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G E O P H Y S I C S

The Effects ofDeglaciation

Our knowledge of the motionof Earth’s major tectonic plates,which move at velocities of upto several centimeters per year,has come primarily from obser-vations and dating of marinemagnetic anomalies.As mid-ocean ridge magmas cooled,they locked in magnetic signa-tures that recorded the orienta-tion of Earth’s magnetic fieldand allowed the spreading andthe geometry of ocean crust tobe reconstructed. More recent-ly, global positioning system(GPS) measurements, whichhave been made for about 20years, have provided compara-ble resolution and perhaps abetter indication of currentplate motions.

Sella et al. provide a synthesisof GPS data from the pastdecade and compare it with thebest estimates from magneticanomalies. Overall, the agree-ment is good, although there

are differences in the motionsacross the North American,South American, and Caribbeanplate boundaries.The resolutionof the GPS data is sufficient toshow that the current plate mo-tion for the Antarctic, NorthAmerican, and Eurasian plateshas been perturbed by the melt-ing of the major ice sheets dur-ing the past 15,000 years. — BH

J. Geophys. Res. 107,10.1029/2000JB000033 (2002).

E V O L U T I O N

A Northerly Migration

Many groups of organisms havemembers in both Africa andSouth America.These disjunctdistributions are commonly at-tributed either to an origin onthe Gondwanan supercontinentbefore its breakup around 100million years ago (Ma) or tolong-distance dispersal acrossthe widening Atlantic Ocean.

In a molecular phylogeneticanalysis of the plant familyMalpighiaceae, Davis et al. pro-

vide evidence for a newscenario.The Malpighi-aceae are a family of trees,shrubs, and lianas occur-ring throughout the trop-ics, with a peak of diversityin the New World.Their

analysis indicates that the fami-ly originated in what is nownorthern South America about64 Ma, well after the breakup ofGondwana; at least six separatetransoceanic dispersal eventswould have to have taken placeto produce the current distribu-tion, which seems unlikely. In-stead, Davis et al. propose thatthe early Malpighiaceae spreadmostly overland across the

Northern HemisphereLaurasian supercontinent, dur-ing the climatically favorablethermal maxima in the Eoceneand Miocene epochs. — AMS

Proc. Natl. Acad. Sci.U.S.A.10.1073/pnas.102175899 (2002).

M I C R O B I O L O G Y

Getting Enough Airto Survive

After infection by tubercle bac-teria, the host battles the in-vader to a standoff, at least aslong as the immune systemfunctions effectively. Upon en-try, the pathogen subversivelyinvades the immune systemthrough inactivated macro-phages; the distressed and in-fected host cells then emit sig-nals that recruit activatedmacrophages to encapsulatethe site of infection in a char-acteristic granuloma, or tuber-cle. Nitric oxide (NO) producedby these macrophages controlspathogen growth by inhibitingaerobic respiration in the bac-teria and contributing to theformation of toxic peroxyni-trite. Tubercle bacteria do nothave flavohemoglobins (whichother pathogens use to detoxi-fy NO) but instead have threetypes of distinctive truncatedhemoglobins (trHbs). Ouelletet al. have found that thetrHbs act in concert in my-cobacteria to acquire enoughoxygen to support the conver-sion of NO into innocuousnitrate. — CAProc. Natl. Acad. Sci. U.S.A. 99, 5902 (2002).

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CONTINUED ON PAGE 1205

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EDITORS’ CHOICE

H I G H L I G H T S O F T H E R E C E N T L I T E R A T U R E

G E O L O G Y

A Texas-Sized Collision

The supercontinent Rodinia, an accretion of all the conti-nents, existed between 1200 and 600 million years ago

(Ma). One piece of ground truth for this tectonic puz-zle is the Grenville Province, a region of metamor-phism and deformation that runs from Labrador tothe Llano Uplift in Texas and formed the coastlineof Laurentia.About 1200 Ma, the Grenvilleprovince collided with a cratonic piece (Amazonia)of South America.The rocks in the uplift provideevidence for this long-ago collision; however, it has

been difficult to identify which piece of Amazoniacollided with this proto-Texas region.

Tohver et al. studied a large sequence of gabbrosand basalts from the Nova Floresta formation of Rondô-

nia in western Brazil.These rocks are part of the Amazoncraton, and Ar-Ar dating indicates that they are 1200 million

years old. Magnetic characteristics of the rockswere used to determine the paleomagnetic poleand to fix the paleogeographic position of Amazo-nia. It appears that around 1200 Ma, one corner ofthis craton (oriented roughly 180 degrees from its

present-day position) collided with the proto-Texas region and created the Llano Uplift. — LREarth Planet. Sci. Lett. 199, 185 (2002).

