Binding of [3H]estradiol- and [3H]H1285-receptor complexes to rabbit uterine chromatin

In the present study we investigated the binding characteristics of estrogen and antiestrogen-receptor complexes to rabbit uterine chromatin. Activated or nonactivated estrogen receptors were partially purified by DEAE-cellulose chromatography using low (1 mM) or high (10 mM) concentrations of sodium molybdate. Activated [³H]estradiol-receptor complexes showed enhanced binding to chromatin acceptor sites unmasked by 1 M, 4 M and 6 M guanidine hydrochloride. We also examined the chromatin-binding characteristics of the estrogen receptors when bound by the high-affinity triphenylethylene antiestrogen, H1285. The acceptor site activity for the [³H]H1285-receptor complexes was markedly decreased at sites unmasked by 4 M and 6 M guanidine hydrochloride. Further, the nonactivated receptor complexes showed very low binding to deproteinized chromatin. The estrogen-receptor chromatin-acceptor sites were tissue specific and saturable. These chromatin acceptor sites differ in their affinity and capacity (number of binding sites per cell) for the estrogen- and antiestrogen-receptor complexes. Thus, we suggest that the differences in the physiological and physicochemical properties of estrogens and antiestrogens may be related to their differential interaction with uterine chromatin subfractions.     

Dynamics of lipid-protein interactions. Interaction of apolipoprotein A-II from human plasma high density lipoproteins with dimyristoylphosphatidylcholine

ApoA-II and dimyristoylphosphatidylcholine (DMPC) spontaneously associate to give three different complexes whose structures are determined by the initial reactant concentration and by the reaction temperature with respect to Tc (23.9 degrees C), the gel to liquid crystalline transition temperature of DMPC. At an initial lipid to protein ratio of 45/1, a single complex (2.29 x 10(5) daltons) is quantitatively formed at all temperatures between Tc - 4 degrees C and Tc + 6 degrees C. When the 45/1 complex is mixed with DMPC liposomes there is lipid exchange but no net transfer of lipid, so that the structure of the complex remains unaltered. At an initial molar ratio of 100 to 300:1, the reaction scheme is more complex. At 24 degrees C a 240/1 complex (1.5 x 10(6) daltons) is formed from a precursor 75/1 complex (3.43 x 10(5) daltons) if excess (approximately 300 mol/mol) lipid is present. The 75/1 complex exhibits lipid exchange in the presence of added DMPC liposomes at 24 degrees C, and both the 75/1 and the 240/1 complex can be converted to smaller protein-rich complexes in the presence of added apoA-II. These results suggest that the initial lipid/protein ratio and the physical state of a lipid or lipid . protein complex determines the composition and structure of the resulting complex and support the view that lipid-protein interactions are stronger than protein-protein or lipid-lipid interactions.     

Adenovirus early gene products may control viral mRNA accumulation and translation in vivo

The mechanisms controlling early adenovirus gene expression in vivo have been studied using inhibitors of protein synthesis. When inhibitors were added shortly before or at the onset of infection, viral mRNA from all early regions was transcribed, spliced and accumulated over a 7 hr period. After longer pretreatment, accumulation of several early mRNAs were suppressed. Addition of inhibitors 1 hr after infection enhanced the accumulation of viral mRNA in the cytoplasm. Translation of early mRNA selected on adenovirus DNA in a cell-free system reflected the amount of viral mRNA present. A viral coded product may therefore control accumulation of viral mRNA.A different pattern emerged when inhibitors of protein synthesis were removed at 5 hr postinfection and cells were pulse-labeled in vivo. If inhibitors were introduced at or before infection, early viral proteins were synthesized only after a lag of 1–3 hr. However, if treatment was introduced 1 hr post-infection, reversion of the protein synthesis block was instantaneous. It appears that protein synthesis inhibitors reveal an in vivo translational block for viral mRNA. This block could be overcome by preinfection with a related virus. Furthermore, no block was observed in a virus-transformed human embryonic kidney cell line (293) which expresses early region 1 of the viral genome. Viral gene product(s) encoded in early region 1 may control translation of early adenovirus messenger RNA in vivo.     

Tubulin subunit carboxyl termini determine polymerization efficiency

Cleavage of tubulin by subtilisin removes a small (Mr less than 2000) fragment from the C-terminal end of both alpha and beta subunits. The resulting protein is much reduced in negative charge. The cleaved, less acidic protein retains its competence to polymerize in a GTP-dependent and cold-, GDP-, and podophyllotoxin-sensitive manner and assembles into sheets or bundles of twisted filaments. The critical concentration for polymerization of the cleaved protein is about 50-fold lower than that for intact tubulin. It is proposed that the C termini of the subunits normally impede polymerization.     

Primary production and accumulation of nutrients by the ground layer community of cerrado vegetation of central Brazil

Summary Net aerial primary production and accumulation of nutrients by the grasses and nongrasses of the ground layer community of a cerrado vegetation in central Brazil were determined in burnt and unburnt areas. The net aerial primary production of the ground layer community was 327 gm^–2 in the unburnt area and only 242 gm^–2 in the burnt area during the first year after fire. Grasses contributed 68 to 78% of the aerial biomass of the ground layer in the unburnt area. The live biomass in the burnt and unburnt areas was comparable by the end of the first dry period after the fire. The major part of N, P and K in the aerial biomass was in the grasses. The concentration of all nutrients in the aerial biomass was generally higher in the burnt area during the first year after the fire.     

Emergence of blackflies (Diptera: Simuliidae) from a small forested stream in Ontario

Three species of blackflies were found in emergence-trap samples taken overa period of 8 months from a second-order, forested, cold-stenothermal stream in southern Ontario. The emergence phenologies of the two common species, Prosimulium mixtum and Stegopterna mutata, are described and compared with their phenologies in other streams in North America. Hypotheses are presented for the poor faunal diversity and prolonged emergence of the two species in the stream studied. Emergence data are used to describe the pupal distribution in the stream.Wing-length measurements showed a distinct dimorphism in P. mixtum: females were larger than males. Adult size (except of P. mixtum females) varied among sampling sites in the stream and, in the case of S. mutata, this variation was time dependent.     

Structural localization of the sequence alpha 235-242 of the nicotinic acetylcholine receptor

Two monoclonal antibodies (mAb 254 and 255) were obtained against a synthetic peptide corresponding to the sequence 235-242 of the alpha-subunit of Torpedo acetylcholine receptor. These mAbs could bind to receptor in native membrane vesicles only when these vesicles were permeabilized, suggesting that the sequence alpha 235-242 is exposed on the cytoplasmic surface of the receptor. Further evidence for the cytoplasmic localization of this sequence was partial competition for binding between these mAbs and mAbs previously demonstrated to bind to the cytoplasmic part of the receptor. A model is proposed which accounts for all the experimental data obtained thus far on the transmembrane orientation of the subunit polypeptide chains.