Restriction endonuclease mapping of ribosomal RNA genes: Sequence divergence and the origin of the tetraploid treefrog Hyla versicolor

Hyla chrysoscelis (2n=24) and H. versicolor (2n=48) are a diploid-tetraploid species pair of treefrogs. Restriction endonuclease mapping of ribosomal RNA (rRNA) gene repeat units of diploids collected from eastern and western populations reveals no differences within rRNA gene coding regions but distinctive differences within the nontranscribed spacers. A minimum of two physical maps is required to construct an rRNA gene map for the tetraploid, whose repeat units appear to be a composite, with about 50% of the elements resembling the western diploid population and about 50% resembling the eastern population. These results imply that this population of the tetraploid species may have arisen from a genetically hybrid diploid. Alternatively, the dual level of sequence heterogeneity in H. versicolor may reflect some type of gene flow between the two species. The coding region of the rRNA genes in the tetraploid differs from that in either diploid in about 20% of all repeat units, as exemplified by a BamHI site located near the 5 terminus of the 28 S rRNA gene. If the 20% variant class of 28 S rRNA gene coding sequences is expressed, then there must be two structural classes of ribosomes; if only the 80% sequence class is expressed, then a genetic control mechanism must be capable of distinguishing between the two different sequence variants. It is postulated that the 20% variant sequence class may be correlated with a partial functional diploidization of rRNA genes in the tetraploid species.This research was supported, in part, by NSF Grants CDP-8002341 and PRM-8106947 and by faculty research grants from Miami University to J.C.V.     

Human chorionic gonadotropin binding to rat testis receptors is inhibited by a thymus factor

An interrelationship between immune and reproductive systems has been postulated, and involves, among others, bidirectional effects between gonads and thymus. To this effect a rat thymus fraction of about 28000 mol wt has been reported to inhibit the effect of hCG on in vitro suspension of Leydig cells. We have investigated the antigonadotropin activity of thymus extracts on rat testis receptors. Acetonic powder obtained from thymus of 14 day-old rats was separated by molecular sieve chromatography. The effect of the collected fractions on the 125I-hCG binding to receptor sites in rat testes was evaluated. A fraction corresponding to 27000-28000 mol wt named thymus factor (TF), was found to inhibit the binding activity of 125I-hCG to its testicular receptor. The inhibitory effect of TF on hCG binding is dose related. By Scatchard analysis a competitive interaction at the receptor level between TF and hCG was demonstrated. The Ka values of hCG binding were diminished in the presence of TF while no significative changes were detected in the number of receptor sites. Present results strongly suggest a modulation function of TF at the testis receptor level.     

Uridine enhances the cytotoxic effect of D-glucosamine in rat C6 glioma cells

This paper studies the influence of uridine on the effects exerted by D-glucosamine in rat C6 glioma cells. 2 mM uridine increased markedly both the cytotoxic effect of the aminosugar and the inhibition of thymidine incorporation into acid-insoluble fraction. Furthermore the complete resumption of the capacity to incorporate either 3H-thymidine or 3H-mannose which was observed after the removal of the aminosugar, was impeded when the cells were treated contemporaneously with D-glucosamine and uridine. An exposure for 4 hr to 20 mM glucosamine alone enhanced about 15-fold the cellular pool of UDP-N-acetylhexosamines; the addition of 2 mM uridine intensified the expansion of this pool, which became about 35-fold the control value. The findings suggest a connection between the accumulation of UDP-N-acetylhexosamines in the cells and the appearance of D-glucosamine cytotoxicity.     

