Changes In Nitrogen Source Research Articles

Increase in Agricultural-Derived NHx and Decrease in Coal Combustion-Derived NOx Result in Atmospheric Particulate N-NH4+ Surpassing N-NO3- in the South China Sea.

The atmospheric deposition of anthropogenic active nitrogen significantly influences marine primary productivity and contributes to eutrophication. The form of nitrogen deposition has been evolving annually, alongside changes in human activities. A disparity arises between observation results and simulation conclusions due to the limited field observation and research in the ocean. To address this gap, our study undertook three field cruises in the South China Sea in 2021, the largest marginal sea of China. The objective was to investigate the latest atmospheric particulate inorganic nitrogen deposition pattern and changes in nitrogen sources, employing nitrogen-stable isotopes of nitrate (δ15N-NO3-) and ammonia (δ15N-NH4+) linked to a mixing model. The findings reveal that the N-NH4+ deposition generally surpasses N-NO3- deposition, attributed to a decline in the level of NOx emission from coal combustion and an upswing in the level of NHx emission from agricultural sources. The disparity in deposition between N-NH4+ and N-NO3- intensifies from the coast to the offshore, establishing N-NH4+ as the primary contributor to oceanic nitrogen deposition, particularly in ocean background regions. Fertilizer (33 ± 21%) and livestock (20 ± 6%) emerge as the primary sources of N-NH4+. While coal combustion continues to be a significant contributor to marine atmospheric N-NO3-, its proportion has diminished to 22 (Northern Coast)-35% (background area) due to effective NOx emission controls by the countries surrounding the South China Sea, especially the Chinese Government. As coal combustion's contribution dwindles, the significance of vessel and marine biogenic emissions grows. The daytime higher atmospheric N-NO3- concentration and lower δ15N-NO3- compared with nighttime further underscore the substantial role of marine biogenic emissions.

Effect of nitrogen source and spray conditions on the integral biotrickling filter operation for hydrophobic volatile organic compounds

A nutrient solution containing major and trace elements plays a crucial role in biotrickling filter (BTF) operation in practical applications. This study conducted a series of experiments on the BTF packed with integral filters using two types of nitrogen sources under different spray conditions. The rapid start-up was observed in the BTF feeding with ammonium tartrate-N (AT-N) nutrient solution. Heterotrophic nitrification and denitrification were observed by monitoring the concentration of ammonia-N, nitrite-N, nitrate-N, PO43− and SO42− in the nutrient solution. The BTF performance under two different types of the nutrient solutions were detected up to ∼90% at the inlet toluene concentration of 300–700 mg∙m−3. The BTF packed with integral filters exhibited the stable performance with the change of nitrogen source and could be steadily operated at a wide pH range of 5.5–8.5. To illustrate the effect of the two types of nitrogen sources on the biofilm, metagenomic sequencing was used to assess the microbial diversity and functional genes of biofilm on the integral filters. The results showed that the application of AT-N nutrient solution increased the functional genes of denitrification in the microorganisms on the filters. Furthermore, the more stable performance of BTF was achieved by using the AT-N nutrient solutions, which might be beneficial to the long-term operation of BTF packed with honeycomb integral packing.

Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ18O-NO3- and δ15N-NO3- observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO3- isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ18O-NO3- (+13‰) compared with δ15N-NO3- (−2‰) on average during groundwater table rise was due to a combination of factors, including high 18O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and 18O-enriched O2 produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate.

Regulation of Lateral Root Development by Shoot-Sensed Far-Red Light via HY5 Is Nitrate-Dependent and Involves the NRT2.1 Nitrate Transporter.

