Species-specific algal responses to zooplankton: experimental and field observations in three nutrient-limited lakes

A series of 4-day manipulations of zooplankton biomass and nutrientavailability was performed in enclosures in three lakes to determinespecies-specific algal responses to herbivory and nutrient enrichment.Algal performance in enclosures was compared to the relationshipsbetween weekly algal growth rates and the zooplankton in situ.When in situ growth rates were significant functions of zooplanktonbiomass, the responses were generally consistent with responsesin the enclosure experiments. The importance of both nutrientsand zooplankton in mediating algal growth was demonstrated bynumerous observations: strong algal community response to enrichment,unimodal or positive responses of certain algal taxa to zooplanktonbiomass, differences in degree of nutrient limitation amongthe algal response types, lack of nutrient limitation of non-grazedalgal taxa and a preponderance of taxa with no net responseto increasing zooplankton biomass. Variation in the zooplanktoncommunity may be the largest source of variability in nutrientsupply rate during summer in stratified lakes, and causes substationalvariability in the algae. Algae responded more strongly to changesin zooplankton composition than to changes in zooplankton biomass.We conclude that, due to the close coupling of phytoplanktonand zooplankton communities in these nutrient-limited lakes,major compositional changes in the zooplankton have greatereffects on the algae than do changes in biomass of grazers alreadypresent. ¹Present address: Division of Environmental Studies, Universityof California, Davis, CA 95616, USA ²Present address: Division of Biological Sciences, Universityof California, Davis, CA 95616, USA     

A mouse model for poliovirus neurovirulence identifies mutations that attenuate the virus for humans.

A mutation in the genome of poliovirus type 3 that is known to reduce neurovirulence in humans similarly reduces neurovirulence in mice when incorporated into a mouse-adapted-human poliovirus recombinant. Viral recombinants with a uracil at nucleotide position 472 in the 5'-noncoding regions of their genomes are unable to replicate in the mouse brain. Viral recombinants with a cytosine at this position are neurovirulent in mice. Neurovirulence of poliovirus in mice may therefore prove to be a useful indicator of the genetic stability of new attenuating mutations created by site-directed mutagenesis.     

G6PD Punjab,a dialysis sensitive variant of human glucose-6-phosphate dehydrogenase

Erythrocyte samples from 101 individuals, originally from Punjab and living at the time of investigation in England, were screened for glucose-6-phosphate dehydrogenase (G6PD) variants by Beutler’s fluorescent spot test and standard cellulose acetate gel (Cellogel) electrophoresis. All but 2 of the 40 males in the study were found to be indistinguishable from normal G6PD B. One of the variants had 2% of the normal activity and resembled G6PD Mediterranean in electrophoretic behaviour. The other variant showed 52% of the normal activity and migrated slower than G6PD B in Cellogel with about half of the normal band intensity. A set of physicochemical characteristics of the variant determined by conventional methods distinguished it from the variants reported so far. It was designated as G6PD Punjab, and the corresponding allele asG6PD PUN. The most striking feature of G6PD Punjab is a remarkable alteration in its electrophoretic behaviour after dialysis.     

Role of de novo protein synthesis in the interconversion of glucose transport systems in the yeast Pichia ohmeri

Glucose-repressed cells of the yeast Pichia ohmeri IGC 2879 transported glucose by facilitated diffusion. Derepression led to the formation of a glucose/proton symport and the simultaneous reduction of the facilitated diffusion capacity by about 70%. Cycloheximide prevented this interconversion indicating its dependence on de novo protein synthesis (proteosynthetic interconversion). In buffer with 2% glucose the glucose/proton symport suffered irreversible inactivation while the facilitated diffusion system was simultaneously restored. This reverse interconversion process did not require de novo protein synthesis as indicated by its lack of sensitivity to cycloheximide (degradative interconversion). Thus the glucose/proton symport system appeared to consist of about 70% of the facilitated diffusion proteins turned silent through association with additional protein(s) the latter being sensitive to glucose-induced repression and glucose-induced inactivation.     

Several nodulins of soybean share structural domains but differ in their subcellular locations.

