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<jats:title>Cracking Up</jats:title> <jats:p> Hydraulic fracturing by fluids at high pressure results in damage or breakage along cracks in deep rocks, a process that in some cases causes earthquakes. This process can occur naturally when the hydrologic setting is just right, or can be induced by human activity when fluids are pumped at high pressure into deep aquifers. By studying the fault along which the 1999 magnitude 7.6 Chi-Chi earthquake occurred in Taiwan, where there are currently low tectonic stresses following the large earthquake, <jats:bold> Ma <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="459" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1222119">459</jats:related-article> ) observed an unusual type of earthquake-like event that they attribute to natural hydraulic fracturing. </jats:p>

<jats:title>Tricky Tryps</jats:title> <jats:p> African trypanosomes, responsible for human sleeping sickness, are known for their powerful strategies of immune evasion, in particular antigenic variation. Adding another facet to this adaptive potential, <jats:bold> Salmon <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="463" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1222753">463</jats:related-article> , published online 14 June; see the cover) now show that early after infection, these parasites subvert the first line of innate host defense by inhibiting tumor necrosis factor-α synthesis in myeloid cells. This occurs through the stress-induced synthesis and release of cyclic adenosine monophosphate by phagocytosed parasites. The findings provide a long-sought function for the abundant and diverse adenylate cyclases in salivarian trypanosomes. Furthermore, this altruistic host colonization strategy, in which a proportion of parasites are sacrificed so that others can thrive, also highlights the selective advantage of population behavior in infection. </jats:p>

<jats:title>Resisting Arsenic</jats:title> <jats:p> The discovery of a bacterium living in the extreme conditions of Mono Lake, California, created a major controversy because it was claimed to be able to grow solely on arsenic and could substitute arsenate for phosphate in its key macromolecules, including DNA. Working with the same <jats:italic>Halomonas</jats:italic> spp. bacterium, known as GFAJ-1, and ultrapure reagents, <jats:bold> Erb <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="467" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1218455">467</jats:related-article> ) found that the bacterium needed a low level of phosphate (1.6 µM) to grow at all. Rather than significant specific arsenic incorporation, when the organism was grown in 40 mM arsenic, its nucleic acids acquired a trace of arsenic. Similarly, <jats:bold> Reaves <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="470" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1219861">470</jats:related-article> ) found that GFAJ-1 could not grow in the absence of phosphate and, moreover, that its growth was not stimulated by the addition of arsenate, although a trace amount of arsenic was also detected in DNA. Thus, GFAJ-1 shows no particular facility to substitute arsenic for phosphate, when phosphate is limiting, but it can tolerate high concentrations of the poison while efficiently scavenging phosphate. </jats:p>

<jats:title>Resisting Arsenic</jats:title> <jats:p> The discovery of a bacterium living in the extreme conditions of Mono Lake, California, created a major controversy because it was claimed to be able to grow solely on arsenic and could substitute arsenate for phosphate in its key macromolecules, including DNA. Working with the same <jats:italic>Halomonas</jats:italic> spp. bacterium, known as GFAJ-1, and ultrapure reagents, <jats:bold> Erb <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="467" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1218455">467</jats:related-article> ) found that the bacterium needed a low level of phosphate (1.6 µM) to grow at all. Rather than significant specific arsenic incorporation, when the organism was grown in 40 mM arsenic, its nucleic acids acquired a trace of arsenic. Similarly, <jats:bold> Reaves <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="470" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1219861">470</jats:related-article> ) found that GFAJ-1 could not grow in the absence of phosphate and, moreover, that its growth was not stimulated by the addition of arsenate, although a trace amount of arsenic was also detected in DNA. Thus, GFAJ-1 shows no particular facility to substitute arsenic for phosphate, when phosphate is limiting, but it can tolerate high concentrations of the poison while efficiently scavenging phosphate. </jats:p>

<jats:title>View of a Sodium Pump</jats:title> <jats:p> Membrane-integral pyrophosphatases (M-PPases) found in plants, protozoans, bacteria, and archaea, link pyrophosphate hydrolysis or synthesis to sodium or proton pumping and contribute to generating an electrochemical potential across the membrane. <jats:bold> Kellosalo <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="473" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1222505">473</jats:related-article> ) report the structure of the sodium pumping M-PPase from <jats:italic>Thermotoga maritima</jats:italic> in the resting state with product bound. The structures reveal the conformational changes that are likely to accompany pyrophosphate binding and provide insight into the ion-pumping mechanism. </jats:p>

<jats:title>Netting the Bad Guys</jats:title> <jats:p> Antimicrobial peptides are an evolutionarily conserved component of innate immunity in the intestine. One family, α-defensins, typically exert their antimicrobial effects through microbicidal activity against bacteria. Humans express only two α-defensins, human defensin 5 (HD5) and HD6. HD5 exhibits bactericidal activity and plays a role in shaping the bacterial composition of the gut. HD6, on the other hand, does not show bactericidal activity and its function in the gut is unclear. Now, <jats:bold> Chu <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="477" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1218831">477</jats:related-article> , published online 21 June; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6093" page="420" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1225906">Ouellette and Selsted</jats:related-article> </jats:bold> ) show that HD6 protects against bacterial pathogens. Rather than killing them directly, HD6 binds to bacteria surface proteins and, through a process of self-assembly, forms fibrils and nanonets that ensnare invading bacterial pathogens. </jats:p>

<jats:title>Sarm-Assisted Suicide</jats:title> <jats:p> Neurodegenerative disease or nerve lesions cause axons and synapses to disintegrate through a process known as Wallerian degeneration, which may involve an active “axon death program.” <jats:bold> Osterloh <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="481" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1223899">481</jats:related-article> , published online 7 June; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6093" page="418" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1226150">Yu and Luo</jats:related-article> </jats:bold> ) identify loss-of-function mutations in <jats:italic>Drosophila</jats:italic> dSarm that are capable of blocking the degeneration of severed axons for the fly life span. Deletion of mouse Sarm1 provides similar protection to severed axons for weeks after injury, which suggests that Sarm is part of an ancient axonal death signaling cascade. </jats:p>

<jats:title>New for Old?</jats:title> <jats:p> When confronted with a visual stimulus, such as an object or a scene, the brain decides whether it is new, and thus deserves to be encoded as a new memory, or old, which triggers the retrieval of the previously encoded memory. When shown a series of stimuli, some of which are similar to, but not identical to, old memories, it becomes necessary to switch back and forth between encoding and retrieval processing. Using behavioral tasks performed by human test subjects, <jats:bold> Duncan <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="485" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1221936">485</jats:related-article> ) took advantage of the fact that switching takes a second or two and found that a subsequent object, presented before switching has occurred, was more likely to be identified as new when the previous object was new, and identified as old when the previous object was old. </jats:p>

<jats:p>The show includes self-assembly at the nanoscale, suicide-bombing termites, what's next for NASA's planetary program, and more.</jats:p>

<jats:p>A weekly roundup of information on newly offered instrumentation, apparatus, and laboratory materials of potential interest to researchers.</jats:p>