Plants are very effective in responding to environmental changes during competition for light and nutrients. Low Red:Far-Red (low R:FR)-mediated neighbor detection allows plants to compete successfully with other plants for available light. This above-ground signal can also reduce lateral root growth by inhibiting lateral root emergence, a process that might help the plant invest resources in shoot growth. Nitrate is an essential nutrient for plant growth and Arabidopsis thaliana responds to low nitrate conditions by enhancing nutrient uptake and reducing lateral and main root growth. There are indications that low R:FR signaling and low nitrate signaling can affect each other. It is unknown which response is prioritized when low R:FR light- and low nitrate signaling co-occur. We investigated the effect of low nitrate conditions on the low R:FR response of the A. thaliana root system in agar plate media, combined with the application of supplemental Far-Red (FR) light to the shoot. We observed that under low nitrate conditions main and lateral root growth was reduced, but more importantly, that the response of the root system to low R:FR was not present. Consistently, a loss-of-function mutant of a nitrate transporter gene NRT2.1 lacked low R:FR-induced lateral root reduction and its root growth was hypersensitive to low nitrate. ELONGATED HYPOCOTYL5 (HY5) plays an important role in the root response to low R:FR and we found that it was less sensitive to low nitrate conditions with regards to lateral root growth. In addition, we found that low R:FR increases NRT2.1 expression and that low nitrate enhances HY5 expression. HY5 also affects NRT2.1 expression, however, it depended on the presence of ammonium in which direction this effect was. Replacing part of the nitrogen source with ammonium also removed the effect of low R:FR on the root system, showing that changes in nitrogen sources can be crucial for root plasticity. Together our results show that nitrate signaling can repress low R:FR responses and that this involves signaling via HY5 and NRT2.1.

The C2 domain of the ubiquitin ligase Rsp5 is required for ubiquitination of the endocytic protein Rvs167 upon change of nitrogen source.

Ubiquitination is a key signal for endocytosis of proteins on the plasma membrane. The ubiquitin ligase Rsp5 of Saccharomyces cerevisiae, which contains an amino-terminal membrane-binding C2 domain, three substrate-recognizing tryptophan-tryptophan (WW) domains and a carboxyl-terminal catalytic homologous to the E6-AP carboxyl terminus (HECT) domain, can ubiquitinate plasma membrane proteins directing them for endocytosis. Here, we examined the roles of the C2 domain in endocytosis for the downregulation of the general amino acid permease Gap1, which is one of nitrogen-regulated permeases in S. cerevisiae. First, we constructed several rsp5 mutants producing Rsp5 variants without the C2 domain or with amino acid changes of membrane-binding lysine residues. These mutants showed defects in endocytosis of Gap1 in response to a preferred nitrogen source. Intriguingly, we found that ubiquitination of Gap1 in these mutant cells was highly similar to that in wild-type cells during endocytosis. These results indicate that the C2 domain is essential for endocytosis but not for ubiquitination of substrates such as Gap1. Moreover, genetic and biochemical analyses showed that the endocytic protein Rvs167 was ubiquitinated via Rsp5 and the C2 domain was required for efficient ubiquitination in response to a preferred nitrogen source. Here, we propose a mechanism for the C2 domain-mediated endocytosis of plasma membrane permeases.

Similar Glacial‐Interglacial δ15N Variations in Two MIS 13–10 Sediment Sequences in the Western North Atlantic Ocean: Changes in Nitrogen Sources, Denitrification, or Diagenesis?

AbstractWe have determined total organic carbon‐mass accumulation rates (TOC‐MARs) and δ15N values in Marine Isotope Stages (MIS) 13–10 sediments from two oligotrophic parts of the western North Atlantic Ocean—Ocean Drilling Program (ODP) Site 1063 on the Bermuda Rise and Site 1058 on the Blake Outer Ridge. Both TOC‐MARs and δ15N values vary significantly in these depositional records. TOC‐MARs are highest in sediments deposited at Site 1063 during MIS 12 and MIS 10, implying marine productivity or TOC preservation increased at this location during glacial stages, and low TOC‐MARs during most of MIS 11 in Site 1063 sediment imply low rates during this warm interval. In contrast to the Site 1063 record, productivity or preservation appears to have been greater at Site 1058 during parts of the MIS 11 interglacial than during the glacial intervals. Unlike the TOC‐MAR records, δ15N values at both locations exhibit similar glacial‐interglacial alternations. Values peak during substage 11.3 and gradually decrease toward the terminations of MIS 12 and MIS 10. The similarity of the δ15N patterns at these two sites implies that nitrogen cycling at these locations is largely independent of changes in local surface production of organic matter, and the higher δ15N values are unlikely to result from denitrification. Instead, the δ15N alternations likely result from glacial‐interglacial alternations between higher rates of sediment delivery to these western North Atlantic locations during glacial periods that favored TOC preservation and lower rates during interglacials that allowed more TOC degradation and associated diagenetic alteration of its nitrogen isotopic composition, leading to higher sediment δ15N values.

Recent Changes in Nitrogen Sources and Load Components to Estuaries of the Contiguous United States.

