(Table 1) Os and Sr isotopes in low-temperature hydrothermal fluids from the Juan de Fuca Ridge at ODP Site 168-1026B

We present Os and Sr isotopes and Mg, Os, and Sr concentrations for ridge-crest high-temperature and diffuse hydrothermal fluids, plume fluids and ridge-flank warm spring fluids from the Juan de Fuca Ridge. The data are used to evaluate the extent to which (1) the high- and low-temperature hydrothermal alteration of mid-ocean ridge basalts (MORBs) provides Os to the deep oceans, and (2) hydrothermal contributions of non-radiogenic Os and Sr to the oceans are coupled. The Os and Sr isotopic ratios of the high-temperature fluids (265-353°C) are dominated by basalts (187Os/188Os = 0.2; 87Sr/86Sr = 0.704) but the concentrations of these elements are buffered approximately at their seawater values. The 187Os/188Os of the hydrothermal plume fluids collected ~1 m above the orifice of Hulk vent is close to the seawater value (=1.05). The low-temperature diffuse fluids (10-40°C) associated with ridge-crest high-temperature hydrothermal systems on average have [Os] = 31 fmol/kg, 187Os/188Os = 0.9 and [Sr] = 86 µmol/kg, 87Sr/86Sr = 0.709. They appear to result from mixing of a high-temperature fluid and a seawater component. The ridge-flank warm spring fluids (10-62°C) on average yield [Os] = 22 fmol/kg, 187Os/188Os = 0.8 and [Sr] = 115 µmol/kg, 87Sr/86Sr = 0.708. The data are consistent with isotopic exchange of Os and Sr between basalt and circulating seawater during low-temperature hydrothermal alteration. The average Sr concentration in these fluids appears to be similar to seawater and consistent with previous studies. In comparison, the average Os concentration is less than seawater by more than a factor of two. If these data are representative they indicate that low-temperature alteration of MORB does not provide adequate non-radiogenic Os and that another source of mantle Os to the oceans must be investigated. At present, the magnitude of non-radiogenic Sr contribution via low-temperature seawater alteration is not well constrained. If non-radiogenic Sr to the oceans is predominantly from the alteration of MORB, our data suggest that there must be a different source of non-radiogenic Os and that the Os and Sr isotope systems in the oceans are decoupled.

Auxin Deprivation Induces Synchronous Golgi Differentiation in Suspension-Cultured Tobacco BY-2 Cells1

To date, the lack of a method for inducing plant cells and their Golgi stacks to differentiate in a synchronous manner has made it difficult to characterize the nature and extent of Golgi retailoring in biochemical terms. Here we report that auxin deprivation can be used to induce a uniform population of suspension-cultured tobacco (Nicotiana tabacum cv BY-2) cells to differentiate synchronously during a 4-d period. Upon removal of auxin, the cells stop dividing, undergo elongation, and differentiate in a manner that mimics the formation of slime-secreting epidermal and peripheral root-cap cells. The morphological changes to the Golgi apparatus include a proportional increase in the number of trans-Golgi cisternae, a switch to larger-sized secretory vesicles that bud from the trans-Golgi cisternae, and an increase in osmium staining of the secretory products. Biochemical alterations include an increase in large, fucosylated, mucin-type glycoproteins, changes in the types of secreted arabinogalactan proteins, and an increase in the amounts and types of molecules containing the peripheral root-cap-cell-specific epitope JIM 13. Taken together, these findings support the hypothesis that auxin deprivation can be used to induce tobacco BY-2 cells to differentiate synchronously into mucilage-secreting cells.

Base excision of oxidative purine and pyrimidine DNA damage in Saccharomyces cerevisiae by a DNA glycosylase with sequence similarity to endonuclease III from Escherichia coli.

