In vitro translation of the upstream open reading frame in the mammalian mRNA encoding S-adenosylmethionine decarboxylase

The upstream open reading frame (uORF) in the mRNA encoding S-adenosylmethionine decarboxylase is a polyamine-responsive element that suppresses translation of the associated downstream cistron in vivo. In this paper, we provide the first direct evidence of peptide synthesis from the S-adenosylmethionine decarboxylase uORF using an in vitro translation system. We examine both the influence of cation concentration on peptide synthesis and the effect of altering the uORF sequence on peptide synthesis. Synthesis of wild type and altered peptides was similar at all concentrations of magnesium tested. In contrast, synthesis of the wild type peptide was more sensitive than that of altered peptides to elevated concentrations of the naturally occurring polyamines, spermidine and spermine, as well as several polyamine analogs. The sensitivity of in vitro synthesis to spermidine was influenced by both the amino acid sequence and the length of the peptide product of the uORF. Findings from the present study correlate with the effects of the uORF and polyamines on translation of a downstream cistron in vivo and support the hypothesis that polyamines and the structure of the nascent peptide create a rate-limiting step in uORF translation, perhaps through a ribosome stalling mechanism.     

A stop transfer sequence recognizes receptors for nascent chain translocation across the endoplasmic reticulum membrane

A stop transfer sequence derived from the extreme carboxyl terminus of membrane IgM heavy chain has been shown to confer predictable transmembrane orientation to secretory proteins by aborting translocation of subsequently synthesized protein domains. Here we demonstrate that, in certain peptide sequence contexts, the same stop transfer sequence is also capable of initiating domain translocation across the endoplasmic reticulum (ER) membrane. Translocation directed by a stop transfer sequence is similar to, but distinguishable from, the action of a conventional signal sequence. Translocation is dependent on participation of the ribosome and protein receptors both in the cytoplasm and in the ER membrane. Moreover, both amino- and carboxy-terminal flanking protein domains can be translocated. Unlike a signal sequence, the stop transfer sequence is not itself translocated across the membrane. These results have implications for the nature of signal sequences, stop transfer sequences, and their receptor interactions.     

Habitat correlates with the spatial distribution of ectoparasites on Peromyscus leucopus in southern Michigan

The goal of this study was to evaluate the role of habitat in determining ectoparasite distribution of Peromyscus leucopus. We tested the hypothesis that ectoparasite occurrence is associated with particular host environments and this association is stronger for ectoparasites with limited interactions (i.e., ticks) than those with frequent interactions (i.e., lice). Ectoparasites from three different groups (Acari, Siphonaptera, and Phthiraptera) were collected from P. leucopus inhabiting a number of forested habitats in southern Michigan. Measurements of plant species structure and composition were collected and models were developed using quadratic discriminant function analysis to determine if habitats associated with ectoparasite presence were different from those associated with their absence. Mice parasitized by ticks were more likely to be found in areas having undergone a recent disturbance. Mice parasitized by ticks, fleas, and lice were more likely to be found in areas having tree species associated with dry soils. Our results show there is a distinct difference in habitats associated with the presence of ectoparasites, though we did not observe a stronger association of host habitat for ticks than for fleas or lice. This implies habitat should be included as an important component of assessments of the spatial distribution of ectoparasites.     

Clonidine and morphine increase atrial natriuretic peptide secretion in anesthetized rats

In order to determine whether the activity of central alpha 2-adrenergic and opioid receptors influence plasma atrial natriuretic peptide (ANP) levels, clonidine and morphine were infused into the lateral cerebral ventricle for 45 min in anesthetized Sprague-Dawley rats. The central administration of a low dose of clonidine (10 ng/min) caused a significant increase in plasma ANP without changing arterial blood pressure or central venous pressure. Pretreatment with yohimbine (5 micrograms/min) completely blocked the effect of clonidine. Central infusion of morphine (100 ng/min) also elevated plasma ANP levels and naloxone (5 micrograms/min) blunted this effect. Intravenous infusion of the same dose of clonidine or morphine did not affect plasma ANP levels. Moreover, the effect of clonidine on plasma ANP was partially blocked by pretreatment with naloxone (5 micrograms/min). These results suggest that central alpha 2-adrenergic and opioid receptors may be involved in ANP secretion.     

Dopaminergic modulation of some central actions of angiotensin II in vivo

Studies were performed in 12 conscious sheep of both sexes to determine if a brain dopaminergic pathway is involved in modulating the central actions of angiotensin II (Ang II) in regulating body temperature and plasma renin activity (PRA). Previous data showed that intracerebroventricular (ICV) infusion of Ang II significantly decreased PRA and body temperature. In contrast, converting enzyme inhibitor SQ 20881 (SQ) or dopamine (DA) significantly increased PRA and body temperature of sheep. In the present study, ICV infusion of the DA antagonist metoclopramide (MCP) (20 micrograms/min) significantly decreased PRA to 68 +/- 5% of the basal level. When sheep were pretreated with ICV MCP (20 micrograms/min) for 2 hr and then infused ICV with MCP (20 micrograms/min) plus DA (20 micrograms/min), Ang II (25 ng/min), or SQ (1 microgram/min), the PRA and temperature responses to DA, Ang II, or SQ were all abolished or attenuated significantly. The converse did not hold. Sheep pretreated with SQ (1 microgram/min) still showed a significant increase in body temperature (0.43 +/- 0.05 degree C) when infused with DA (20 micrograms/min). These results support the hypothesis that a central DA pathway is involved in the modulation of the actions of centrally administered Ang II on temperature and PRA.