Contrasting effects of Escherichia coli single-stranded DNA binding protein on synthesis by T7 DNA polymerase and Escherichia coli DNA polymerase I (large fragment). Evidence that binding protein inhibits trans-lesion synthesis by polymerase I

The effect of Escherichia coli single-stranded DNA binding protein (SSB) on DNA synthesis by T7 DNA polymerase and E. coli DNA polymerase I (large fragment) using native or aminofluorene-modified M13 templates was evaluated by in vitro DNA synthesis assays and polyacrylamide gel electrophoresis analysis. The two polymerase enzymes displayed differential responses to the addition of SSB. T7 DNA polymerase, a enzyme required for the replication of the T7 chromosome, was stimulated by the addition of SSB whether native or modified templates were used. On the other hand, E. coli DNA polymerase I was slightly stimulated by the addition of SSB to the native template but substantially inhibited on modified templates. This result suggests that DNA polymerase I may be able to synthesize past an aminofluorene adduct but that the presence of SSB inhibited this trans-lesion synthesis. Polyacrylamide gels of the products of DNA synthesis by polymerase I supported this inference since SSB caused a substantial increase in the accumulation of shorter DNA chains induced by blockage at the aminofluorene adduct sites.     

Impact de l'échantillonnage sur la mésure des nucléotides adényliques (ATP,ADP, AMP) du microplancton

The influence of sampling and sample treatment upon adenylic nucleotide (ATP, ADP, AMP) content of microplankton is studied. Changes in light conditions during nigh-sampling and extracting do not induce significant variations, in the adenylic nucleotide content of microplankton or in energy charge values.The contribution of zooplankton (size up to 200 µm) to microplankton adenosine values can be neglected for inshore surface waters and traditional sample volumes (about one liter). This result can been explained by the low density of zooplankton in such a small sample volume and by differences in efficiency of the extracting method used.

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Impact de l'échantillonnage sur la mésure des nucléotides adényliques (ATP, ADP, AMP) du microplancton

     

Sensitivity of Saccharomyces cerevisiae vegetative cells and spores to antimicrobial compounds

The sensitivity of Saccharomyces cerevisiae spores and vegetative cells to various antimicrobial compounds was compared. Sulphur dioxide, benzoic acid, potassium sorbate, salicylic acid, nystatin, actidione and pimaricin were tested. Generally, the Saccharomyces spores were more resistant than the corresponding vegetative cells. It was also observed that this greater resistance shown by the spores varied with the antimicrobial compound used. Only potassium sorbate was not selective and killed both vegetative cells and spores at about the same rate.     

d-Glucose Transport System of Zymomonas mobilis

The properties of the d-glucose transport system of Zymomonas mobilis were determined by measuring the uptake of nonmetabolizable analogs (2-deoxy-d-glucose and d-xylose) by wild-type cells and the uptake of d-glucose itself by a mutant lacking glucokinase. d-Glucose was transported by a constitutive, stereospecific, carrier-mediated facilitated diffusion system, whereby its intracellular concentration quickly reached a plateau close to but not above the external concentration. d-Xylose was transported by the d-glucose system, as evidenced by inhibition of its uptake by d-glucose. d-Fructose was not an efficient competitive inhibitor of d-glucose uptake, indicating that it has a low affinity for the d-glucose transport system. The apparent K(m) of d-glucose transport was in the range of 5 to 15 mM, with a V(max) of 200 to 300 nmol min mg of protein. The K(m) of Z. mobilis glucokinase (0.25 to 0.4 mM) was 1 order of magnitude lower than the K(m) for d-glucose transport, although the V(max) values for transport and phosphorylation were similar. Thus, glucose transport cannot be expected to be rate limiting at concentrations of extracellular glucose normally used in fermentation processes, which greatly exceed the K(m) for the transport system. The low-affinity, high-velocity, nonconcentrative system for d-glucose transport described here is consistent with the natural occurrence of Z. mobilis in high-sugar environments and with the capacity of Z. mobilis for rapid conversion of glucose to metabolic products with low energetic yield.     

Sensitivity of Saccharomyces cerevisiae vegetative cells and spores to antimicrobial compounds

The sensitivity of Saccharomyces cerevisiae spores and vegetative cells to various antimicrobial compounds was compared. Sulphur dioxide, benzoic acid, potassium sorbate, salicylic acid, nystatin, actidione and pimaricin were tested. Generally, the Saccharomyces spores were more resistant than the corresponding vegetative cells. It was also observed that this greater resistance shown by the spores varied with the antimicrobial compound used. Only potassium sorbate was not selective and killed both vegetative cells and spores at about the same rate.