Regional Spatially Referenced Regressions on Watershed models were used to update 2002 delivered nitrogen (N) loads to estuaries of the contiguous US for 2011, supplemented by direct estuarine atmospheric deposition from the Community Multiscale Air Quality Model. Median 2011 watershed N yields were greatest for the Puget Trough, Virginian. and Oregon-Washington-Vancouver Coast marine ecoregions (MEs; 13. 7, 11.0, and 9.9 kg N/ha watershed/year, respectively); intermediate for the Floridian, Southern California Bight, and Northern California MEs (4.4-6.3 kg N/ha watershed/year); and lowest for the Northern Gulf of Mexico, Carolinian, and Gulf of Maine MEs (2.4-3.2 kg N/ha watershed/year). Dominant sources varied across marine ecoregions, with direct atmospheric deposition as the dominant source only in the far northern Gulf of Maine ME. Delivered N loads from atmospheric deposition have significantly decreased (p < 0.05) for most estuaries on the Atlantic and Gulf coasts for 2002-2012. Estimated point source delivered N loads for 2002-2012 increased for most estuaries with upstream treatment plants, with estimated loads to only seven estuaries decreasing by more than 50%. Urban runoff increased for most estuaries in the Puget Trough and Carolinian MEs and either increased or had no significant trend for the remaining marine ecoregions. The magnitude of change in total N delivered loads is uncertain due to incomplete monitoring for most minor dischargers. In areas with increased population growth and decreases in agricultural land, decreasing agricultural fertilizer inputs have been insufficient to offset increases in urban runoff.

Coral Skeleton \u03b415N as a Tracer of Historic Nutrient Loading to a Coral Reef in Maui, Hawaii

Excess nutrient loading to nearshore environments has been linked to declining water quality and ecosystem health. Macro-algal blooms, eutrophication, and reduction in coral cover have been observed in West Maui, Hawaii, and linked to nutrient inputs from coastal submarine groundwater seeps. Here, we present a forty-year record of nitrogen isotopes (δ15N) of intra-crystalline coral skeletal organic matter in three coral cores collected at this site and evaluate the record in terms of changes in nitrogen sources. Our results show a dramatic increase in coral δ15N values after 1995, corresponding with the implementation of biological nutrient removal at the nearby Lahaina Wastewater Reclamation Facility (LWRF). High δ15N values are known to be strongly indicative of denitrification and sewage effluent, corroborating a previously suggested link between local wastewater injection and degradation of the reef environment. This record demonstrates the power of coral skeletal δ15N as a tool for evaluating nutrient dynamics within coral reef environments.

Sugar-stimulated CO2 sequestration by the green microalga Chlorella vulgaris

To convert waste CO2 from flue gases of power plants into value-added products, bio-mitigation technologies show promise. In this study, we cultivated a fast-growing species of green microalgae, Chlorella vulgaris, in different sizes of photobioreactors (PBRs) and developed a strategy using small doses of sugars for enhancing CO2 sequestration under light-emitting diode illumination. Glucose supplementation at low levels resulted in an increase of photoautotrophic growth-driven biomass generation as well as CO2 capture by 10% and its enhancement corresponded to an increase of supplied photon flux. The utilization of urea instead of nitrate as the sole nitrogen source increased photoautotrophic growth by 14%, but change of nitrogen source didn't compromise glucose-induced enhancement of photoautotrophic growth. The optimized biomass productivity achieved was 30.4% higher than the initial productivity of purely photoautotrophic culture. The major pigments in the obtained algal biomass were found comparable to its photoautotrophic counterpart and a high neutral lipids productivity of 516.6 mg/(L·day) was achieved after optimization. A techno-economic model was also developed, indicating that LED-based PBRs represent a feasible strategy for converting CO2 into value-added algal biomass.

Effect of the change of nitrogen source on photosynthesis and respiration of Scots pine (Pinus silvestris L.) seedlings

Rates of photosynthesis and respiration of Scots pine seedlings, grown in water culture under conditions of nitrogen deficiency and supplied with nitrogen in ammonium or nitrate form, were investigated. Besides, the rates of photosynthesis and respiration of the seedlings were measured, after changing the form of supplied nitrogen. A rapid and different response of nitrogen deficient seedlings was found independent on the form of supplied nitrogen. The investigations also showed a change in rates of photosynthesis and respiration of seedlings after changing the form of nitrogen.

