The nuclear matrix: Three-dimensional architecture and protein composition

The structural filament network of the nucleus is prepared while still connected to the cytoskeleton. The relatively gentle procedure removes about 98% of the DNA and at least 86% of the histones. The matrix is bounded by an outer nuclear lamina connected to the cytoskeletal framework, as well as the inner filaments. The filaments range in diameter from 3 to 22 nm, and are organized in a three-dimensional anastomosing network in which nucleoli are enmeshed. The nuclear matrix is separated from the cytoskeletal framework by a double detergent and then partitioned into a chromatin fraction and a matrix fraction by nuclease and high salt. Two-dimensional gel electrophoresis shows that the proteins of the cytoskeleton, chromatin and nuclear matrix are very different. A major protein found in all fractions cofocuses with actin. Vimentin is largely associated with the nuclear matrix, probably as a corona external of filaments.     

Enzymatic formation of the bisfuran structure in aflatoxin biosynthesis.

A relatively stable enzyme system that converts versiconal hemiacetal acetate to versicolorin A was isolated from the soluble fraction of the homogenized cells of Aspergillus parasiticus ATCC 15517. The cell-free preparation did not require oxygen or oxidized nicotinamide adenine dinucleotide phosphate for activity, nor did it require dithiothreitol, polyclar (polyvinyl pyrrolidone), or glycerol for stabilization of activity. It was susceptible to inhibition by dichlorvos and cysteine. Isotope tracer studies revealed involvement of several intermediates in the conversion of versiconal hemiacetal acetate to versicolorin A. These findings confirm the biogenetic relationship of versiconal hemiacetal acetate and versicolorin A, and they confirm that the bisfuran ring structure in aflatoxins and related fungal metabolites is derived from the hemiacetal structure of versiconal hemiacetal acetate.     

Effects of enkephalin in analogues on prolactin release in the rat.

The effects of nineteen enkephalin analogues on the circulating levels of prolactin in the male rat following intraventricular injection of the peptides were determined and compared with that of Met- and Leu-enkephalin. Eleven of the 19 analogues stimulated prolactin secretion. It was found, in general, that the structure activity relationship for enkephalin stimulation of prolactin secretion was similar to that for opiate receptor activity. Analogues which contained a [DAla²] substitution were generally effective in stimulating prolonged prolactin release. Some, but not all analogues containing [DTrp²] or [DLeu⁵] were active. Analogues containing the [DTrp¹], [DPhe⁴] or [DMet⁵] substitutions were ineffective. The prolactin releasing effect of intravenous Tyr-DAla-Gly-Phe-DLeu was reversed by naloxone. Naloxone had no effect on the haloperidol- and alpha-methylparatyrosine induced increases in plasma prolactin levels. The results of these studies are discussed in the light of the suggestion that the enkephalins may function as neuroendocrine modulators.     

Estimation of whole-plant resistance to gaseous exchange independent of leaf temperature measurement

For studies into the uptake of mercury vapor by wheat (Triticum aestivum), a simple theory and plant chamber were employed to estimate total leaf resistance of whole plants to water vapor exchange. The estimates were independent of leaf temperature, for which mean values were indirectly determined. The approach involved the measurement, at steady-state conditions, of the net change in water vapor flux per unit of leaf surface (Δq_v) in response to a small induced change in absolute humidity (ΔC_a). Assuming that total leaf resistance (r_l) was constant and that change in leaf temperature (T_l) was negligible, total leaf resistance was calculated from the equation, [Formula: see text]     

小麦根系特征对干旱胁迫的响应

干旱胁迫时, 小麦(Triticum aestivum)根系率先产生应激响应, 同时向地上部发出信号, 诱导地上部发生生理反应, 从而提高植株抗旱能力。根系构型包括平面几何性状和立体几何结构(即拓扑构型), 具有遗传稳定性和可塑性。干旱胁迫影响根系理化特性, 如根源化学信号、根系细胞酶类和根系渗透作用的响应。根系通过调整其解剖学结构和水分吸收动力等来适应干旱胁迫。该文从根系构型、理化特性和解剖学结构3个方面, 系统阐述了小麦根系特征对干旱胁迫的响应, 并探讨了其与干旱胁迫的关系和当前研究中存在的问题, 以期为相关研究提供参考。     

DNA methylation patterns of human pachytene spermatocytes

A study has been made of the possibility that methylation occurs during germ cell production in the human testis. Utilizing the immunoperoxidase method with an antibody to 5-methylcytidine, it has been demonstrated that the number of immunoreactive sites on bivalents increased between early and mid/late pachytene.     

Broad-host-range properties of plasmid RK2: importance of overlapping genes encoding the plasmid replication initiation protein TrfA.

