Genetic variation in the genus Leiopelma and relationships to other primitive frogs

Allozyme variation was studied the three living species of Leiopelma. L. hamiltoni and L. archeyi are shown to be closely related to each other although L. hamiltoni is slightly more divergent relative to L. hochstetteri. This parallels previous cytoenetic data. The rarity and insularity of L. hamiltoni enables the calculation of a mutation rate based on genetic variance and population size. A mutation rate per generation of 2.7times 10^--⁶ is sufficient to account for the observed levels of variation. Six populations of L. hochstetteri show a pattern of genetic divergence that also closely parallels previously detected cytogenetic variation. L. hochstetteri is genetically distant from its congeneric species while all species of Leiopelma are at an extreme genetic distance from Ascaphus truei, the only other living amphicoelous fro At the limits of resolution of the allozme technique, Ascaphus clusters with the more morphologically advanced frogs, Discogiossus and Bombina, rather than with Leiopelma. Taken with other evidence, this supports recognition of two families, Leiopelmatidae anl Ascaphidae, with Leiopelma the probable sister group of all other frogs.     

Thromboxane A2 biosynthesis in human disease

Thromboxane A2 (TxA2), the predominant cyclooxygenase product of human platelets, is a potent vasoconstrictor and platelet agonist. Although its biological properties are readily appreciable in vitro, it has been difficult to define its biological importance in vivo. To a large extent this reflected the problems associated with efforts to monitor biosynthesis of this eicosanoid and the lack of selective pharmacological probes that prevented the synthesis of TxA2 or antagonized its biological action in vivo. Recently the analysis of urinary metabolites of TxB2 has become simplified so that the methodology is readily applicable to clinical studies. This provides a noninvasive, time-integrated index of Tx biosynthesis. Although one cannot definitively establish a tissue of origin for metabolites measured in urine, indirect evidence suggests that urinary TxB2 derives primarily from the kidney whereas its dinor metabolite predominantly reflects platelet biosynthesis under physiological conditions. Although plasma concentrations of TxB2 are readily confounded by platelet activation ex vivo, the enzymatic metabolites formed from TxB2 have recently been identified and appear to bypass this problem. Combined analysis of long-lived (e.g., 11-dehydro-TxB2) and short-lived (e.g., 2,3-dinor-TxB2) metabolites in plasma promise to more accurately localize phasic increases in the biosynthesis of TxA2 and have been paralleled by the development of antagonists of the TxA2/prostaglandin endoperoxide receptor and their study of humans. The use of such specific probes in conditions characterized by abnormal biosynthesis of TxA2 promises to define the biological role of this mediator for humans.     

Acquisition of glucose by Rickettsia prowazekii through the nucleotide intermediate uridine 5''-diphosphoglucose.

The ability of Rickettsia prowazekii to transport potential sources of the glucose moiety of bacterial polysaccharides was determined. Transport was determined both by filtration assays and by centrifugation through nonaqueous layers. Uridine 5'-diphosphoglucose (UDPG) was transported, whereas glucose was not transported; the uptake of glucose phosphates, although greater than that for glucose, was markedly lower than the transport of UDPG. Furthermore, the activities of hexokinase and phosphoglucomutase, enzymes required for the metabolism of glucose and glucose 6-phosphate, were undetectable in rickettsial extracts. The uptake of UDPG had an extended time course and did not reach a plateau until 60 min. The maximum rate of uptake was 340 pmol/min per mg of protein, and the rate was half-maximal at a UDPG concentration of 220 microM. Measurement of true influx of UDPG was complicated by the low activity of this transport system and the metabolism of the UDPG. The uptake of labeled UDPG was markedly inhibited by a 10-fold excess of uridine monophosphate, uridine diphospho-N-acetylglucosamine, and uridine diphospho-N-acetylgalactosamine but not by a variety of other structurally related compounds.     

Monoclonal antibodies to Escherichia coli 50S ribosomes.

Hybridoma cell lines that produce monoclonal antibodies directed against 50S Ribosomal proteins have been isolated. Spleen cells (from BALB/c mice immunized with 50S ribosomal subunits extracted from Escherichia coli) were fused to mouse myeloma cell line SP2/O-Ag 14. The initial screening for antibody producing hybridomas was carried out by a double antibody sandwich method; hybridomas were subsequently cloned in soft agar. Antibodies were characterized by their specific binding to individual 50S ribsomal proteins separated on phosphocellulose columns and in two-dimensional polyacrylamide gels. The assignments were confirmed with purified single ribosomal proteins. Of four clones analyzed thus far, two are identical with specificity for r-protein L5. The other clones produce two different antibodies directed against r-protein L20. Each monoclonal antibody formed ribosome dimers visualizable in the electron microscope. Dimers could be reacted with a different second antibody to form chains containing 8 or more ribosomes, which may be useful for structural studies.     

The Function of Urease in Citrullus Seeds

Urease is present in considerable quantity in the cotyledonsof Citrullus, though elsewhere in the plant it is present onlyin traces or is absent; urease activity in the cotyledons changesduring growth, showing an initial rise followed by an abruptdrop almost to zero. These changes, under a wide variety ofconditions, are not correlated with those in the major nitrogenfractions; they are, however, closely correlated with cell extensionand the associated changes in water content and respiration.A connexion with chlorophyll formation is possible but unlikely.It is suggested that the changes in cotyledonary urease constitutemerely one aspect of the ‘protoplasmic differentiation’that takes place as a cell matures.     

The DAL81 gene product is required for induced expression of two differently regulated nitrogen catabolic genes in Saccharomyces cerevisiae.

We demonstrate that the DAL81 gene, previously thought to be specifically required for induced expression of the allantoin pathway genes in Saccharomyces cerevisiae, functions in a more global manner. The data presented show it to be required for utilization of 4-aminobutyrate as a nitrogen source and for 4-aminobutyrate-induced increases in the steady-state levels of UGA1 mRNA. The DAL81 gene encodes a 970-amino-acid protein containing sequences homologous to the Zn(II)2Cys6 motif and two stretches of polyglutamine residues. Deletion of sequences homologous to the Zn(II)2Cys6 motif did not result in a detectable loss of function. On the other hand, loss of one of the polyglutamine stretches, but not the other, resulted in a 50% loss of DAL81 function.