Conversion of post consumer polyethylene to the biodegradable polymer polyhydroxyalkanoate

A process for the conversion of post consumer (agricultural) polyethylene (PE) waste to the biodegradable polymer medium chain length polyhydroxyalkanoate (mcl-PHA) is reported here. The thermal treatment of PE in the absence of air (pyrolysis) generated a complex mixture of low molecular weight paraffins with carbon chain lengths from C8 to C32 (PE pyrolysis wax). Several bacterial strains were able to grow and produce PHA from this PE pyrolysis wax. The addition of biosurfactant (rhamnolipids) allowed for greater bacterial growth and PHA accumulation of the tested strains. Some strains were only capable of growth and PHA accumulation in the presence of the biosurfactant. Pseudomonas aeruginosa PAO-1 accumulated the highest level of PHA with almost 25% of the cell dry weight as PHA when supplied with the PE pyrolysis wax in the presence of rhamnolipids. The change of nitrogen source from ammonium chloride to ammonium nitrate resulted in faster bacterial growth and the earlier onset of PHA accumulation. To our knowledge, this is the first report where PE is used as a starting material for production of a biodegradable polymer.

NITROGEN SOURCE EFFECT ON COPPER SPECIATION WITHIN ECTOMYCORRHIZAL FUNGUS RHIZOPOGON RUBESCENS

Aim. The aim of this work was to study the effects of nitrogen source on the speciation of copper accumulated by fungi and ectomycorrhizas grown in the presence of copper phosphate. Materials and Methods. Ectomycorrhizal fungus Rhizopogon rubescens and its symbiotic ectomycorrhizal association with Scots Pine were grown in the presence of copper phosphate at different nitrogen sources: either ammonium or nitrate. The coordination of copper released from copper phosphate and bioaccumulated by fungus and ectomycorrhiza was determined by using synchrotron-based X-ray absorption spectroscopy (XAS). Results. XAS data showed that the change of nitrogen source from ammonium to nitrate was shifting copper coordination by mixed oxygen ligands towards oxalate coordination of copper. Conclusions. It was found that the conditions of growth of fungi and mycorrhizas may change the nature of ligands available for sequestering toxic metals and coordination of copper within ectomycorrhizal fungus R. rubescens and its symbiotic association with pine roots depended on nitrogen source.

Potential of δ13C and δ15N of cladoceran subfossil exoskeletons for paleo-ecological studies

Stable isotope analyses on cladoceran subfossil exoskeletons retrieved from sediment cores could allow the reconstruction of past changes in lake food webs provided the δ13C and δ15N values of the exoskeletons reflect those of the organisms’ whole body. The relationships between the C and N stable isotope compositions of the exoskeletons and those of the whole body were investigated for two freshwater cladoceran taxa (Bosmina sp. and Daphnia sp.) from modern samples. The C and N stable isotope compositions of the exoskeleton and those of the whole body were strongly correlated. Exoskeleton δ13C was similar to the whole body δ13C for both taxa. Daphnia exoskeletons were strongly depleted in 15N (−7.9‰) compared to the whole body. Stable isotope analyses were thereafter performed on cladoceran remains from five downcore samples from Lake Annecy, France. Results showed that Bosmina δ15N values increased by more than 4‰, between the early twentieth and twenty first centuries. Such changes might be the result of changes in nitrogen sources or cycling in the lake and/or of major shifts in Bosmina trophic position within the lake food web. This study sets up the potential of stable isotope analyses performed on cladoceran subfossil remains for paleo-ecological purposes.

Seasonality in lake pelagic 15N values: patterns, possible explanations, and implications for food web baselines

