EDITORS’CHOICEEDITED BY GILBERT CHIN AND JAKE YESTON

mechanism explains the results, whereas in thehydrodynamic regime the experiments suggest
another, collision-dependent mechanism, whichlacks a thorough theoretical description. — JS
Phys. Rev. Lett. 103, 265301 (2009).
CANCER

MOLECULAR BIOLOGY

Untranslated Regulators
A variety of short noncoding RNA molecules—microRNAs, small interfering RNAs, and Piwiinteracting RNAs (piRNAs)—play regulatory roles in eukaryotes. Many piRNAs are derived
from transposon-related sequences and, through complementary sequence interactions anda “ping-pong” amplification process, act to silence those selfish and potentially mutagenic
elements in germline cells.
However, Robine et al. show that a substantial population of piRNAs found in a Drosophila
somatic ovarian cell line are in fact derived from a distinct subset of genes, and also that thebulk of these piRNAs arise directly from the 3’ untranslated regions (3’ UTRs) of the sense
strands. This suggests that the complementary targets of these piRNAs may not be theparental transcripts. Ping-pong amplification is not required for the generation of these 3’
UTR piRNAs, nor does it appear as if they are aberrant products of the primary piRNA processing pathway. Furthermore, Saito et al. have found that the Drosophila gene traffic jam (tj)gives rise to 3’ UTR piRNAs and that one of its targets is the fasciclin III gene transcript, andRobine et al. note that the subset of functional categories of mRNAs that gives rise to the3’ UTR piRNAs is broadly conserved between fruit flies and mice. — GRCurr. Biol. 19, 2066 (2009); Nature 461, 1296 (2009).

PHYSICS

A Second Second Sound?
Superfluids, such as 4He below its transition temperature around 2 K, make for a great sciencedemonstration: They flow without viscosity
through tiny holes, creep up vertical walls, andrefuse to boil. This last property arises because
heat, which normally propagates by diffusion,
travels through a superfluid following a wave
equation (often referred to as the “secondsound”), resulting in extremely high thermalconductivity. Meppelink et al. realize an analog
of this phenomenon in a weakly interacting

394

Bose-Einstein condensate of sodium atoms byphysically separating the condensed (superfluid)from the thermal (normal) portion of the gas.
They then cause out-of-phase oscillations of thetwo components during which the center of mass
remains stationary, much as in the case of thesecond sound. The amplitude of these oscillations is damped as the condensate passes
through the thermal cloud, leading to energy
exchange between the two (Landau damping).
The damping rate is measured as a function of
the temperature and confinement of the gas. In
the collisionless regime (gas cloud size smaller
than the mean free path), the Landau damping

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Chromosomal translocations, whereby nonadjacent regions of chromosomes are inappropriately
joined, are a hallmark of several different typesof cancer. They can result in the generation offusion proteins that bypass normal control mechanisms and can wreak havoc in cells. It has been
assumed that translocation events occur randomly; whether more ordered processes areinvolved has been unclear.
The androgen receptor is a transcription factor that regulates gene expression during
prostate development. Fusions between an
androgen receptor target gene and genes encoding members of the ETS family of transcription
factors place ETS proteins under the control ofandrogen and are associated with prostate cancer. Using androgen-responsive prostate cells,
Lin et al. could produce cancer-specific genefusions by inducing genotoxic stress and adding
androgen. The activation of the androgen receptor promoted encounters between physically separate genes in the nuclei of prostate cells, andgenotoxic stress induced double-stranded breaks
in intronic regions of these genes. Furthermore,androgen and genotoxic stress recruited proteins
involved in sensing DNA breaks and mediating
chromosomal rearrangements. — HP
Cell 139, 1069 (2009).
CHEMISTRY

Jumping off the Page
Few marriages of analytical methods have beenas successful as that of chromatography andmass spectrometry;
together they cantease out thechemical composition of extraordinarily complex
mixtures. A typicalapparatus incorporates a gas or liquid chromatograph, in whichanalytes travel
through a separation column, and awww.sciencemag.org

