PERSPECTIVES

MOLECULAR BIOLOGY An enzyme that cleaves RNA is converted to a DNA-cleaving enzyme during programmed cell Dicer’s Cut and Switch death in Caenorhabditis elegans. Qinghua Liu and Zain Paroo

poptosis is an essential develop- mental process in which a highly Aconcerted series of biochemical RNase DNase events directs cell disassembly and death. function Dicer function Degradation of chromosomal DNA is a defi ning characteristic of apoptosis, involv- Cleavage by ing distinct deoxyribonucleases that initi- CED-3 Truncated ate DNA fragmentation (initiator DNases) Dicer and metabolize residual DNA (progressive P DNases). On page 327 of this issue, Naka- Dicer P gawa et al. ( 1) report an unexpected role in P P dsRNA dsRBP DNA apoptotic DNA degradation for an enzyme that was previously known for its role in Initiates DNA RNA interference (RNAi). cleavage The first advance in understanding the Cleaved DNA mechanistic basis of apoptosis was the iden- OH OH tifi cation of ced-3, the gene that encodes a Small RNA cysteine protease (caspase) in the nematode P OH on April 15, 2010 Caenorhabditis elegans ( 2, 3). CED-3 and OH P Progressive DNases other caspases function in a cascade to acti- vate a spectrum of effectors for cellular disso- DNA fragments lution. Although progressive DNases are com- mon to nematodes and mammals, initiators of DNA cleavage in C. elegans have been elu- sive. In mammals, DNA fragmentation fac- tor (DFF40) [also known as caspase-activated Silence gene expression Cell death deoxyribonuclease (CAD)] is sequestered in a www.sciencemag.org complex with DFF45 [also called inhibitor of CAD (ICAD)] (4 , 5). Upon induction of apop- RNA and DNA substrate crossover. The RNase III Dicer functions with a double-stranded RNA binding tosis, activated caspases cleave the inhibitory protein (dsRBP) in processing long double-stranded RNA (dsRNA) to small RNAs. In C. elegans, the CED-3 DFF45 subunit, thereby freeing the DFF40 protease cleaves Dicer (or a Dicer-dsRBP complex), which inactivates its RNase function. The truncated Dicer subunit to initiate chromosome cleavage, gen- product gains DNase function and initiates breaks on chromosomal DNA. Progressive DNases further metabo- erating 3′ hydroxyl DNA breaks. lize DNA fragments to complete apoptotic DNA degradation. To identify the functional equivalent of Downloaded from mammalian DFF in C. elegans, Nakagawa by examining two different nematode strains domain, producing a truncated Dcr-1 con- et al. conducted a targeted RNAi screen. lacking the dcr-1 gene, both of which exhib- sisting of only an RNase IIIb domain and a Candidate initiator DNases were selected ited a stronger apoptotic phenotype. His- dsRNA binding domain. Because both RNase through informatics searches and not lim- torically, loss-of-function experiments that III domains are required for dsRNA process- ited solely to annotated DNases. Surpris- involve RNAi components have been used ing ( 10–12 ), this cleavage abolished RNase ingly, a gene (dcr-1) encoding a ribonu- to infer functional roles for small RNAs (9 ). activity. Remarkably, truncated Dcr-1, but clease called Dicer was identifi ed as a weak Given the wide spectrum of biological pro- not full-length Dcr-1, exhibited DNase activ- hit in the screen. Dicers are highly conserved cesses governed by such regulatory RNAs, ity, generating 3′ hydroxyl DNA breaks much RNase III enzymes that function in tandem it seemed plausible that small RNAs might like initiator DNases in mammalian cells. with double-stranded RNA binding proteins be involved in apoptotic DNA degradation. Moreover, mutation of catalytic RNase resi- (dsRBPs) in processing dsRNA to small However, animals defective for other RNAi dues in full-length Dcr-1 abolished both RNA regulatory RNA (6 , 7) (see the fi gure). These components did not produce the same pheno- and DNA cleavage activity. These fi ndings small RNAs effect RNAi by programm- type as that of the dcr-1 mutants, indicating an indicate that CED-3 switches Dcr-1 from an ing Argonaute proteins to guide sequence- RNAi-independent role for Dcr-1 in apoptotic RNase to a DNase. specifi c silencing of messenger RNA ( 8). DNA fragmentation. Through a series of elegant genetic exper-

Recognizing that the targeting of dcr-1 Nakagawa et al. postulated that Dcr-1 iments, Nakagawa et al. further demonstrate SCIENCE by RNAi was achieved with low effi ciency, might be a direct target of the CED-3 caspase that cleavage of Dcr-1 by CED-3 is required Nakagawa et al. confi rmed the initial fi nding because the lack of functional Dcr-1 reduced for promoting apoptosis. In addition to defi - the number of cell deaths induced by consti- ciencies in apoptosis, dcr-1 mutants exhibit Department of Biochemistry, University of Texas Southwest- ern Medical Center, Dallas, TX 75390, USA. E-mail: qinghua. tutively activated CED-3. Indeed, recombi- aberrant vulvar anatomy due to defective

[email protected]; [email protected] nant CED-3 cleaved Dcr-1 in its RNase IIIa production of microRNA. In dcr-1 mutants, GREENMAN/ Y. CREDIT:

314 16 APRIL 2010 VOL 328 SCIENCE www.sciencemag.org Published by AAAS PERSPECTIVES expression of a dcr-1 transgene that lacks a DFF45 complex in mammals. Cleavage of through which this switching may occur. cleavage site for CED-3 rescued the devel- Dcr-1 by CED-3 activates a functional DNase References opmental (vulva) defect, but not the apop- that generates 3′ hydroxyl DNA breaks, 1. A. Nakagawa, Y. Shi, E. Kage-Nakadai, S. Mitani, D. Xue, totic phenotype. Conversely, transgenic which are subsequently resolved by progres- Science 328, 327 (2010); published online 11 March 2010 (10.1126/science.1182374). expression of the truncated form of Dcr-1 in sive DNases. Conservation of this enzymatic 2. J. Yuan, S. Shaham, S. Ledoux, H. M. Ellis, H. R. Horvitz, dcr-1 mutants rescued apoptosis but not the switch may mean that Dicer also serves as Cell 75, 641 (1993). vulvar phenotype. an apoptotic DNase in higher organisms. 3. M. Miura et al., Cell 75, 653 (1993). This highly unexpected role for Dcr-1 in The DNase activity of Dicer may also gov- 4. X. Liu, H. Zou, C. Slaughter, X. Wang, Cell 89, 175 (1997). 5. H. Sakahira, M. Enari, S. Nagata, Nature 391, 96 (1998). early apoptotic DNA degradation has numer- ern other aspects of chromosome dynamics. 6. E. Bernstein et al., Nature 409, 363 (2001). ous important implications. Caspase-medi- Thus, it may be prudent to consider RNAi- 7. Q. Liu et al., Science 301, 1921 (2003). ated activation of DNA degradation now independent mechanisms underlying the phe- 8. J. Liu et al., Science 305, 1437 (2004). dicer 9. E. Bernstein et al., Nat. Genet. 35, 215 (2003). appears conserved from nematodes to mam- notypes of mutants. Perhaps most excit- 10. H. Zhang et al., Cell 118, 57 (2004). mals. Full-length Dcr-1 (and/or Dcr-1 in ing will be determining the extent to which 11. I. J. MacRae et al., Science 311, 195 (2006). complex with a dsRBP) maintains an inac- regulators of nucleic acid function exhibit 12. X. Ye, Z. Paroo, Q. Liu, J. Biol. Chem. 282, 28373 (2007). tive DNase in a manner similar to the DFF40- substrate crossover, and the mechanisms 10.1126/science.1189245

CHEMISTRY

A durable molecular catalyst for water oxidation In Pursuit of Water Oxidation has been made from readily available and Catalysts for Solar Fuel Production relatively inexpensive materials. on April 15, 2010 James K. Hurst

oughly three-fourths of the power that must be carefully synchronized with the permit maximal dispersion of catalytic cen- generated globally comes from burn- removal of protons and electrons if the forma- ters (and hence maximal reactivity), and in Ring fossil fuels. For solar energy to tion of rate-retarding, high-energy intermedi- most cases, many well-characterized vari- compete directly as a replacement, technol- ates is to be avoided. Moreover, to be of use ants can be synthesized with different ligand ogies are needed to capture this energy in a in large-scale applications, the catalysts must environments, All of these features foster chemical form—as a fuel—so it can be used stay active for extended periods of time or be performance optimization. However, cur- when sunlight is not available. One bottle- easily regenerated, and must be made from rently available catalysts tend to rapidly lose www.sciencemag.org neck in the development of practical solar readily available and inexpensive materials. their catalytic capabilities because they con- fuels is the water oxidation reaction. Water is Both homogeneous and heterogeneous tain heterocyclic organic ligands that easily the only potential source of electrons capable catalysts have advantages and drawbacks degrade (2 –4 ) in the hostile environments of reducing protons to H2 or CO2 to carbona- for water oxidation. Homogeneous catalysts required to oxidize water. These catalysts are ceous fuels on a global scale. Thus, while there may be many options for reduction catalysts, A B redox cycling inevitably requires the coupling Downloaded from of reduction reactions to water oxidation (see Light energy the fi gure, panel A). On page 342 of this issue, Yin et al. (1 ) report on a water-soluble water oxidation catalyst that has a reaction center containing four cobalt (Co) atoms. Its sur- 2H (or CH O + H O) WOC 2 2 2 rounding ligands are not organic groups but 2H2O 9− e– are polyoxotungstate (PW9O34 ) anions that 4 resemble the solid oxide supports of hetero- + 4H (+ CO2) geneous catalysts. This catalyst weds the best O + 4H+ features of extant heterogeneous and homo- 2 geneous catalysts while remedying many of Photoactive their respective disadvantages. element The task of developing a catalyst that meets the diverse criteria for practical applications is daunting. The half-reaction comprising water Artificial photosynthesis. (A) A water oxidation catalyst (WOC) transfers electrons from water to photoactive assemblies that promote photoinitiated charge separation for reduction of protons to hydrogen or carbon oxidation, 2H O → O + 4H+ + 4e−, is a chem- 2 2 dioxide to organic compounds. In this generic scheme, the photoactive device comprising a photosensitizer ist’s nightmare, in that it involves multiple and reduction catalyst might be assembled on an electrode, suspensions of small particles, or conducting bond rearrangements between two molecules organic or inorganic membranes capable of directing charge transport. Formaldehyde is shown as one of

several possible CO2 reduction products. (B) A schematic drawing of the water oxidation catalyst reported et al Department of Chemistry, Washington State University, by Yin . depicts the Co4O16 core with cobalt atoms in purple and oxygen atoms in red. The WO6 units are Pullman, WA 99164–4630, USA. E-mail: [email protected] depicted as gray octahedra, and phosphate groups are shown as orange tetrahedra.

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