Escherichia coli possesses two distinct nitrite reductase enzymes encoded by the nrfA and nirB operons. The expression of each operon is induced during anaerobic cell growth conditions and is further modulated by the presence of either nitrite or nitrate in the cells' environment. To examine how each operon is expressed at low, intermediate, and high levels of either nitrate or nitrite, anaerobic chemostat culture techniques were employed using nrfA-lacZ and nirB-lacZ reporter fusions. Steady-state gene expression studies revealed a differential pattern of nitrite reductase gene expression where optimal nrfA-lacZ expression occurred only at low to intermediate levels of nitrate and where nirB-lacZ expression was induced only by high nitrate conditions. Under these conditions, the presence of high levels of nitrate suppressed nrfA gene expression. While either NarL or NarP was able to induce nrfA-lacZ expression in response to low levels of nitrate, only NarL could repress at high nitrate levels. The different expression profile for the alternative nitrite reductase operon encoded by nirBDC under high-nitrate conditions was due to transcriptional activation by either NarL or NarP. Neither response regulator could repress nirB expression. Nitrite was also an inducer of nirB and nrfA gene expression, but nitrate was always the more potent inducer by >100-fold. Lastly, since nrfA operon expression is only induced under low-nitrate concentrations, the NrfA enzyme is predicted to have a physiological role only where nitrate (or nitrite) is limiting in the cell environment. In contrast, the nirB nitrite reductase is optimally synthesized only when nitrate or nitrite is in excess of the cell's capacity to consume it. Revised regulatory schemes are presented for NarL and NarP in control of the two operons.Escherichia coli possesses two biochemically distinct nitrite reductase enzymes encoded by the nrfABCDEFG and nirBDC operons (4). The NirB nitrite reductase is a soluble sirohemecontaining enzyme that uses NADH as an electron donor to reduce nitrite in the cytoplasm. The NrfA nitrite reductase is a membrane-associated respiratory enzyme that couples to the membrane-associated formate-oxidizing enzymes via quinones in order to generate membrane potential. The abundance of each enzyme is elevated during anaerobic cell growth conditions when either nitrate and/or nitrite is present (14). Nitrite, the substrate for the two enzymes, must either be encountered environmentally or generated by the cell from nitrate reduction by one of the three E. coli nitrate reductases.Expression of the nrfABCDEFG operon (previously described as aeg-93 [3]) and the nirBDC operon is elevated during anaerobic cell growth by the Fnr regulatory protein (1, 10, 14). The addition of nitrite, but not nitrate, is reported to further elevate nrfA expression via either the NarL or NarP response regulators. In contrast, NarL is reported to repress nrfA expression in response to nitrate, whereas NarP cannot (14-16, 23, 24). Expression of the nirB operon...

Escherichia coli synthesizes two biochemically distinct nitrate reductase enzymes, a membrane-bound enzyme encoded by thenarGHJI operon and a periplasmic cytochromec-linked nitrate reductase encoded by thenapFDAGHBC operon. To address why the cell makes these two enzymes, continuous cell culture techniques were used to examinenapF and narG gene expression in response to different concentrations of nitrate and/or nitrite. Expression of thenapF-lacZ and narG-lacZ reporter fusions in strains grown at different steady-state levels of nitrate revealed that the two nitrate reductase operons are differentially expressed in a complementary pattern. The napF operon apparently encodes a “low-substrate-induced” reductase that is maximally expressed only at low levels of nitrate. Expression is suppressed under high-nitrate conditions. In contrast, the narGHJI operon is only weakly expressed at low nitrate levels but is maximally expressed when nitrate is elevated. The narGHJI operon is therefore a “high-substrate-induced” operon that somehow provides a second and distinct role in nitrate metabolism by the cell. Interestingly, nitrite, the end product of each enzyme, had only a minor effect on the expression of either operon. Finally, nitrate, but not nitrite, was essential for repression of napF gene expression. These studies reveal that nitrate rather than nitrite is the primary signal that controls the expression of these two nitrate reductase operons in a differential and complementary fashion. In light of these findings, prior models for the roles of nitrate and nitrite in control ofnarG and napF expression must be reconsidered.

