Biochemical investigations of retinotectal adhesive specificity

The preferential adhesion of chick neural retina cells to surfaces of intact optic tecta has been investigated biochemically. The study uses a collection assay in which single cells from either dorsal or ventral halves of neural retain adhere preferentially to ventral or dorsal halves of optic tecta respectively. The data presented support the following conclusions: (a) The adhesion of ventral retina to dorsal tecta seems to depend on proteins located on ventral retina and on terminal β-N-acetylgalactosamine residues on dorsal tecta. (b) The adhesion of dorsal retina to ventral tecta seems to depend on proteins located on ventral tecta and on terminal β- N-acetylgalactosamine residues on dorsal retina. (c) A double gradient model for retinotectal adhesion along the dorsoventral axis is consistent with the data presented. The model utilizes only two complementary molecules. The molecule suggested to be concentrated dorsally in both retina and tectum seems to require terminal β-N-acetylgalactosamine residues for adhesion. Its activity is not affected by protease. A molecule fitting these qualifications, the ganglioside GM(2), could not be detected in a gradient, but lecithin vesicles containing GM(2) adhered preferentially to ventral tectal surfaces. The second molecule, concentrated ventrally in both retina and tectum, is a protein and seems capable of binding terminal β-N- acetylgalactosamine residues. One enzyme, UDP-galactose:GM(2) galactosyltransferase, has been found to be more concentrated in ventral retina than dorsal, but only by 30 percent.     

Chemical probes reveal no evidence of Hoogsteen base pairing in complexes formed between echinomycin and DNA in solution

Five different DNA fragments have been treated with a range of conformationally sensitive reagents in an effort to probe structural changes in DNA associated with binding of the bis-intercalating antibiotic echinomycin. For each probe, the intensity and pattern of its reactivity with DNA have been analyzed in order to elucidate the effect of antibiotic binding on the accessibility of a specific site or sites to chemical attack. It was found that in one of the DNA fragments, pTyr2 DNA, several purine residues exhibit enhanced reactivity to diethyl pyrocarbonate (DEPC) in the absence of bound antibiotic, and that this strongly sequence specific reaction is enhanced in the presence of quite low echinomycin concentrations. The echinomycin-dependent reactivities towards DEPC of three homologous DNA fragments, chosen for their subtly different antibiotic binding characteristics, were also investigated. It was found that small changes in base sequence generate striking changes in susceptibility to modification by DEPC. The abolition of one antibiotic binding site leads to the creation of a new, intense DEPC-reactive site. In the presence of moderate concentrations of echinomycin, specific thymidine residues exhibit enhanced reactivity towards osmium tetroxide. No differences in the reactivities of the DNA fragments towards bromoacetaldehyde, S1 nuclease, dimethyl sulphate or potassium tetrachloropalladinate were observed in the presence of the antibiotic. DEPC reactions were performed on tubercidin (7-deaza-adenosine) to determine the DEPC reactive positions in situation where N-7 is inaccessible. Tubercidin was found to be generally resistant to attack by DEPC followed by treatment with base. We conclude that the bulk of structural changes induced by the binding of echinomycin to DNA do not involve Hoogsteen base pairing, but rather are due to sequence-specific unwinding of the helix in a manner which is strongly dependent on the nature of surrounding nucleotide sequences.     

The cytological responses of tomato-seedling cotyledons to chilling and the influence of membrane modifications upon these responses

Summary Tomato seedlings were exposed to 3 days of chilling at 5 °C in both the presence and absence of 5 mM ethanolamine. An array of pathological responses could be seen at both the light and electron microscope levels: apparent loss of cell turgor, vacuolization, reduction in the apparent volume of both the cytoplasm and the vacuolar protein bodies, apparent deposition of new material in the cell walls, general disorganization of organelles, especially plastids, and a general loss of cytoplasmic structure. Chilling injury was less pronounced in ethanolamine-treated cotyledonary tissues which appeared to have normal turgor and cell shape at the light microscope level and better preserved organelles at the ultrastructural level. Protection by ethanolamine at the ultrastructural level was most pronounced for the cytoplasm, the mitochondria, and the cell walls. Dead cells were seen with both treatments but were 2–3 times as numerous in the chilled-only tissue.     

DNA bis-intercalation: Theoretical analysis,including cooperativity of the interaction of echinomycin analogs with DNA

A theoretical analysis of the binding curves for the bifunctional peptide antibiotic echinomycin and its analogs interacting with a variety of DNAs is presented. The method is an extension of our previous work using the sequence-generating-function technique and has been modified to allow for consideration of cooperativity within the framework of the neighbor-exclusion rule. Binding by both single and double intercalation is included, and the results are, in many cases, superior to the analysis based on single-mode binding. In all cases the data are consistent with the neighbor-exclusion principle for intercalation when analyzed with this approach, while data analysis, using a simpler method, often led to violation of this rule. Positive cooperativity is considered in the treatment of data for binding to poly(dA-dT). Finally, we discuss the relationship between base specificity and binding site size.     

Parental behaviour in Mongolian gerbils (Meriones unguiculatus). II. Parental interactions

Twelve pairs of Mongolian gerbils were observed raising 15 litters. Both parents performed parental activities. Within pairs, associations were found between the behaviours of the parents. If one parent was active outside the nest, the other was likely to be relatively inactive in the nest. These non-random behavioural associations existed even though overall activity tendencies of members of pairs were similar. As a consequence of these patterns of behaviour, the pups were seldom left unattended, and the adaptive significance of parental presence is discussed.