Extraction of bile acids from rat feces containing cholestyramine

The fecal extraction procedure described by Evrard and Janssen (1) was inadequate for the complete extraction of conjugated bile acids from feces containing the bile acid sequestrant, cholestyramine. As judged by gas-liquid chromatographic analysis, substitution of 0.5 n HCl in absolute ethanol for glacial acetic acid allowed for complete recovery (98-104%) of three different conjugated bile salts in the presence of the resin.     

Slipped-strand mispairing: a major mechanism for DNA sequence evolution

Simple repetitive DNA sequences are a widespread and abundant feature of genomic DNA. The following several features characterize such sequences: (1) they typically consist of a variety of repeated motifs of 1-10 bases--but may include much larger repeats as well; (2) larger repeat units often include shorter ones within them; (3) long polypyrimidine and poly-CA tracts are often found; and (4) tandem arrangements of closely related motifs are often found. We propose that slipped-strand mispairing events, in concert with unequal crossing- over, can readily account for all of these features. The frequent occurrence of long tandem repeats of particular motifs (polypyrimidine and poly-CA tracts) appears to result from nonrandom patterns of nucleotide substitution. We argue that the intrahelical process of slipped-strand mispairing is much more likely to be the major factor in the initial expansion of short repeated motifs and that, after initial expansion, simple tandem repeats may be predisposed to further expansion by unequal crossing-over or other interhelical events because of their propensity to mispair. Evidence is presented that single-base repeats (the shortest possible motifs) are represented by longer runs in mammalian introns than would be expected on a random basis, supporting the idea that SSM may be a ubiquitous force in the evolution of the eukaryotic genome. Simple repetitive sequences may therefore represent a natural ground state of DNA unselected for coding functions.      

Complete sequences of the rRNA genes of Drosophila melanogaster

In this, the first of three papers, we present the sequence of the ribosomal RNA (rRNA) genes of Drosophila melanogaster. The gene regions of D. melanogaster rDNA encode four individual rRNAs: 18S (1,995 nt), 5.8S (123 nt), 2S (30 nt), and 28S (3,945 nt). The ribosomal DNA (rDNA) repeat of D. melanogaster is AT rich (65.9% overall), with the spacers being particularly AT rich. Analysis of DNA simplicity reveals that, in contrast to the intergenic spacer (IGS) and the external transcribed spacer (ETS), most of the rRNA gene regions have been refractory to the action of slippage-like events, with the exception of the 28S rRNA gene expansion segments. It would seem that the 28S rRNA can accommodate the products of slippage-like events without loss of activity. In the following two papers we analyze the effects of sequence divergence on the evolution of (1) the 28S gene "expansion segments" and (2) the 28S and 18S rRNA secondary structures among eukaryotic species, respectively. Our detailed analyses reveal, in addition to unequal crossing-over, (1) the involvement of slippage and biased mutation in the evolution of the rDNA multigene family and (2) the molecular coevolution of both expansion segments and the nucleotides involved with compensatory changes required to maintain secondary structures of RNA.      

Cyanide-resistant reduction of nitroblue tetrazolium and hydrogen peroxide production by the rabbit blastocyst.

The ability of the rabbit blastocyst to reduce nitroblue tetrazolium (NBT) to formazan in the presence of cyanide was assayed as an indicator of extramitochondrial oxidase activity capable of generating the superoxide radical. A cytochemical method initially developed for the detection and localization of hydrogen peroxide production at the ultrastructural level in phagocytosing leukocytes (Briggs et al.: J Cell Biol 67:566, 1975) was also applied to the blastocyst. The results demonstrate that the rabbit blastocyst acquires the ability to reduce NBT by a cyanide-insensitive process and to generate hydrogen peroxide between the fourth and fifth days postcoitum. The enzymatic activity responsible is apparently an NAD(P)H-dependent oxidase in the outer, microvillous plasma membrane of the trophoblast.     

Synthesis of RNA containing polyadenylic acid sequences in preimplantation rabbit embryos

Messenger RNA synthesis has been estimated by assaying polyadenylic acid (poly A)-rich sequences in heterogeneous RNA from preimplantation rabbit embryos. Poly A containing RNAs are synthesized at least as early as the 16-cell stage and continue to be made through blastocyst formation and maturation. Sixty to 78% of the heterogeneous polysomal RNA in blastocysts contain poly A sequences. The portion of the heterogeneous RNA containing poly A sequences does not appear to change markedly between cleavage and blastocyst stages of development. Poly Arich sequences are greater than 4 S and consist of at least 84% adenine residues. RNA molecules ranging from 6 S to greater than 28 S contain poly A sequences.     

Morphology of the site of sperm entry on the frog egg.

We located the site of sperm entry on the frog (Rana pipiens) egg using light and scanning electron microscopy. The site changed from a small, microvilli-free structure at 20 min after insemination to a clump of elongated microvilli at 40 min, to a distinct patch of microvilli by 2 hr. The sperm entry site responded differentially to hypertonicity. At 40 min, the site was found in a circular depression which was due to the sperm aster and which expanded to a rigid area in the animal half. We consider how these observations may be related to gray crescent formation.     

Small scale vertical gradients of Arctic ice algal photophysiological properties

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (F _v/F _m), maximum relative electron transport rate (rETR_max), photosynthetic efficiency (α), and the photoadaptive index (E _k). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0–5 cm from ice–water interface) expressed low F _v/F _m (0.331–0.426) and low α (0.098–0.130 (μmol photons m⁻²s⁻¹)⁻¹) in December. F _v/F _m and α increased in March and May (0.468–0.588 and 0.141–0.438 (μmol photons m⁻²s⁻¹)⁻¹, respectively) indicating increased photosynthetic activity. In addition, increases in rETR_max (3.3–16.4 a.u.) and E _k (20–88 μmol photons m⁻² s⁻¹) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion, photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).     

Effects of different fire intensities on chemical and biological soil components and related feedbacks on a Mediterranean shrub (<Emphasis Type="Italic">Phillyrea angustifolia</Emphasis> L.)

In July 2000, six plots of Mediterranean maquis in the Castel Volturno Nature Reserve were burnt at two intensity levels to examine the effects of fire intensities on chemical and biological soil components and their relationships with ecophysiological processes of Phillyrea angustifolia L. Net photosynthesis and stomatal conductance, as well as P availability, were higher in burnt plots than in control plots, even 2 years after fire; the TM density of total soil microfungi was significantly lower in the first 8 months after fire, while xerotolerant and heat-stimulated soil microfungi were still higher 2 years after fire. Significant correlations between photosynthesis and stomatal conductance in resprouts and mycorrhizal status, as well as changes in the soil fungal components of the communities, suggest that both soil and mycorrhizal fungi play a role in immobilizing and translocating nutrients temporarily released in the below-ground system by fire. Nutrient balance interacts with physiological processes, and a feedback mechanism is well represented by stomatal conductance, which allows both the influx of water and mineral nutrients from the soil; moreover, the post-fire increase in photosynthetic activity promotes vigorous resprouting and may lead to increased availability of carbohydrates for soil biota and, consequently, to enhanced vegetation resilience.