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8 APRIL 2011 VOL 332 SCIENCE www.sciencemag.org

B I O C H E M I S T R Y

Enzymes Aren’t Perfect

Cells have effi cient quality-control systems to

detect and repair errors in polymerization of DNA

or RNA. Tagliabracci et al. reveal the biological

effects of errors by another polymerase, glycogen

synthase, which creates glycogen—branched

chains of glucose that serve as an important

energy store in cells. About 1 in 1000 of the

glucose residues in normal glycogen contain

covalently linked phosphate. The authors found

that the major enzymatic activity from mouse

muscle catalyzing such incorporation of phos-

phate was in fact glycogen synthase. Although

the incorporated phosphate may serve a useful

function, current evidence indicates it is probably

a mistake. A phosphatase that removes phos-

phate from glycogen is known as laforin because

mutations in the enzyme are associated with La-

fora disease, a deadly human disease that causes

neurodegeneration and epilepsy. The disease

appears to result from insolubility of excessively

phosphorylated glycogen. Thus, laforin is likely

required to prevent deleterious effects of catalytic

errors made by glycogen synthase. — LBR

Cell Metab. 13, 274 (2011).

M A T E R I A L S S C I E N C E

Flat Pack

Superlattices combining colloidal particles or

nanocrystals with two distinct sizes can exhibit

unusual magnetic and electronic properties. It

has proven easier to make three-dimensional

rather than two-dimensional structures of

this sort, particularly for samples that extend

over large areas. Dong et al. used a liquid-air

fabrication method with a low concentration of

nanocrystals in the feedstock to form single and

binary superlattices that extend over cm2 areas.

The nanocrystals were deposited from hexane

onto the surface of diethylene glycol and allowed

to dry. Cocrystallization of Fe3O4 with Au, Fe3O4

with FePt, and NaYF4:Yb/Er

with Fe3O4 nanocrystals

showcased the general-

ity of the method. The

superlattice structure

could be controlled

by changing the

ratio of the diameter

of the particles, with

both AB- and AB2-type

lattices obtained. In hex-

agonally packed AB2 lattices,

the smaller particles occupied the interstices of

three larger neighboring particles. Shifting to

A2B3 bilayer structures (above), the authors found

miRNAs normally act cell-autonomously, but

increasing evidence suggests that they might

also act far from their site of synthesis.

High-density lipoprotein (HDL) particles

facilitate the transport of lipids, and other

biomolecules, in the bloodstream. Given that

lipid-based transport of RNAs can be used

as a systemic delivery system, Vickers et al.

investigated the nucleic acid component of

HDL and found that they are able to carry

miRNAs. The profi le of these miRNAs differed

between healthy individuals and individuals

suffering from familial hypercholesterolemia

(which can lead to atherosclerosis), an effect

also seen in a mouse model for atherosclerosis.

that the smaller particles sat on only half of the

larger particles, and this anisotropy drove the

formation of a beltlike morphology rather than a

membrane. The method was extended to ternary

ABC2 structures using a mixture of three types of

particles, although in this case the structure was

more prone to defects. — MSL

Nano Lett. 11, 10.1021/nl200468p (2011).

C E L L B I O L O G Y

Have miRNA, Will Travel

MicroRNAs (miRNAs), small noncoding RNAs en-

coded in the genomes of many eukaryotes, have

a pervasive role in regulating gene expression.

Two-and-a-half billion years ago, cyanobacteria may have caused the fi rst mass extinction on

Earth by inventing oxygenic photosynthesis. During their slow evolution, these prokaryotes

donated their light-harvesting organelles to plants, developed a nitrogen-harvesting mecha-

nism, and, not least, adopted many multicellular morphotypes. Few cyanobacteria have been

sequenced, however, and as a result, their evolutionary history and phylogeny are largely

unknown. Schirrmeister et al. used multiple phylogenetic analyses on the limited number of

16S rDNA cyanobacterial sequences available to compare the resulting trees with established

morphologically derived clades and to the scant fossil record. They obtained a monophyletic

tree with Gloeobacter being the nearest outgroup and Chlamydia as the closest eubacterium.

