Lett. 84, 3394 (2004).


    Seeing Two Spots

    1. Gilbert J. Chin

    More than a few decades ago, when paper electrophoresis was in vogue, two unusual nucleotides (ppGpp and pppGpp) were identified and called magic spots I and II. These spots—collectively called (p)ppGpp—appeared when Escherichia coli were starved of amino acids and ran into problems making proteins. Subsequent work established that the ribosome-associated enzyme RelA synthesized (p)ppGpp when uncharged transfer RNAs (depleted of their cognate amino acids) entered the ribosome and that (p)ppGpp served as a pleiotropic regulator of gene expression.

    Hogg et al. describe structural and biochemical studies on two conformations of a Rel/Spo homolog from Streptococcus; Spo degrades (p)ppGpp and has sequence similarity to Rel. They find two catalytic sites, one for (p)ppGpp synthesis and one for hydrolysis, that are active in mutually exclusive fashion. Artsimovitch et al. observe two orientations of (p)ppGpp binding to the six-subunit RNA polymerase from Thermus thermophilus. They suggest that the differential impact of these orientations on binding of Mg2+, nucleotides, and the nontemplate DNA strand may account for the inhibitory and stimulatory effects of (p)ppGpp on transcription. —GJC

    Cell 117, 57; 299 (2004).