We studied the seasonal δ 15 N patterns in particulate organic matter (POM <48 μm) and crustacean zooplankton collected from a moderately eutrophic lake. Time series of POM and zooplankton showed consistent differences in δ 15 N values between POM and different zooplankton taxa, but similar trends of steadily increasing δ 15 N values through summer with peak values in autumn. The observed annual range in δ 15 N was over 25 %o in POM and almost 17 %o in cladocera. We relate these marked seasonal changes in δ 15 N values to temporal variation in inorganic nitrogen source for pelagic primary production. Ammonium (NH 4 + ) and nitrate (NO,) showed distinct δ 15 N values, ammonium being considerably 15 N-enriched (δ 15 N values ∼20-30%o) compared to nitrate (<10%o). The peak δ 15 N values in POM and zooplankton can hence be explained by a shift from nitrate uptake to ammonium uptake by phytoplankton, as ammonium concentrations in the Jyvasjarvi epilimnion increase after lake turnover in autumn. However, such elevated δ 15 N values were not observed during the high NH 4 + concentrations after spring lake turnover, suggesting that the phytoplankton spring bloom was primarily supported by nitrate. These apparent seasonal differences in nitrogen utilisation reflect variation in species composition of the phytoplankton community. We conclude that seasonal changes in nitrogen source for phytoplankton can produce considerable differences in their δ 15 N values, which are transmitted up the food chain to crustacean zooplankton. This has important implications for many food web studies using nitrogen stable isotopes, especially when δ 15 N values have to be assigned to a trophic baseline.

USING STABLE ISOTOPES TO MONITOR ANTHROPOGENIC NITROGEN INPUTS TO ESTUARIES

Use of stable nitrogen isotope ratios is one method that has been proposed to indicate anthropogenic nutrient enrichment in estuarine systems. However, the role of stable isotopes as a tool in long-term ecosystem monitoring has not been fully developed. Resident producer and consumer species were collected from marshes dominated by Spartina alterniflora and subject to a range of anthropogenic impacts in Cape Cod, Massachusetts, and in Great South Bay and Jamaica Bay, New York. Tissue isotope ratios of Spartina alterniflora, Ulva lactuca, Fundulus heteroclitus, and Geukensia demissa were analyzed in order to determine which organisms are the most sensitive indicators of changes in anthropogenic nitrogen source and loading. Power analysis was used to determine the sample sizes necessary to detect change in nutrient source using the species sampled. Relationships between the delta15N values of the species sampled and watershed population density and residential development were evaluated. Population density was a better indicator of anthropogenic nitrogen impact than residential development, since most anthropogenic nitrogen in the study marshes was derived from wastewater. Consumer species demonstrated lower within-site variability than producer species and would therefore require smaller sample sizes to detect changes in nitrogen source and loading.

DIGESTIBILIDADE DA FRAÇÃO NITROGENADA EM BOVINOS ALIMENTADOS COM RAÇÕES CONTENDO DIFERENTES FONTES DE NITROGENIO

O objetivo do presente trabalho foi estudar a digestibilidade da fração nitrogenada em animais alimentados com diferentes fontes de nitrogênio (N). As dietas foram fornecidas a bezerros da raça Holandesa, canulados no abomaso, com idade aproximada de 8 meses e peso médio de 187 kg, totalizando 21 animais escolhidos aleatoriamente, formando o grupo de repetições experimentais. As rações experimentais foram constituídas de 40 % de volumoso (feno de Capim-de-Rhodes) e 60 % de concentrado (milho, farelo de algodão, levedura, uréia, melaço e minerais), sendo: RA: ração com farelo de algodão; RL: ração com levedura; RU: ração com uréia. A quantidade de N ingerido em g/dia e em relação ao peso metabólico não apresentou diferenças significativas (P &gt; 0,05) à medida que se variou a fonte nitrogenada das dietas. O N presente no abomaso (g/ dia) também não apresentou diferenças significativas entre as dietas (P &gt; 0,05). A digestibilidade aparente do N foi superior (P &lt; 0,05) na dieta RU (53,61 %) em relação às dietas RA e RL (46,66 e 46,37 %, respectivamente). As dietas RA e RL, apesar de apresentarem maiores quantidades de nitrogênio não-amoniacol (NNA) presentes no abomaso (71,75 e 72,38 g/dia respectivamente) em relação à dieta RU (P &lt; 0,05), também foram as que apresentaram maiores quantidades eliminadas nas fezes (37,74 e 41,20 g/dia, respectivamente); isto contribuiu para que a quantidade de NNA absorvida no intestino (g/dia) não apresentasse diferenças significativas entre as dietas (P &gt; 0,05). As fontes nitrogenadas das dietas interferiram sobre a digestibilidade aparente do nitrogênio.

Structure theorems and the dynamics of nitrogen catabolite repression in yeast.