Rates of evolution among the cyanobacteria may be slow, but they organized into multicel-

lular forms that displayed some functional specialization several times—and well before

eukaryotes undertook this major transition. Since then, cyanobacteria appear merely to have

evolved into three major clades, which have shifted between multicellular and unicellular

morphotypes. Most modern lineages appear to have multicellular common ancestors, includ-

ing the important marine genera Synechococcus and Prochlorococcus. — CA

BMC Evol. Biol. 11, 45 (2011).

E V O L U T I O N

Shape-Shifting Cyanobacteria

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www.sciencemag.org SCIENCE VOL 332 8 APRIL 2011

HDL loaded with exogenous miRNAs was able to modulate the expression of specifi c miRNA target genes when added to tissue-culture cells. Indeed, the changes in gene expression in cells exposed to HDL from individuals with familial hypercholesterolemia were enriched for genes involved in lipid metabolism, infl ammation, and atherosclerosis, which suggests that some of the HDL miRNAs may play a direct role in disease progression. — GR

Nat. Cell Biol. 13, 10.1038/ncb2210 (2011).

P O L I C Y

Framing the Climate Debate

How concerned are Americans on the whole about global warming? Yeager et al. have found that the answer may depend on what exactly the question is. Pollsters regularly pose a “most important problem” (MIP) question originally devised by George Gallup in the 1930s: “What do you think is the most important problem facing the country today?” Only 1 to 2% of respondents to this question in three surveys conducted by the authors offered global warm-ing or the environment as an answer. However, if instead the question posed was “What do you think will be the most serious problem facing the world in the future if nothing is done to stop it?” global warming/environment emerged as the most frequent response, its percentage more than 10-fold higher than before. Shuldt et al. examined the partisan subtleties of word-ing choice. They found that the Web sites of conservative think tanks use the phrase “global warming” more frequently than “climate change,” whereas the reverse was true of liberal think tank sites. They then surveyed a sample of Americans to probe the impact of these distinct phrases and found that self-identifi ed Repub-licans were more likely (by a ~3:2 margin) to consider climate change a real phenomenon than global warming. Democrats were not affected by the wording, nor did educational attainment appear to favor one response over the other. — BJ

Public Opin. Q. 75, 125;115 (2011).

M A T E R I A L S S C I E N C E

The (S)lowdown on Crystallization

One way to resolve the many steps that occur during crystallization is to use small droplets of solution, in part to avoid heterogeneous nucle-ation by impurities. Stephens et al. created pico-liter droplets by using self-assembled monolay-ers (SAMs) that contained hydrophilic (carboxylic acid terminated) islands inside hydrophobic

(fl uorous terminated) layers. These islands sup-ported hemispherical droplets, with radii varying from 4 to 10 µm (0.04 to 2 pl in volume), that were supersaturated with calcium carbonate. Bulk precipitation created many rhombohedral crystals, but in 90% of the droplets, a single smaller tetrahedral crystal formed. Redissolution of these tetrahedral crystals in undersaturated solution, or further growth from bulk solution, initiated recrystallization into rhombohedral crystals. The SAMs appear to slow down crystal-lization and capture the tetrahedron as an early intermediate crystal form. — PDS

J. Am. Chem. Soc. 133, 10.1021/

ja200309m (2011).

B I O M E D I C I N E

Axon Damage Illuminated

Pathogenesis of the autoimmune disease multi-ple sclerosis (MS) is associated with progressive deterioration of the myelin sheath surrounding neuronal axons; however, axon damage may also contribute to MS-associated neurodegen-eration. Nikic et al. used in vivo imaging and electron microscopy to examine axon damage in a mouse model of MS (EAE, experimental au-toimmune encephalitis). In EAE mice, swelling in discrete sites on axons was observed, which was then followed by axon fragmentation. In many cases, damaged axons retained myelin; in

some cases, axon damage was reversible. Axon damage was preceded by mitochondrial pathol-ogy, which was associated with the presence of microglia and the production of reactive oxygen and nitrogen species. Induction of oxidative or nitrosative stress was suffi cient to induce mitochondrial pathology and axon damage in normal mice, and their blockade in EAE mice alleviated axon damage. Lesion biopsies from MS patients also showed similar axon (above, right) and mitochondrial damage, which sug-gests that reversing axon damage may be an important therapeutic strategy in the treatment of MS. — KLM

Nat. Med. 17, 10.1038/nm.2324 (2011).

Normal-appearing white matter

Lesion

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