CREDITS (TOP TO BOTTOM): ROBINE ET AL., CURR. BIOL. 19, 2066 (2009); © SCIENCEPHOTOS/ALAMY

Unfaithful Encounters

EDITORS’CHOICE
downstream detector where their masses aremeasured. An early, no-frills variant of chromatography involved spotting samples on paper,an inexpensive and highly portable support
medium. Wang et al. now show that this simple
material can also be used as an ionization platform for introducing samples into mass spectrometers. Samples such as blood are spotted on
the paper, which is then cut to a sharp triangularpoint. The paper is wet with a methanol-watersolution, and a high positive bias (4.5 kV) isapplied to the paper relative to thenearby inlet of a tandem mass spectrometry under ambient conditions; the precise
mechanism for ion release remains somewhat unclear. The authors demonstrate thedetection of drugs such as Gleevec inblood, as well as picogram quantities ofcocaine swabbed from a surface. Chromatographic methods can also be used toseparate components in a sample along
the paper, which can then be cut into separate pieces for further analysis. — PDS
Angew. Chem. Int. Ed. 49,10.1002/anie.200906314 (2010).
C L I M AT E S C I E N C E

CREDIT: NÊSIĆ ET AL., NAT. STRUCT. MOL. BIOL. 17, 130 (2010)

Model Behavior
Most of what we believe about how climate willchange in the future is based on projections
made by sophisticated climate models. There are
currently multiple highly complex mathematicalrepresentations of the climate system, andalthough most of them generally make similar
predictions about the future course of climate,they differ significantly in many particulars.
Because it is not normally clear which models’scenarios are likely to be the most realistic, thequestion arises of which specific models tobelieve and why. Knutti et al. discuss some major
sources of differences between models, how theirpredictive skill can be evaluated, and how confidence in their projections can be improved by
combining the results of multiple models. As climate models become more complex, and theamount of data from them increases, it becomeseven more important to have quantitative methods available through which to extract and synthesize information needed to guide climate policy decisions. — HJSJ. Clim. 10.1175/2009JCLI3361.1 (2009).
BIOCHEMISTRY

Uninhibited Mimicry
Helicobacter pylori is a pathogen that causes
gastric disease and is associated epidemiologically with gastric cancer. This bacterium injects
its protein CagA into epithelial cells lining thewww.sciencemag.org

stomach in order to disrupt cellular functions,and a domain of CagA that contains contiguous
repeats acts to inhibit the PAR1-MARK family ofprotein serine-threonine kinases.
Neŝić et al. describe the 2.2 Å crystal structure of the human kinase MARK2 in complex
with a 120-residue repeatcontaining subdomain ofCagA. Unusually, thekinase adopts anactive conformationeven though there isno magnesium or ATP
present, and a 14residue CagA segment was observed
to occupy theMARK2 substrate-bindingsite. This peptide appeared
to promote andstabilize the active
conformation of MARK2
by mimicking conserved features of the host substrates of thisprotein kinase family in a fashion reminiscent of
the inhibition of the cAMP-dependent protein
kinase by the endogenous peptide PKI. — VV
Nat. Struct. Mol. Biol. 17, 130 (2010).
BIOMEDICINE

Calculating UnmeasurablesIn assessing an epidemic, it would be helpful toknow how many people were being infectedeach day. The objective data, however, are more
likely to reflect how many people either became
ill or died on a given day, time-lagged tallies
that represent subgroups of the total infected
population. For latent infections that simmer foryears before producing symptoms, such as HIV,or for acute infections where time to death isvariable, derivations of the desired incidence
curve can be uncertain.
Goldstein et al. apply a mathematicalmethod that was originally used for the purpose of extracting images from blur. Information about the time to death and the deaths
per day was combined to calculate the incidence distribution, and the authors applied
their approach to the influenza epidemic thatstruck Philadelphia in 1918. Their analysis
suggests that in the few days between when
the size of the epidemic became clear andwhen the city enacted closure of public gathering places, the spread of influenza was alreadybeing slowed significantly, probably bychanges in individual behavior. — PJH

SCIENCE

Proc. Natl. Acad. Sci. U.S.A. 106, 21825 (2009).

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