Four soybean cDNA nodule-specific clones encoding nodulin-23, -26b, -27 and -44 were observed to cross-hybridize under low stringency conditions. Nucleotide sequence analysis revealed that the cDNAs contain three distinct domains: two domains with 70 to 95% homology separated by a third domain unique to each cDNA. Despite a number of nucleotide insertions and deletions, the protein sequences are conserved in the two domains which correlate with the homologous nucleotide domains. The amino terminal domain of each nodulin contains putative signal sequences for membrane translocation, although only two (nodulin-23 and -44) meet all the criteria for a functional signal. Immuno-precipitation of hybrid-release translation products of the four cDNAs revealed that nodulin-23 is associated with the peribacteroid membrane while nodulin-27 is in the cytoplasmic fraction of the nodule. These four nodulins are members of a diverse family with conserved structural features and the genes encoding them appear to have recently evolved from a common ancestor.     

Nodulin-26, a peribacteroid membrane nodulin is expressed independently of the development of the peribacteroid compartment.

The peribacteroid membrane (pbm) of root nodules is derived from the plant cell plasma membrane but contains in addition several nodule-specific host proteins (nodulins). Antibodies raised against purified pbm of soybean were used to immunoprecipitate polysomes to isolate an RNA fraction that served as a template for the synthesis of a cDNA probe for screening a nodule-specific cDNA library. Clone p1B1 was found to encode a 26.5 kDa polypeptide (nodulin-26) which is immunoprecipitable specifically with the anti-pbm serum. Nodulin-26 has features of a transmembrane protein and its structure differs from that of nodulin-24 which appears to be a surface protein of pbm. The expression of these two pbm nodulins was examined in nodules induced by Bradyrhizobium japonicum Tn5 mutants that arrest nodule development at different stages of pbm biosynthesis. Nodules that do not show release of bacteria from the infection thread express nodulin-24 at a very low level. In contrast, the expression of nodulin-26 occurs fully in nodules that form infection threads only and is not affected by the release of bacteria from the threads.     

High-dose,unlabeled, nonspecific antibody pretreatment: influence on specific antibody localization to human melanoma xenografts

Summary Nonspecific uptake of radiolabeled monoclonal antibodies in normal tissues is a significant problem for tumor imaging. A potential means of decreasing nonspecific antibody binding is to blockade nonspecific antibody binding sites by predosing with cold, nonspecific isotypematched antibody, before injecting specific antibody. Nontumor-specific murine monoclonal antibody LK2H10 (IgG1) or Ab-1 (IgG2a) was given i.v. at doses of 0 to 3.5 mg to nude mice with xenografts of human melanoma. These mice were then given i.v. 4 g of ¹³¹I anti-high molecular weight antigen of melanoma (HMWMAA) monoclonal antibody 763.24T (IgG1) or 225.28S (IgG2a), respectively. These mice were also given a tracer dose of ¹²⁵I LK2H10 or Ab-1, respectively. Specific tumor uptake of anti-HMWMAA antibodies was see in all cases. No drop in tumor or nontumor uptake was demonstrated for either of the tumor-specific or nonspecific monoclonal antibodies due to nonspecific monoclonal antibody pretreatment. These data suggest that high doses of isotype-matched unlabeled nonspecific monoclonal antibody given before ¹³¹I tumor-specific monoclonal antibody, will not enhance tumor imaging. Present address: Hybritech, San Diego, CA, USA     

UV irradiation inhibits initiation of DNA replication from oriC in Escherichia coli

Summary Irradiation of Escherichia coli with UV light causes a transient inhibition of DNA replication. This effect is generally thought to be accounted for by blockage of the elongation of DNA replication by UV-induced lesions in the DNA (a cis effect). However, by introducing an unirradiated E. coli origin (oriC)-dependent replicon into UV-irradiated cells, we have been able to show that the environment of a UV-irradiated cell inhibits initiation of replication from oriC on a dimer-free replicon. We therefore conclude that UV-irradiation of E. coli leads to a trans-acting inhibition of initiation of replication. The inhibition is transient and does not appear to be an SOS function.