Energetics of cell-cell and cell-biopolymer interactions

The energy vs distance balance of cell suspensions (in the presence and in the absence of extracellular biopolymer solutions) is studied, not only in the light of the classical Derjaguin-Landau-Verwey-Over-beek (DLVO) theory (which considered just the electrostatic (EL) and Lifshitz-van der Waals (LW) interactions), but also by taking electron-acceptor/electron-donor, or Lewis acid-base (AB) and osmotic (OS) interactions into account. Since cell surfaces, as well as many biopolymers tend to have strong monopolar electron-donor properties, they are able to engage in a strong mutual AB repulsion when immersed in a polar liquid such as water. The effects of that repulsion have been observed earlier in the guise of hydration pressure. The AB repulsion is, at close range, typically one or two orders of magnitude stronger than the EL repulsion, but its rate of decay is much steeper. In most cases, AB interactions are quantitatively the dominant factor in cell stability (when repulsive) and in “hydrophobic interactions” (when attractive). OS interactions exerted by extracellularly dissolved biopolymers are weak, but their rate of decay is very gradual, so OS repulsions engendered by biopolymer solutions may be of importance in certain long-range interactions. OS interactions exerted by biopolymers attached to cells or particles (e.g., by glycocalix glycoproteins), are very short-ranged and usually are negligibly small in comparison with the other interaction forces, in aqueous media.     

The Site Specificity of Two Species of Gregarina in Tenebrio molitor Larvae

The site specificity of the apicomplexans Gregarina cuneata and Gregarina sleini , in larval Tenebrio molitor was investigated. Gregarina cuneata was found to inhabit the anteriormost region of the larval midgut, while G. steini was restricted to the posterior portion of the intestine. The site specificity of the pair was conserved in single and concurrent infections. Interspecific interactions do not seem to be presently responsible for the resource partitioning by the 2 gregarine species. Key words. Gregarina, site specificity, Tenebrio molitor.     

The Ubiquitin System in Higher and Lower Plants — Pathways in Protein Metabolism

The multiple biological functions of the small polypeptide ubiquitin are mirrored by its unparalleled conservation on the amino acid and gene organization level. During the last years, it has become widely accepted that ubiquitin is an essential component in the ATP-dependent nonlysosomal protein degradation pathway occurring in all eukaryotic organisms. As turnover, consisting of protein synthesis and disassembly, is a central and vital process for each living cell, ubiquitin-mediated proteolysis is of enormous physiological value. The components of the ubiquitin ligation system have been characterized skillfully in plant and animal cells, but at the moment many questions remain as to how the high degree of specificity that is necessary for the regulation of intracellular breakdown is ensured. The recent hypotheses and models proposed for the basic mechanisms of protein recognition, conjugation and degradation will be discussed in detail. The existence of ubiquitin-protein conjugates which are not rapidly degraded clearly suggested that the role of ubiquitin is not restricted in its implication for protein turnover. Alterations of DNA structure, specific cell recognition mechanisms and cytoskeletal variations were observed as further ubiquitin-dependent processes which are not directly coupled to protein degradation.     

Development and specificity of inhibitory terminal arborizations in the central nervous system

This study examined the morphological development of single inhibitory arborizations in the gerbil central auditory brain stem. Using a brain slice preparation, neurons of the medial nucleus of the trapezoid body (MNTB) were filled with horseradish peroxidase (HRP), and their complete arborizations were analyzed along the tonotopic axis of the lateral superior olive (LSO). The projections in neonatal animals displayed well-defined arbors that were ordered appropriately within the LSO. It was evident from the axonal pathways that the MNTB afferents could correct for projection errors after reaching the postsynaptic population. As development progressed, a number of arbors established diffuse or inappropriate projections within the LSO. These immature arborizations were no longer apparent by 18–25 days postnatal. The anatomical specificity of arbors at 12–13 and 18–25 days was quantified by measuring the distance that terminal boutons spread across the frequency axis. There was a significant reduction of this distance in older animals. In addition, there was a significant reduction in the mean number of boutons per arbor between 12–13 days and 18–25 days. The maximum nucleus cross-sectional area continued to increase through 15–16 days, indicating that the refined arbors occupied an even smaller fraction of the postsynaptic structure. Taken together, these observations suggest that central inhibitory arbors form exuberant contacts that must be eliminated during development.     

