NMR study of parallel-stranded tetraplex formation by the hexadeoxynucleotide d(TG4T).

Multistranded DNA structures based upon guanine association have been proposed to be important in the structure of chromosome telomeres and in immunoglobulin class switching. Nucleic acids containing runs of guanine bases form a number of structures in vitro, including fold-back structures (Fig. 1a) and parallel-stranded quadruplex structures in DNA and RNA. The features of fold-back structures have now been determined at high-resolution. The different structures are probably based on a tetrad of hydrogen-bonded guanine bases (Fig. 1b), with buffer conditions and sequence effects mediating isomerization between the different forms. Here we use NMR spectroscopy to investigate the solution structure of the complex formed by the hexadeoxynucleotide d(TG4T) in the presence of sodium ions. We have observed the formation of a parallel-stranded quadruplex containing hydrogen-bonded tetrads of guanine. The parallel-stranded form differs significantly from the fold-back form, with individual nucleotide conformations being closer to those of B-form DNA.     

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.     

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 x 13 x 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of 'ejecta blocks'. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a 'pond'. Properties of the 'ponds' are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.     

Subsurface water and clay mineral formation during the early history of Mars

Clay minerals, recently discovered to be widespread in Mars's Noachian terrains, indicate long-duration interaction between water and rock over 3.7 billion years ago. Analysis of how they formed should indicate what environmental conditions prevailed on early Mars. If clays formed near the surface by weathering, as is common on Earth, their presence would indicate past surface conditions warmer and wetter than at present. However, available data instead indicate substantial Martian clay formation by hydrothermal groundwater circulation and a Noachian rock record dominated by evidence of subsurface waters. Cold, arid conditions with only transient surface water may have characterized Mars's surface for over 4 billion years, since the early-Noachian period, and the longest-duration aqueous, potentially habitable environments may have been in the subsurface.     

Fluorescence energy transfer shows that the four-way DNA junction is a right-handed cross of antiparallel molecules

The four-way junction between DNA helices is the central intermediate in recombination, and the manner of its interaction with resolvase enzymes can determine the genetic outcome of the process. A knowledge of its structure is a prerequisite to understanding the interaction with proteins, and there has been recent progress. Here we use fluorescence energy transfer to determine the relative distances between the ends of a small DNA junction, and hence the path of the strands. Our results are consistent with the geometry of an 'X'. The interconnected helices are juxtaposed so that the continuous strands of each helix generate an antiparallel alignment, and the two interchanged strands do not cross at the centre. The acute angle of the X structure is defined by a right-handed rotation of the helical axes about the axis perpendicular to the X plane, as viewed from the centre of the X.     

Shoemaker crater as the source of most ejecta blocks on the asteroid 433 Eros

The loose material--regolith--on the surfaces of asteroids is thought to represent ballistically emplaced ejecta from impacts but the identification of source craters and the detailed study of the regolith modification have been hampered by the limited spatial resolution and area coverage of the few asteroids imaged by spacecraft. Here we report the results of global mapping of the asteroid 433 Eros from high-resolution images obtained by the NEAR-Shoemaker spacecraft. Based on the images and ejecta-emplacement models, we suggest that most large ejecta blocks on Eros originate from a relatively young 7.6-km-diameter crater. A large fraction of the ejecta from impacts pre-dating that crater has apparently been buried or eroded. The images also show evidence for the action of a variety of sorting environments for regolith particles after they are deposited on the surface.     

The nature of ponded deposits on Eros

One of the surprises of the NEAR-Shoemaker mission was that Eros's surface exhibits a wide variety of landforms, which are indicative of a global covering of loose fragmental debris. At one extreme in roughness is the Shoemaker Regio area, which is characterized by a high density of boulders up to 100 m across, slumps, slides, and finer blanketing material. At the other extreme are distinctive, flat deposits that appear smooth down to a resolution of 1.2 cm per pixel. Here we report the results of global mapping and colour analysis of these smooth deposits. They have formed most efficiently in restricted areas, and appear to be the result of deposition of finer material sorted from the upper portion of the asteroid's regolith. The smooth deposits constitute a family of features with a range of morphologies, but all appear to be the result of sedimentation. The geography of the deposits is consistent with some predicted aspects of photoelectric sorting, but these exotic transport and depositional mechanisms are not well understood. Deposits with the properties seen on Eros have no obvious analogues in previous lunar or asteroid data.