Nitrate, a signal relieving seed dormancy in Arabidopsis.

Nitrate is an important nitrogen source for plants, but also a signal molecule that controls various aspects of plant development. In the present study the role of nitrate on seed dormancy in Arabidopsis was investigated. The effects of either mutations affecting the Arabidopsis nitrate reductase genes or of different nitrate regimes of mother plants on the dormancy of the seeds produced were analysed. Altogether, data show that conditions favouring nitrate accumulation in mother plants and in seeds lead to a lower dormancy of seeds with little other morphological or biochemical differences. Analysis of germination during seed development indicated that nitrate does not prevent the onset of dormancy but rather its maintenance. The effect of an exogenous supply of nitrate on seed germination was tested: nitrate in contrast to glutamine or potassium chloride clearly stimulated the germination of dormant seeds. Data show, moreover, that the Arabidopsis dual affinity nitrate transporter NRT1.1 (CHL1) may be involved in conveying the nitrate signal into seeds. Thus, nitrate provided exogenously or by mother plants to the produced seeds, acts as a signal molecule favouring germination in Arabidopsis. This signalling may involve interaction with the abscisic acid or gibberellin pathway.

Complex gel permeation assays for screening combinatorial libraries.

Gel permeation methods have been commonly used to screen combinatorial libraries synthesized on a solid support. We report here three screens of combinatorial libraries using gel permeation assays. These include a simple enzymatic assay to identify inhibitors of the influenza enzyme neuraminidase, and two more complex assays designed to screen for inhibitors of the interleukin-8 (IL-8)-IL-8 receptor and the urokinase-urokinase receptor interactions, respectively. The IL-8 ligand-receptor assay makes use of IL-8 receptor-expressing cells attached to a membrane, thus enabling washing steps as part of the assay. The urokinase ligand-receptor assay employs an enzyme-linked immunosorbent assay-type format, previously thought to be amenable only to well-based assays. The results of these three screens are reported here, including the discovery of a novel series of acyclic inhibitors of neuraminidase. The development of complex assays in a gel permeation format allows for the routine screening of combinatorially as well as noncombinatorially made compound collections against virtually any kind of target, and is being widely used in our high throughput screening operations.

Role of photoperiod and the pineal gland in T cell-dependent humoral immune reactivity in the Siberian hamster.

The present study tested the hypothesis that antibody production in response to xenoantigen is modulated by daylength and dependent upon the pineal gland. Alter injection of sheep erythrocytes (SRBC), serum immunoglobulin (Ig) concentrations were 5-fold lower in hamsters in short versus long days. Pinealectomy (Pinx) abolished the nocturnal melatonin rhythm, blocked short-day-mediated testis regression, and eliminated the short-day reduction in Ig production after SRBC treatment. Antibody titers in response to SRBC were equivalently augmented in short-day Pinx and long-day sham hamsters. The results indicate that photoperiodic effects on T cell-dependent humoral immunity are dependent upon the pineal gland. These findings raise the possibility that day length-associated changes in some immune system functions are mediated by the pineal melatonin rhythm.

Melatonin rhythm onset in the adult siberian hamster: influence of photoperiod but not 60-Hz magnetic field exposure on melatonin content in the pineal gland and in circulation.

To determine the relationship between pineal melatonin production and its appearance in circulation, the rising phase of the pineal and serum melatonin rhythm was studied in the adult Siberian hamster. Melatonin concentrations increased in the pineal gland and in serum at 1.50 and 1.75 h, respectively, relative to lights off in long days (16 h of light/day) and at 2.00 and 2.75 h, respectively, in short days (10 h of light/day). Thus, a photoperiod-dependent melatonin rise in circulation lagged production by the pineal gland by 0.50 h--a delay of 0.75 h in short-day hamsters versus 0.25 h in long-day hamsters. Following initiation of this rise, concentrations that were typical of the nighttime peak were achieved within 2 h of melatonin rhythm onset, regardless of photoperiod. To determine whether clock control of the rising phase of the melatonin rhythm, in the absence of photoperiod cues, may be disrupted by perturbations in the ambient magnetic field, hamsters in constant darkness were acutely exposed to a 1-Gauss, 60-Hz magnetic field for 15 min or were daily exposed to this treatment for 14 or 21 days. Neither the melatonin rise in pineal content or circulation during subjective night was affected by acute or chronic magnetic field exposures; testes regression similarly occurred in sham and daily magnetic field-exposed hamsters in constant darkness. These findings indicate that magnetic field exposures are unlikely to serve as a zeitgeber for the circadian mechanism that controls onset of the melatonin rhythm; rather, photoperiod is a predominant cue that may differentially regulate the rising phase of melatonin production in the pineal gland and concentration in circulation.

Sequence and characterization of two Arabidopsis thaliana cDNAs isolated by functional complementation of a yeast gln3 gdh1 mutant.

We have isolated two Arabidopsis thaliana cDNAs by complementation of a yeast gln3 gdh1 strain that is affected in the regulation of nitrogen metabolism. The two clones (RGA1 and RGA2) are homologous to each other and to the SCARECROW (SCR) gene that is involved in regulating an asymmetric cell division in plants. RGA1, RGA2 and SCR share several structural features and may define a new family of genes. RGA1 and RGA2 have been mapped, respectively, to chromosome II and I, and their expression in plant is constitutive.