Similarity and divergence among rodent repetitive DNA sequences

Summary We have analyzed the sequence of 63 B1 and 71 B2 repetitive elements from published data base sequences. The sequences conform to previously published consensus sequences, but are not identical to them. The B2 sequences show seven regions of high variability between family members, which we show points to the B2 family containing subfamilies; no similar evidence is found for subfamilies of the B1 family. The comparisons show no evidence for the emergence of species-specific variants of B1 or B2 sequences since the separation of murine and hamster lines of descent, nor of their concerted evolution within species in the last 10 million years.     

Eubacterial isocitrate dehydrogenase with dual specificity for NAD and NADP from Rhodomicrobium vannielii

Abstract Cell-free extracts of the photosynthetic eubacterium Rhodomicrobium vannielii contained both NADP and NAD-linked isocitrate dehydrogenase activities. Apparent K _m values of 12 μM for NADP, 0.75 mM for NAD, 9.3 μM for isocitrate (NADP utilising) and 8.2 μM for isocitrate (NAD utilising) were determined in such extracts. Four lines of evidence indicated that one enzyme was responsible for the two activities; (i) non-additivity of reaction rates in the presence of both NADP and NAD (ii) the presence of one band which stained for activity with both cofactors on non-denaturing polyacrylamide gels (iii) identical heat inactivation kinetics for both activities (iv) co-elution of both activities after ion-exchange and hydrophobic interaction chromatography. This is the first report of a eubacterial isocitrate dehydrogenase with dual cofactor specificity.     

Vectors permitting visual monitoring of simple transposition events

The construction and use of two novel transposon(Tn)-delivery vectors is described. These vectors carry Inc.W or Inc.N broad-host-range transfer functions cloned next to the narrow-host-range replicon of pBR329. The host specificities of pSLX10 and pSLX23 both complement and extend the host specificities of existing Tn delivery vectors. Plasmids pSLX10 and pSLX23 were shown to transfer at high frequency in intergeneric matings. The lux genes which are present on each vector permit the visual monitoring of transconjugants which have retained a Tn element, but are devoid of plasmid molecules. pSLX10 and pLSX23 were efficiently used to generate a range of auxotrophic mutants in various strains of Pseudomonas as well as to clone genes from Serratia liquefaciens. These vectors may have general applicability to identify and clone genes in a wide range of Gram-negative bacteria.     

DNA sequence analysis of spontaneous histidine mutations in a polA1 strain of Escherichia coli K12 suggests a specific role of the GTGG sequence

Summary Spontaneously arising histidine mutations in an Escherichia coli K12 strain deficient for DNA polymerase I were analysed at the DNA sequence level. We screened approximately 150000 colonies and isolated 106 histidine auxotrophs. Of these, 98 were unstable hisC mutations; 12 representative mutants analysed were shown to have arisen by the excision of a single quadruplet repeat in the sequence 5-GCTGGCTGGCTGGCTG-3. Of the eight mutations at other sites, three hisA deletions and one hisD deletion occurred as a consequence of misalignment of tandemly repeated pentamers (hisD) or decamers (hisA). A single hisA point mutation was found to be a missense mutation. Two extended deletions, covering the his operon were not analysed. We could not identify the hisC deletion by sequencing. We conclude that polA1 is a strong imitator that induces mutations mostly of the minus frameshift and deletion type by a Streisinger-type of mispairing in repetitive DNA sequences. Finally, the possible role of a 5-GTGG-3 sequence and its inverted or direct complements, which are found in the vicinity of all the deletions and frameshifts, is discussed.     

Transfer of amino acids and nitrate from the roots into the xylem of Ricinus communis seedlings

The loading of amino acids and nitrate into the xylem was investigated by collection and analysis of root-pressure exudate from the cut hypocotyl stumps of seedlings of Ricinus communis L. Glutamine was found to be the dominant amino acid in the exudate and also to be the amino acid which is transferred to the xylem most rapidly and accumulated to the greatest extent. The comparison between uptake and xylem loading showed significant differences in specificity between these two transport reactions, indicating a different set of transport systems. Nitrate is transferred to the xylem at a higher relative rate than any amino acid despite the great nitrate-storage capacity of the root system. Thus the supply of nitrate to Ricinus plants leads to enhanced nitrogen allocation to the shoots.