The orientational freedom of molecular probes. The orientation factor in intramolecular energy transfer.

The measurement of the efficiency of F?rster long-range resonance energy transfer between donor (D) and acceptor (A) luminophores attached to the same macromolecular substrate can be used to estimate the D-A separation, R. If the D and A transition dipoles sample all orientations with respect to the substrate (the isotropic condition) in a time short compared with the transfer time (the dynamic averaging condition), the average orientation factor less than K2 greater than is 2/3. If the isotropic condition is not satisfied but the dynamic averaging condition is, upper and lower bounds for less than K2 greater than, and thus R, may be obtained from observed D and A depolarizations, and these limits may be further narrowed if the transfer depolarization is also known. This paper offers experimental protocols for obtaining this reorientational information and presents contour plots of less than K2 greater than min and less than K2 greater than max as functions of generally observable depolarizations. This permits an uncertainty to be assigned to the determined value of R. The details of the D and A reoreintational process need not be known, but the orientational distributions are assumed to have at least approximate axial symmetry with respect to a stationary substrate. Average depolarization factors are derived for various orientational distribution functions that demonstrate the effects of various mechanisms for reorientation of the luminophores. It is shown that in general the static averaging regime does not lend itself to determinations of R.     

The interaction of the polyene antibiotic lucensomycin with cholesterol in erythrocyte membranes and in model systems. III. Characterization of spectral parameters.

The variations of optical density and fluorescence of lucensomycin are good indices of the binding of this polyenic antibiotic to membranes. The former parameter reflects more generally the binding to any site present in the membrane, while the latter is more specific for binding to cholesterol. The chromophore of the lucensomycin-cholesterol complex has a relatively long lifetime, is almost immobile in the membrane, and is not accessible to water-soluble fluorescence-quenching agents. The stoichiometry, evaluated fluorometrically, corresponds to about two cholesterol molecules per polyene. In colloidal cholesterol suspensions, the extent of binding as a function of free polyene concentration is described by rectangular hyperbolae, the dissociation constant being, however, dependent on the sterol concentration. In erythrocyte membranes, on the other hand, and even more markedly in model systems containing appropriate solvents, the combination between lucensomycin and the sterol sites is described by sigmoid titration curves, indicative of cooperative effects, and probably due to solvation of cholesterol.     

Transport of amino acids in intact 3T3 and SV3T3 cells. Binding activity for leucine in membrane preparations of ehrlich ascites tumor cells

Transport of amino acids into 3T3 and SV3T3 (SV40 virus-transformed 3T3) cells was measured on glass cover slips. The 3T3 and SV3T3 cells contain both A (alanine preferring) and L (leucine preferring) systems for neutral amino acid transport. Initial rates of uptake of amino acids are about twofold higher in SV3T3 than in 3T3 cells. Other parameters measured, however, do not indicate marked differences in the transport of amino acids by the two cell types. L-system amino acids, such as leucine, are subject to trans-stimulation in both cell lines, whereas A-system amino acids, such as alanine and glycine, are not. Leucine was transported to higher levels in confluent cells than in nonconfluent cells. Glycine, however, shows distinctly less transport activity as the cells become confluent. Ehrlich ascites cell plasma membranes were prepared and assayed for amino acid-binding activity. Leucine-binding activity was detected by equilibrium dialysis in Triton X-100-treated membrane preparations.     

Plasmid-determined beta-lactamase indistinguishable from the chromosomal beta-lactamase of Escherichia coli.

A plasmid, derived from a naturally occurring strain of Proteus mirabilis, conferred resistance to cephalosporins, apparently mediated by a beta-lactamase indistinguishable from that determined by the chromosomal gene of Escherichia coli K-12. There was evidence for a recombination event between the wild-type plasmid and a defective F factor (Fsp) in the Escherichia coli K-12 culture in which it was stored.     

Pollen dimorphism and anther culture in barley

Summary A dimorphism is observed in barley (Hordeum vulgare L., cv. Akka) pollen when stained with acetocarmine from the mid-binucleate stage onwards. The majority of grains have staining cytoplasms, while the remainder have cytoplasms which take up little or no stain (NS grains). The staining dimorphism cannot be detected at the late-uninucleate microspore stage when anthers are normally cultured, but the evidence suggests that the microspores have already diverged at this time and it is the cells destined to become NS grains in vivo that respond in culture to become pollen calluses. Evidence comes from a comparison of the frequencies of NS grains and pollen calluses and from their distribution between and within anthers.     

Inhibition of ion accumulation in maize roots by abscisic acid

Summary An inhibition of root growth, a decrease in the amount of potassium (as ⁸⁶Rb) and phosphate (³²P) accumulation by the root, and a partial depolarization of transmembrane electropotential were observed to develop with a similar time course and to a similar extent when intact maize (Zea mays L.) roots were treated with 10^-5 M abscisic acid (ABA). Potassium uptake was inhibited by ABA when excised, low-salt roots were bathed in KCl, KH₂PO₄, or K₂SO₄. ABA did not affect the ATP content of the tissues, the activity of isolated mitochondria, nor the activity of microsomal K⁺-stimulated ATPases.     

Conversion of covalently mercurated nucleic acids to tritiated and halogenated derivatives.

Mercurated nucleic acids are converted to the corresponding tritiated, brominated, and iodinated derivatives by treatment with sodium borotritiide, N-bromosuccinimide, and elemental iodine, respectively. All three reactions occur under mild conditions in neutral aqueous solutions. Mercury-halogen conversions are essentially quantitative at both the mono- and polynucleotide levels. Tritiation reactions also proceed efficiently with mononucleotides, although polymers undergo incomplete demercuration. In spite of the latter limitation , these reactions provide novel and efficient synthetic routes to radiolabeled nucleic acid derivatives.