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CONTINUED FROM 1203 E D I T O R S ’ C H O I C E

C E L L B I O L O G Y

Mind the Gap

Cells that are subjected to a variety ofstresses, such as heat shock, make funda-mental changes in the way they processmisfolded proteins in order to retrieve,where possible, proteins that have not beendamaged fatally. In looking at the stress re-sponse of cells,VanSlyke and Musil noticedchanges in the way that misfolded proteins

within the endoplasmic reticulum (ER) areprocessed. Generally, misfolded or incom-pletely assembled proteins are transportedout of the ER and into the cytosol for degra-dation by proteasomes. However, in stressedcells, this dislocation reaction appears to beless efficient, allowing proteins that mightpreviously have been removed from the ERto complete their folding and assembly andsubsequently to be transported to the cellsurface.The authors specifically examinedthe folding and transport of components ofgap junctions, and the net effect of stresswas the assembly of excess functional gapjunctions at the cell surface. — SMH

J. Cell Biol. 157, 381 (2002).

C L I M AT E S C I E N C E

Modeling Atmospheric Circulation

The Quasi-Biennial Oscillation (QBO) is acyclic atmospheric process with a periodslightly longer than 2 years; it involves rever-sals of the easterly and westerly phases ofequatorial zonal stratospheric winds.TheQBO dominates the interannual variabilityin the tropical stratosphere and causes asecondary circulation with an associated re-sponse in the temperature distribution andglobal effects on trace gas distributions.Thus, general circulation models must simu-late the QBO accurately if they are to pre-dict climate more reliably. Giorgetta et al.have developed a model that successfullysimulates the QBO by incorporating im-proved spatial resolution, a realistic simula-tion of tropical convection, and the effectsof gravity waves.With these improvements,their model yields a QBO very similar to theobserved one, forced from a broad spectrum

of atmospheric waves (as expected from ob-servations) and without the need for specificad hoc adjustments. — HJS

Geophys. Res. Lett. 29, 10.1029/2002GL014756 (2002).

A P P L I E D P H Y S I C S

Clear Reflections

Optical components such as mirrors can bemade to very high specification. However,because of stresses arising during operation,

mirrors can warp, sag, or bend. For veryhigh-specification systems, adaptiveoptics uses feedback to iron out thedeformations by making tiny mechan-ical adjustments via a network of ac-tuators attached to the underside ofthe mirror.Vdovin et al. describe anactuation system that is relatively in-expensive and may be suitable for ap-plications that are not extremely de-manding.Their demonstrator system

comprises 19 resistors placed in a hexagonalarray underneath a flexible mirror.The resis-tor length is temperature-dependent, andeach resistor is controlled and adjusted by acurrent source.With a response time of 5seconds and actuator motion of about 6 mi-crometers, this simple setup is ideally suitedto correcting slowly varying deformationsand aberrations in mirror systems. — ISO

Opt. Lett. 27, 677 (2002).

E V O L U T I O N

Mixing and Matching, Maybe

The central position of ribosomal RNA(rRNA) in molecular phylogenetics has re-cently been reinforced, both by its demon-strated importance in translation and bythe evolutionary proximity of nucleic acidmachinery (mediating replication and tran-scription) in eukaryotic and archaeal lin-eages. Nevertheless, the heated discussionconcerning metabolic enzymes has beenstoked by the provision of new genome se-quences. Now, Canback et al. suggest thatthe eukaryotic glycolytic enzymes cannothave been obtained en masse from the mi-tochondrial symbiont, because they do notcluster with the enzymes of α-proteobacte-ria. On the other hand, these protein treesand those of other metabolic pathways arenot simply isomorphic with that of rRNA,which may in part be due to lateral genetransfer among the ancestors of extant α-proteobacteria and even between eukaryot-ic hosts and pathogenic bacteria. For a his-torical view of the continuing debate aboutthe origin of the mitochondrion, see Martinet al. — GJC

Proc. Natl. Acad. Sci. U.S.A. 99, 6097 (2002);Biol. Chem. 382, 1521 (2001).

Heat-shocked cells (right) exhibit more Lucifer

yellow–conducting gap junctions than unstressed

cells (left).

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Clear Reflections

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