Presence of a reversible inhibitor for human lymphoid cell DNA synthesisin the extracts of human peripheral lymphocytes.

An endogenous inhibitor of human lymphocyte DNA synthesis contained in extracts of purified human peripheral lymphocytes is described. It was found that the peripheral lymphocyte extract inhibits the DNA synthesis of phytohemagglutinin (PHA) stimulated human peripheral lymphocytes, lymphocytes in mixed lymphocyte culture, and human lymphoid cells in a long-term culture (PGLC-33H). This extract did not inhibit the DNA synthesis of nonlymphoid cells including HeLa and human embryonic lung. The effects of the inhibitor were reversible and noncytotoxic. Initial characterization showed the inhibitor to be thermolabile, DNase resistant, trypsin sensitive, and stable in a pH range 5.4–8.4. It appears that the inhibitor contained in the purified human peripheral lymphocyte extract is similar to a previously described inhibitor extracted from a human lymphoid cell line (PGLC-33H). Quantitation of the inhibitor in various lymphoid cell populations showed the amount of inhibitor per cell to be higher in resting peripheral lymphocytes than in PHA stimulated peripheral lymphocytes or human lymphoid cells in long-term culture (PGLC-33H). This data suggest that the inhibitor described may play a regulatory role in lymphocyte metabolism.     

A reaction-diffusion theory of morphogenesis with inherent pattern invariance under scale variations

In the framework of reaction-diffusion theory we deal with the problem of pattern regulation in morphogenesis. A generic model is proposed where the kinetic terms follow constraints imposed by scale invariance considerations. These constraints allow a class of kinetic schemes to be formulated so that, starting with an initially homogeneous morphogen distribution in the field, a stable gradient is established of the form: S(chi,L) = Lpf(chi/L). Here L is the length of the morphogenetic field, chi is the position variable and f(chi/L) is some monotonic function of the relative distance. With this distribution a scale invariant gradient can be constructed which leads to pattern regulation. A linear stability analysis of the model permits the definition of the parameter values enabling the system to abandon the homogeneous state spontaneously. Simulations of the evolution of the system towards its final stable state result in approximate pattern invariance for different field lengths. The accuracy of this invariance is in agreement with some recent quantitative experimental findings in both developing and regenerating systems.     

A Simple Methodological Approach for Counting and Identifying Culturable Viruses Adsorbed to Cellulose Nitrate Membrane Filters

We identified conditions under which Buffalo green monkey cells grew on the surfaces of cellulose nitrate membrane filters in such a way that they covered the entire surface of each filter and penetrated through the pores. When such conditions were used, poliovirus that had previously been adsorbed on the membranes infected the cells and replicated. A plaque assay method and a quantal method (most probable number of cytopathic units) were used to detect and count the viruses adsorbed on the membrane filters. Polioviruses in aqueous suspensions were then concentrated by adsorption to cellulose membrane filters and were subsequently counted without elution, a step which is necessary when the commonly used methods are employed. The pore size of the membrane filter, the sample contents, and the sample volume were optimized for tap water, seawater, and a 0.25 M glycine buffer solution. The numbers of viruses recovered under the optimized conditions were more than 50% greater than the numbers counted by the standard plaque assay. When ceftazidime was added to the assay medium in addition to the antibiotics which are typically used, the method could be used to study natural samples with low and intermediate levels of microbial pollution without decontamination of the samples. This methodological approach also allowed plaque hybridization either directly on cellulose nitrate membranes or on Hybond N+ membranes after the preparations were transferred.     

Absorption of light by non-refractive spherical shells

The transmittance of pigmented and non-refractive spherical shells is a function of the absorbance along the diameter and the ratio of the shell radii. For equal absorbance, thinner shells transmit less and exert a stronger flattening distortion (sieve effect) on the true absorption bands of the pigment. For equal amounts of absorbing material, shells of equal outer radius but having different radial ratios have identical transmittances and flattening coefficients. The transmittance and the flattening effect of spherical particles are considered as limiting cases of spherical shells with zero cavity. The derived expressions can be applied for the correction of the absorption spectra of suspensions of peripherally pigmented biological particles.     

The Salinity Tolerance of Freshwater Cyanobacterium Synechococcus sp. PCC 7942 is Determined by Its Ability for Osmotic Adjustment and Presence of Osmolyte Sucrose

We investigated the factors that impose an upper limit of salinity tolerance to the unicellular freshwater cyanobacterium Synechococcus sp. PCC 7942. Above approx. 0.4 M NaCl, Synechococcus cells cease to proliferate, after having accumulated 0.3 M sucrose. Cells that pre-accumulated sucrose could tolerate up to 0.5 M NaCl, but not 0.6 M NaCl. After exposure to 0.5 M NaCl or higher, the cells were irreversibly modified becoming unable for osmotic volume adjustments.

     

Effects of chaotropic electrolytes on the structure and electronic excitation coupling of glutaraldehyde- and diimido ester-cross-linked phycobilisomes

The tendency of isolated intact phycobilisomes to disperse into multimeric phycobiliprotein subunits in dilute phosphate buffer is diminished, or even eliminated, by means of covalent polypeptide cross-linking with glutaraldehyde, or dimethyl suberimidate. Employing sensitized fluorescence spectrophotometry in order to detect transitions of the type phycobilisome → phycobiliprotein multimers, and second-derivative absorption spectrophotometry to detect transitions of the type phycobiliprotein multimers → monomers, we investigated the molecular basis of phycobilisome stabilization by these two cross-linkers. A network of intrahexamer and interhexamer covalent cross-links prevents the dissociation of glutaraldehyde-treated phycobilisomes into monomers, even under the strong chaotropic effect of KNO₃ and KSCN solutions, but fails to protect against polypeptide unfolding in concentrated urea solution. Dimethyl suberimidate, on the other hand, introduces only intramonomer cross-links, and thus it does not prevent dissociation to monomers in the presence of KNO₃ and KSCN. Increased hydrophobicity of phycobiliprotein subunits, as a result of the alkylation of ?NH₂ by the diimido ester, or the dialdehyde, is an additional structure-stabilizing factor.     

Module detection in complex networks using integer optimisation

Background  

The detection of modules or community structure is widely used to reveal the underlying properties of complex networks in biology, as well as physical and social sciences. Since the adoption of modularity as a measure of network topological properties, several methodologies for the discovery of community structure based on modularity maximisation have been developed. However, satisfactory partitions of large graphs with modest computational resources are particularly challenging due to the NP-hard nature of the related optimisation problem. Furthermore, it has been suggested that optimising the modularity metric can reach a resolution limit whereby the algorithm fails to detect smaller communities than a specific size in large networks.