By using current biological understanding, a conceptually simple, but mathematically complex, model is proposed for the dynamics of the gene circuit responsible for regulating nitrogen catabolite repression (NCR) in yeast. A variety of mathematical "structure" theorems are described that allow one to determine the asymptotic dynamics of complicated systems under very weak hypotheses. It is shown that these theorems apply to several subcircuits of the full NCR circuit, most importantly to the URE2-GLN3 subcircuit that is independent of the other constituents but governs the switching behavior of the full NCR circuit under changes in nitrogen source. Under hypotheses that are fully consistent with biological data, it is proven that the dynamics of this subcircuit is simple periodic behavior in synchrony with the cell cycle. Although the current mathematical structure theorems do not apply to the full NCR circuit, extensive simulations suggest that the dynamics is constrained in much the same way as that of the URE2-GLN3 subcircuit. This finding leads to the proposal that mathematicians study genetic circuits to find new geometries for which structure theorems may exist.

The stability of the Chlorella nitrate reductase mRNA is determined by the secondary structure of the 5'-UTR: implications for posttranscriptional regulation of nitrate reductase.

Nitrate reductase (NR; EC 1.6.1.1-3) can be controlled at both transcriptional and posttranscriptional levels. Here we describe stability of NR mRNA as a mechanism of control. The NR gene in Chlorella vulgaris (Warburg strain) transcribes a stable mRNA and an unstable mRNA. In-vitro-synthesized transcripts representing these mRNAs show the same stability characteristics. The unstable mRNA is 30 nucleotides longer at the 5'-UTR compared to the stable mRNA. Using an RNA-folding program the 5'-UTR of the longer unstable RNA showed a more extensive stem-loop structure compared to the more linear form of the shorter stable mRNA. Transcripts representing RNAs with intermediate 5'-UTRs folded similarly to the long form and were unstable, or similarly to the short form and were more stable. Thus the secondary structure of the 5'-UTR of NR mRNA is important in the stability of NR transcripts in Chlorella and allows the cell to respond to changes in nitrogen source in an energy-efficient manner.

Mathematical evaluation of plantaricin formation supports an auto-induced production mechanism

The production rate of a bacteriocin, produced by Lactobacillus plantarum TMW1.25 and previously named plantaricin1.25, was studied during pH-constant batch fermentations under various growth media conditions. The growth of L. plantarum and production of bacteriocin during the retardation phase were modelled, using 11 different empirical and mechanistic approaches. The optimal pH for bacteriocin production was 4.5. Among the different nitrogen sources tested, yeast extract was the most important, on the basis of the fact that the maximum growth rate decreased 16% without yeast extract, and only 7.2% or 8.1% without meat extract or peptone respectively. However, the change of nitrogen source did not have a significant effect on bacteriocin production. The progression of plantaricin1.25 production during the retardation phase and growth of L. plantarum TMW1.25 could be described by a structured model in which the bacteriocin concentration induces its own production. Among those models not implementing bacteriocin induction, only the one with an exponential increase of bacteriocin yield per unit biomass was suitable to describe bacteriocin production. Computer-aided evaluation of experimental data appears to be helpful in elucidating the relationship between the growth of lactic acid bacteria and bacteriocin production.

Cloning and characterization of the nitrate reductase-encoding gene from Chlorella vulgaris: structure and identification of transcription start points and initiator sequences

The reduction of nitrate to nitrite catalyzed by nitrate reductase (NR) is considered to be the rate-limiting and regulated step of nitrate assimilation, a major metabolic pathway occurring in a wide range of organisms which in turn supply the nutritional nitrogen requirements for other forms of life. Chlorella vulgaris NR mRNA levels are very responsive to changes in nitrogen source. In the presence of ammonia as the sole nitrogen source, under repressed conditions, NR mRNA is undetectable. Under inducing conditions, the removal of ammonia and addition of nitrate, rapid NR mRNA synthesis occurs. We are studying the elements involved in regulating the expression of this important gene. Two overlapping genomic clones ( NRS1 and NR5′) were isolated from a cosmid library. The two clones were sequenced and their sequences were aligned with that of a full-length NR cDNA. The gene is approximately 8 kb long and consists of 19 exons and 18 introns. Unlike NR isolated from other species, the exons which code for the functional domains of C. vulgaris are separated by introns. Two transcription start points ( tsp) were identified and each is surrounded by potential initiator sequences. No TATA, CAAT or GC-rich promoter elements were located. A time course of NR induction revealed that while transcription initiation from one tsp remains at a constant level from the point of induction through steady state, the level of initiation from another tsp is high upon induction, but decreases as steady state is attained.