Escherichia coli possesses three distinct formate dehydrogenase enzymes encoded by the fdnGHI, fdhF, and fdoGHI operons. To examine how two of the formate dehyrogenase operons (fdnGHI and fdhF) are expressed anaerobically in the presence of low, intermediate, and high levels of nitrate, nitrite, and formate, chemostat culture techniques were employed with fdnG-lacZ and fdhF-lacZ reporter fusions. Complementary patterns of gene expression were seen. Optimal fdhF-lacZ expression occurred only at low to intermediate levels of nitrate, while high nitrate levels caused up to 10-fold inhibition of gene expression. In contrast, fdnG-lacZ expression was induced 25-fold in the presence of intermediate to high nitrate concentrations. Consistent with prior reports, NarL was able to induce fdnG-lacZ expression. However, NarP could not induce expression; rather, it functioned as an antagonist of fdnG-lacZ expression under low-nitrate conditions (i.e., it was a negative regulator). Nitrite, a reported signal for the Nar sensory system, was unable to stimulate or suppress expression of either formate dehydrogenase operon via NarL and NarP. The different gene expression profiles of the alternative formate dehydrogenase operons suggest that the two enzymes have complementary physiological roles under environmental conditions when nitrate and formate levels are changing. Revised regulatory schemes for NarL-and NarP-dependent nitrate control are presented for each operon.Escherichia coli synthesizes three formate dehydrogenase enzymes encoded by the fdnGHI, fdhF, and fdoGHI genes (4). Each enzyme oxidizes formate to CO 2 and passes electrons to either the quinone pool or other proteins for subsequent reduction of anaerobic respiratory substrates, including nitrate, nitrite, trimethylamine N-oxide, dimethyl sulfoxide, and fumarate (13,14). These alternative electron transfer reactions allow cellular energy conservation and ATP generation via the proton-translocating ATPase.The formate dehydrogenase N (Fdh-N) enzyme is encoded by the fdnGHI operon located at 32 min on the E. coli chromosome. Synthesis of this membrane-bound enzyme is maximal under anaerobic cell growth conditions when nitrate is present (6, 12). The enzyme functions in the formate-nitrate respiratory chain by coupling to the NarG nitrate reductase for reduction of nitrate to nitrite. The nitrate induction of fdnG expression occurs at the level of transcription control, where NarL is a strong activator and NarP is a weak activator (11,20).Formate dehydrogenase H (Fdh-H) is encoded by the fdhF gene located at 92 min on the E. coli chromosome. The 80-kDa selenopolypeptide is part of the formate-hydrogen lyase complex and either is located in the cell cytoplasm or is loosely associated with the inner surface of the cytoplasmic membrane. Optimal Fdh-H synthesis requires anaerobic conditions and the presence of formate. Induction of fdhF transcription occurs via the formate-dependent FHLA regulator in combination with sigma-54 polymerase (3). It has also been proposed tha...

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is widespread in non-poikilitic regions, occurring interstitially between olivine and pyroxene grains. It is the main REE carrier in GRV 99027 and has relatively higher REEs (200-1000 × CI) than that of other lherzolitic shergottites. A REE budget calculation for GRV 99027 yields a whole rock REE pattern very similar to that of other lherzolites. It is characterized by the distinctive light REE depletion and a smooth increase from light REEs to heavy REEs. REE microdistributions in GRV 99027 strongly support the idea that all lherzolitic shergottites formed by identical igneous processes, probably from the same magma chamber on Mars. Despite many similarities in mineralogy, petrography, and trace element geochemistry, subtle differences exist between GRV 99027 and other lherzolitic shergottites. GRV 99027 has relatively uniform mineral compositions (both major elements and REEs), implying that it suffered a higher degree of sub-solidus equilibration than the other three lherzolites. It is notable that GRV 99027 has experienced terrestrial weathering in the Antarctic environment, as its olivine and pyroxenes commonly display a light REE enrichment and a negative Ce anomaly. Caution needs to be taken in future chronological studies.

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Ice cores recovered from polar ice sheet received and preserved sulfuric acid fallout from explosive volcanic eruptions. DT263 ice core was retrieved from an east Antarctic location. The ice core is dated using a combination of annual layer counting and volcanic time stratigraphic horizon as 780 years (1215-1996 A.D.). The ice core record demonstrates that during the period of approximately 1460-1800 A.D., the accumulation is sharply lower than the levels prior to and after this period. This period coincides with the most recent neoglacial climatic episode, the "Little Ice Age (LIA)", that has been found in numerous Northern Hemisphere proxy and historic records.The non-sea-salt 2 4 SO − concentrations indicate seventeen volcanic events in DT263 ice core. Compared with those from previous Antarctic ice cores, significant discrepancies are found between these records in relative volcanic flux of several well-known events. The discrepancies among these records may be explained by the differences in surface topography, accumulation rate, snow drift and distribution which highlight the potential impact of local glaciology on ice core volcanic records, analytical techniques used for sulfate measurement, etc. Volcanic eruptions in middle and high southern latitudes affect volcanic records in Antarctic snow more intensively than those in the low latitudes.

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