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Elemental Composition: Orbital and in situ Surface Measurements
C. N. Foley, Department of Terrestrial Magnetism, Carnegie Institute of Washington, 5241 Broad Branch Road, NW Washington, DC 20015-1305, USA,
T. E. Economou, Laboratory for Astrophysics & Space Res. University of Chicago, 933 East 56th Street, Chicago, IL 60637, USA,
R. N. Clayton, Enrico Fermi Institute, 5640 S. Ellis Avenue, RI 440 Chicago, IL 60637, USA,
J. Brückner, Geochemistry Department, Max Planck Institut für Chemie, PO Box 3060, Mainz D-55020, Germany,
G. Dreibus, Cosmochemistry Deparment, Max Planck Institut für Chemie, PO Box 3060, Mainz D-55020, Germany,
R. Rieder, Cosmochemistry Deparment, Max Planck Institut für Chemie, PO Box 3060, Mainz D-55020, Germany,
H. Wänke, Abteilung Kosmochemie Max Planck Institut für Chemie, PO Box 3060, Mainz D-55020, Germany
The Mars Pathfinder Alpha Proton X-ray Spectrometer (APXS) was utilized to determine the major and minor elemental abundances of rocks and soils at the 1997 landing site in Ares Vallis. The determined abundances suggest that: (1) the rocks are covered with various amounts of soil; (2) the Soil-Free Rock (SFR) chemistry is similar to that of an evolved SNC-like (SNC – Shergottite, Nakhlite, and Chassignite) igneous tholeiitic basalt-andesite to andesite that is minimally altered (possibly similar to Type 2 TES material); (3) the carbon content is below detection limits for all samples, implying < 5% as MgCO3 (Brückner et al., 1999); (4) the α-mode oxygen abundance indicates that mineral-bound water, above the value for igneous rocks, is present in some rocks and is therefore indicative of some nonigneous alteration and therefore possibly rock-rinds that obscure the petrology of the SFR; and (5) the Pathfinder soils are similar to the Viking fines and may be composed of mafic igneous material like the SNC meteorites and of volatiles deposited from volcanic emissions, as previously suggested by Clark (1993) for the Viking soils.
Recognition between leguminous plants and the specific rhizobial strains that nodulate them is mediated via a regulon of nodulation (nod) genes present in the bacteria. These nod genes are induced by flavonoids secreted from legume roots. Many of the nod gene products are involved in the synthesis of host-specific signals that are recognised by appropriate legume hosts. Recently (Lerouge et al., 1990), the signal molecule made by one strain of Rhizobium meliloti was identified as an acylated and sulphated, tetraglucosamine glycolipid and there is strong evidence that Rhizobium leguminosarum makes related but structurally distinct signals.
On the basis of these observations it is now possible to make sense of several similarities that have been recognised between nod gene products and enzymes of known function. Thus, for example, it appears that the nodM gene product is involved in the formation of glucosamine precursors of the signal molecule, whilst other gene products are likely to be involved in specific substitutions that confer host specificity to the signal molecule.
In addition to those nod gene products that are involved in the synthesis of the glycolipid, it is evident that there are other genes which may carry out a different role. Of particular interest is the nodO gene which encodes a secreted Ca2+-binding protein that has the potential to interact directly with plant cells. In the absence of the nodFEL genes, nodO is necessary for nodulation, indicating that the NodO protein can compensate for the loss of nodFEL function during infection.
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