The trfA gene, encoding the essential replication initiation protein of the broad-host-range plasmid RK2, possesses an in-frame overlapping arrangement. This results in the production of TrfA proteins of 33 and 44 kDa, respectively. Utilizing deletion and site-specific mutagenesis to alter the trfA operon, we compared the replication of an RK2-origin plasmid in several distantly related gram-negative bacteria when supported by both TrfA-44 and TrfA-33, TrfA-33 alone, or TrfA-44/98L (a mutant form of the TrfA-44 protein) alone. TrfA-44/98L is identical to wild-type TrfA-44 with the exception of a single conservative amino acid alteration from methionine to leucine at codon 98; this alteration removes the translational start codon for the TrfA-33 protein. Copy number and stability were virtually identical for plasmids containing both TrfA-44 and TrfA-33 proteins or TrfA-44/98L alone in Pseudomonas aeruginosa and Agrobacterium tumefaciens, two unrelated bacteria in which TrfA-33 is poorly functional. This, along with recent in vitro studies comparing TrfA-44, TrfA-33, and TrfA-44/98L, suggests that the functional activity of TrfA-44 is not significantly affected by the 98L mutation. Analysis of minimal RK2 derivatives in certain gram-negative bacterial hosts suggests a role of the overlapping arrangement of trfA in facilitating the broad host range of RK2. RK2 derivatives encoding TrfA-44/98L alone demonstrated decreased copy number and stability in Escherichia coli and Azotobacter vinelandii when compared with derivatives specifying both TrfA-44 and TrfA-33. A strategy employing the trfA-44/98L mutant gene and in vivo homologous recombination was used to eliminate the internal translational start codon of trfA in the intact RK2 plasmid. The mutant intact RK2 plasmid produced only TrfA-44/98L. A small reduction in copy number and beta-lactamase expression resulted in E. coli, suggesting that overlapping trfA genes also enhance the efficiency of replication of the intact RK2 plasmid.     

Freezing of phosphocholine headgroup in fully hydrated sphingomyelin bilayers and its effect on the dynamics of nonfreezable water at subzero temperatures.

Differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spectroscopy are applied to characterize the nonfreezable water molecules in fully hydrated D2O/sphingomyelin at temperatures below 0 degrees C. Upon cooling, DSC thermogram displays two thermal transitions peaked at -11 and -34 degrees C. The high-temperature exothermic transition corresponds to the freezing of the bulk D2O, and the low-temperature transition, which has not previously been reported, can be ascribed to the freezing of the phosphocholine headgroup in the lipid bilayer. The dynamics of nonfreezable water are also studied by 2H NMR T1 (spin-lattice relaxation time) and T2e (spin-spin relaxation time obtained by two pulse echo) measurements at 30.7 MHz and at temperatures down to -110 degrees C. The temperature dependence of the T1 relaxation time is characterized by a distinct minimum value of 2.1 +/- 0.1 ms at -30 degrees C. T2e is discontinuous at temperature around -70 degrees C, indicating another freezing-like event for the bound water at this temperature. Analysis of the relaxation data suggest that nonfreezable water undergoes both fast and slow motions at characteristic NMR time scales. The slow motions are affected when the lipid headgroup freezes.     

Membrane-microfilament interactions in ascites tumor cell microvilli. Identification and isolation of a large microfilament-associated membrane glycoprotein complex

[14C]Glucosamine metabolic labeling and concanavalin A blots were used to identify four major glycoprotein species associated with ascites tumor cell microvillar microfilament cores and with a transmembrane complex containing actin. Phalloidin shift analysis of glucosamine-labeled microvilli showed that glycoproteins of 110-120, 80, 65, and 55 kDa are stably associated with the microfilament cores. Analysis of large (greater than 10(6) kDa) transmembrane complexes from microvillar membranes made under microfilament-depolymerizing conditions (Carraway, C. A. C., Jung, G., and Carraway, K. L. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 430-434) revealed glycoproteins of the same Mr values, showing the same relative staining or labeling patterns as those observed with the microfilament cores. Gel filtration of high salt, high pH extracts of intact microvilli, microfilament cores, or transmembrane complexes showed that in all of these fractions the glycoproteins are associated in a very large, stable complex. The glycoprotein multimer was isolated essentially free of actin and other components by Sephacryl S-1000 chromatography of microvilli, microvillar membranes prepared at pH 11, microfilament cores, or transmembrane complex fractions in Triton X-100, 1 M KCl, glycine, pH 9.5. Purified glycoprotein complex bound actin when incubated under polymerizing conditions. The presence of the glycoprotein heteromultimer in both microfilament cores and transmembrane complex from isolated membranes and the association of the purified glycoprotein complex with actin are consistent with our hypothesis that the glycoprotein-containing transmembrane complex is an association site for microfilaments at the plasma membrane.