One gene locus on chromosome I in Saccharomyces cerevisiae encodes a protein (YAB5_YEAST; accession no. P31378) with local sequence similarity to the DNA repair glycosylase endonuclease III from Escherichia coli. We have analyzed the function of this gene, now assigned NTG1 (endonuclease three-like glycosylase 1), by cloning, mutant analysis, and gene expression in E. coli. Targeted gene disruption of NTG1 produces a mutant that is sensitive to H2O2 and menadione, indicating that NTG1 is required for repair of oxidative DNA damage in vivo. Northern blot analysis and expression studies of a NTG1-lacZ gene fusion showed that NTG1 is induced by cell exposure to different DNA damaging agents, particularly menadione, and hence belongs to the DNA damage-inducible regulon in S. cerevisiae. When expressed in E. coli, the NTG1 gene product cleaves plasmid DNA damaged by osmium tetroxide, thus, indicating specificity for thymine glycols in DNA similarly as is the case for EndoIII. However, NTG1 also releases formamidopyrimidines from DNA with high efficiency and, hence, represents a glycosylase with a novel range of substrate recognition. Sequences similar to NTG1 from other eukaryotes, including Caenorhabditis elegans, Schizosaccharomyces pombe, and mammals, have recently been entered in the GenBank suggesting the universal presence of NTG1-like genes in higher organisms. S. cerevisiae NTG1 does not have the [4Fe-4S] cluster DNA binding domain characteristic of the other members of this family.

(Table 1) Concentrations of Os, Ir, Pt and 187Os/188Os ratio in ODP Hole 130-806C samples

New osmium (Os) isotope and platinum group element (PGE) concentration data are used in conjunction with published 3He and Th isotope data to determine the relative proportions of lithogenic, extraterrestrial and hydrogenous iridium (Ir) in a Pacific pelagic carbonate sequence from the Ocean Drilling Program (ODP) Site 806 on the Ontong Java Plateau (OJP). These calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary Ir budget are minor, while hydrogenous Ir accounts for roughly 85% of the total Ir. Application of analogous partitioning calculations to previously reported data from a North Pacific red clay sequence (LL44-GPC3) yields very similar results. Total Ir burial fluxes at Site 806 and LL44-GPC3 are also similar, 45 and 30 pg/cm**2/kyr, respectively. Average Ir/3He and Ir/xs230Th_initial ratios calculated from the entire Site 806 data set are similar to those reported earlier for Pacific sites. In general, down-core profiles of Ir, 3He and xs230Th_initial, are not well correlated with one another. However, all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two. While these results indicate that Ir concentration has potential as a point-paleoflux tracer in pelagic carbonates, Ir-based paleoflux estimates are likely subject to uncertainties that are similar to those associated with Co-based paleoflux estimates. Consequently, local calibration of Ir flux in space and time will be required to fully assess the potential of Ir as a point paleoflux tracer. Measured 187Os/188Os of the OJP sediments are systematically lower than the inferred 187Os/188Os of contemporaneous seawater and a clear glacial-interglacial 187Os/188Os variation is lacking. Mixing calculations suggest Os contributions from lithogenic sources are insufficient to explain the observed 187Os/188Os variations. The difference between the 187Os/188Os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic Os carried by particulate extraterrestrial material. Down-core variations of 187Os/188Os with Pt/Ir and Os/Ir also point to contributions from extraterrestrial particles. Mixing calculations for each set of several triplicate analyses suggest that the unradiogenic Os end member cannot be characterized by primary extraterrestrial particles of chondritic composition. It is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions, 187Os/188Os of pelagic carbonates has greater potential compared to abundances of PGE. An attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial Os in the OJP samples and previously reported extraterrestrial Os flux. Throughout most of the OJP record, Os isotope-based paleoflux estimates are within a factor of two of those derived using other constant flux tracers. Meaningful flux estimates cannot be made during glacial maxima because the OJP sediments do not record the low 187Os/188Os reported previously. We speculate that this discrepancy may be related to focusing of extraterrestrial particles at the OJP, as has been suggested to explain down-core 3He variations.