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Spatial organization of wild-type (strain U1) tobacco mosaic virus (TMV) and of the temperature-sensitive TMV ts21-66 mutant was compared by tritium planigraphy. The ts21-66 mutant contains two substitutions in the coat protein (Ile21 fi Thr and Asp66 fi Gly) and, in contrast with U1, induces a hypersensitive response (formation of necroses) on the leaves of plants bearing a host resistance gene N¢ (for example Nicotiana sylvestris); TMV U1 induces systemic infection (mosaic) on the leaves of such plants. Tritium distribution along the coat protein (CP) polypeptide chain was determined after labelling of both isolated CP preparations and intact virions. In the case of the isolated low-order (3-4S) CP aggregates no reliable differences in tritium distribution between U1 and ts21-66 were found. But in labelling of the intact virions a significant difference between the wild-type and mutant CPs was observed: the N-terminal region of ts21-66 CP incorporated half the amount of tritium than the corresponding region of U1 CP. This means that in U1 virions the CP N-terminal segment is more exposed on the virion surface than in ts21-66 virions. The possibility of direct participation of the N-terminal tail of U1 CP subunits in the process of the N¢ hypersensitive response suppression is discussed.Keywords: coat protein; hypersensitive response; tobacco mosaic virus; tritium planigraphy.Systems in which defined modifications in a pathogen structure cause specific changes in a host response are highly useful in studies of molecular mechanisms of pathogenesis. One of a small number of plant virology models which satisfy this criterion is the combination of tobacco mosaic virus (TMV) with plants bearing the host resistance gene N¢. In hosts with N¢ many TMV strains and coat protein (CP) mutants induce formation of necroses (small necrotic lesions) on infected leaves [hypersensitive response (HR)], whereas wild-type (strain U1) TMV avoids the induction of plant defence necrotic response machinery and produces systemic infection (yellow-green mosaic) [1][2][3]. With the help of spontaneous and site-directed mutagenesis it has been shown that defined single substitutions in U1 CP deprive the virus of the ability to escape the N¢ plant defence system and result in necroses formation on plants of, for example, Nicotiana sylvestris N¢ [4,5].Several years ago we isolated and characterized a moderately thermosensitive TMV CP mutant, ts21-66, which induces HR in N. sylvestris [6][7][8]. This mutant contains two well-known substitutions, Ile21 fi Thr and Asp66 fi Gly, in its CP molecule, each of which induces HR in N¢ hosts [9,10]. Whereas the positions of amino acid substitutions in ts21-66 are distant in the protein sequence, they are close in the known TMV CP tertiary structure, both being located at a distance of about 70 Å from the virion (disk) axis [10][11][12]. Many other classical temperaturesensitive and N¢ necrotic mutations in TMV CP are also located in this region: residues 19-21 (TMV CP mutants Ni103, Ni118, Ni511, Ni696...

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Synthesis of compounds containing a fragment similar to the guanidine group of L-arginine, which is a substrate of nitric oxide synthase (NOS), is the main direction in creating NOS inhibitors. The inhibitory effect of such compounds is caused not only by their competition with the substrate for the L-arginine-binding site and/or oxidizing center of the enzyme (heme) but also by interaction with peptide motifs of the enzyme that influence its dimerization, affinity for cofactors, and interaction with associated proteins. Structures, activities, and relative in vitro and in vivo specificities of various NOS inhibitors (amino acid and non-amino acid) with linear or cyclic structure and containing guanidine, amidine, or isothiuronium group are considered. These properties are mainly analyzed by comparison with effects of the inhibitors on the inducible NOS.

The scintillation phase procedure based on measuring the counting rate of compounds labeled with tritium in a two-layer scintillation cocktail3aqueous solution system was modified. Previously this procedure was used primarily for studying sorption of proteins at the phase boundary. After certain modification, it became applicable to studying the kinetics of sorption and the equilibrium sorption of various compounds at the organic scintillation cocktail3aqueous solution boundary and to evaluating the distribution coefficients of compounds in this system. Two experimental modes are proposed: common two-layer system and drop system. The suitability of these procedures for determining the surface activity and hydrophobicity of organic compounds and for studying the permeability of the molecular layers adsorbed at the phase boundary is discussed.Procedures for studying dynamics of formation of adsorption layers and of redistribution of compounds between phases are being actively developed today [13 4]. By using tritium-labeled compounds and measuring the radioactivity with liquid scintillation spectrometers it is possible to directly monitor formation of adsorption layers at the phase boundary and to determine the rate of supply of a labeled compound into the organic phase in the scintillation cocktail3 aqueous solution system. This approach (the so-called scintillation phase procedure) was used previously to study the adsorption layers of bovine serum albumin at the aqueous solution3toluene boundary [537]. In these studies, the experiments were carried out in 20-ml standard glass vials and the dependence of the equilibrium counting rate on the organic phase volume was determined to evaluate the surface and volume contributions to the counting rate. The following disadvantages of this procedure should be noted:(1) An aqueous phase in a glass vial, wetting its walls, forms a meniscus, which can affect the results, and (2) the surface and volume counting rates are determined by extrapolation to a [zero volume] of the scintillation cocktail, which can cause serious errors at high concentrations of the labeled compound in the organic phase. In this study we improved the scintillation phase procedure to expand its application areas. EXPERIMENTALToluene containing 2,5-diphenyloxazole (0.45 wt %) was carefully added over the layer of an aqueous solution of a labeled compound in a 7-ml scintillation vial of polyethylene (1.5 cm in diameter, Wallac Oy, Finland). Then the vial was placed in a RackBeta 1215 liquid scintillation spectrometer (LKB Wallac, Finland), and its activity was measured at desired time intervals. In the course of measurements, the counting rate increased owing to redistribution of the tritiumlabeled compound between the phases or its concentration at the phase boundary, if this compound exhibited surface activity. Based on changes in the counting rate, the kinetics of redistribution of the labeled compound between the phases was evaluated, and, after attainment of the equilibrium, the equilibrium concentra...