Discovery of selective, small-molecule inhibitors of RNA complexes--I. The Tat protein/TAR RNA complexes required for HIV-1 transcription.

We have developed a therapeutic program focusing on the inhibition of a human immunodeficiency virus-1 specific protein-RNA interaction. This program begins with a search for small organic molecules that would interfere with the binding of Tat protein to TAR RNA. The methodologies chosen to study the HIV-1 Tat-TAR interaction and inhibition include gel mobility shift assays, scintillation proximity assays, filtration assays, and mass spectrometry. These methods helped establish in vitro high-throughput screening assays which rapidly identified Tat-TAR inhibitors from our corporate compound library. Tat-activated reporter gene assays were then used to investigate the cellular activities of the Tat-TAR inhibitors. The cellular activity, selectivity, and toxicity data for select Tat-TAR inhibitors were determined. Evaluation of both the cellular data and the Tat-TAR inhibition results led to further testing in anti-HIV-1 infection assays.

Inhibition of self-splicing group I intron RNA: high-throughput screening assays.

High-throughput screening assays have been developed to rapidly identify small molecule inhibitors targeting catalytic group I introns. Biochemical reactions catalyzed by a self-splicing group I intron derived from Pneumocystis carinii or from bacteriophage T4 have been investigated. In vitro biochemical assays amenable to high-throughput screening have been established. Small molecules that inhibit the functions of group I introns have been identified. These inhibitors should be useful in better understanding ribozyme catalysis or in therapeutic intervention of group I intron-containing microorganisms.

The tomato nia gene promoter functions in fission yeast but not in budding yeast.

A fragment comprising 1 kb of the 5' region and the 81 first nucleotides of the coding region of the tomato nitrate reductase nia gene was placed in translational fusion with the lacZ reporter gene. This construct was introduced in budding and in fission yeast using a derivative of the Saccharomyces cerevisiae/Schizosaccharomyces pombe autonomously replicating vector pUZL. Beta-galactosidase activity was detected in S. pombe but not in S. cerevisiae. Primer extension experiments show that in fission yeast transcripts are initiated at the same starting point as in tomato, indicating for the first time that a plant promoter can be correctly recognized in fission yeast.

NIT2, the nitrogen regulatory protein of Neurospora crassa, binds upstream of nia, the tomato nitrate reductase gene, in vitro.

The nit-2 gene of Neurospora crassa encodes a trans-acting regulatory protein that activates the expression of a number of structural genes which code for nitrogen catabolic enzymes, including nitrate reductase. The NIT2 protein contains a Cys2/Cys2-type zinc-finger DNA-binding domain that recognizes promoter regions of the Neurospora nitrogen-related genes. The NIT2 zinc-finger domain/beta-Gal fusion protein was shown to recognize and bind in a specific manner to two upstream fragments of the nia gene of Lycopersicon esculentum (tomato) in vitro, whereas two mutant NIT2 proteins failed to bind to the same fragments. The dissociation kinetics of the complexes formed between the NIT2 protein and the Neurospora nit-3 and the tomato nia gene promoters were examined; NIT2 binds more strongly to the nit-3 promoter DNA fragment than it does to fragments derived from the plant nitrate reductase gene itself. The observed specificity of the binding suggests the existence of a NIT2-like homolog which regulates the expression of the nitrate assimilation pathway of higher plants.

Characteristics of Nicotiana tabacum nitrate reductase protein produced in Saccharomyces cerevisiae.

Tobacco nitrate reductase (NR) produced in yeast retains cytochrome c reductase activity, but not NR activity. Biochemical data suggest that the haem and FAD domains are functional, and that the molybdenum cofactor (MoCo) domain is inactive owing to the absence of MoCo in yeast. The native form of the produced NR is dimeric. Thus MoCo is not involved in NR dimerization in higher plants, contrary to current assumptions.

Translated translational operator in Escherichia coli. Auto-regulation in the infC-rpmI-rplT operon.

The genes coding for translation initiation factor IF3 (infC) and for the ribosomal proteins L35 (rpmI) and L20 (rplT) are transcribed in that order from a promoter in front of infC. The last two cistrons of the operon (rpmI and rplT) can be transcribed from a weak secondary promoter situated within the first cistron (infC). Previous experiments have shown that the expression of infC, the first cistron of the operon, is negatively autoregulated at the translational level and that the abnormal AUU initiation codon of infC is responsible for the control. We show that the expression of the last cistron (rplT) is also autoregulated at the posttranscriptional level. The L20 concentration regulates the level of rplT expression by acting in trans at a site located within the first cistron (infC) and thus different from that at which IF3 is known to act. This regulatory site, several hundred nucleotides upstream from the target gene (rplT), was identified through deletions, insertions and a point mutation. Thus, the expression of the operon is controlled in trans by the products of two different cistrons acting at two different sites. The localization within an open reading frame (infC) of a regulatory site acting in cis on the translation of a downstream gene (rplT) is new and was unforeseen since ribosomes translating through the regulatory site might be expected to impair either the binding of L20 or the mRNA secondary structure change caused by the binding. The possible competition between translation of the regions acting in cis and the regulation of the expression of the target gene is discussed.