Host Subunit of Qβ Replicase is Translation Control Factor i

THE tetrameric phage Qβ replicase is composed of three pre-existing E. coli proteins in addition to the phage coded synthetase^1,2. The host subunits also appear to form part of the f2 replicase³. We have found that the cistron specific factor i⁴ cross reacts immunologically and coelectrophoresis on SDS-acrylamide gel with the largest replicase subunit.     

Cistron Specific Translation Control Protein in <Emphasis Type="Italic">Escherichia coli</Emphasis>

GENE expression may be controlled during translation by ribosomal selection of mRNAs or even individual cistrons. Escherichia coli initiation factors associated with ribosomes affect the binding of ribosomes to mRNA^1,2; initiation factor IF3, for instance, influences the specificity of mRNA-ribosome interaction^3,4. IF3 activity has been separated into several fractions which show various specificities for different mRNA cistrons^4–9. An important problem is the possibility of intracellular changes in IF3 activity^10–12. From uninfected E. coli, we have now isolated a protein which changes the specificity of IF3 toward different mRNAs; we call this interference factor i. Pure factor i binds to IF3 and specifically affects the translation of T4 and MS2 RNA. Whereas the initiation of translation of the MS2 coat protein cistron is inhibited by factor i, the synthetase cistron—when available—is more rapidly initiated in the presence of factor i. The overall translation of T4 mRNA appears unchanged by factor i, but certain cistrons are stimulated at the expense of others. Interfering factors such as factor i could be important in controlling translation in E. coli.     

Effect of a Purified Initiation Factor F3 (B) on the Selection of Ribosomal Binding Sites on Phage MS2 RNA

STUDIES with T4 mRNA showed that initiation factor F2 (C) promotes the attachment of ribosomes to mRNA¹. On the 30S ribosomal subunit this effect is independent of the function of F2 in the binding of formylmethionyl tRNA², whereas formation of a 70S-mRNA complex depends on the binding of fMet-tRNA³. Template competition experiments⁴ showed that, with F2 (C), the ribosome seems to have the same affinity for synthetic polynucleotides as for natural mRNA. Addition of initiation factor F3 (B), however, leads to preferential binding of ribosomes to the natural mRNA. This suggests⁴ that while factor F2 (C) binds the ribosome to any site on the mRNA, the function of factor F3 (B) is to recognize some specific signal in natural mRNA corresponding, perhaps, to the beginning of a cistron. Fractionation of initiation factor F3 (B) into several species differing in their specificity for different mRNA templates⁵ gave further support to the hypothesis that this protein can select binding sites. An excellent system to demonstrate this effect of F3 (B) would be the binding of ribosomes to RNA from E. coli RNA bacteriophages, since Steitz⁶ has analysed and determined the nucleotide sequence of the three binding sites corresponding to the three cistrons of R17 mRNA. Experiments were thus undertaken to study the effect of a purified fraction of F3 (B) on the binding of ribosomes to the different sites of such a phage RNA.     

A tale of two pesticides: how common insecticides affect aquatic communities

1. Recent ecotoxicology studies show that pesticide exposure can alter community composition, structure and function. Generally, community responses to pesticides are driven by trait‐ and density‐mediated indirect effects resulting from sublethal and lethal effects of pesticide exposure on vulnerable taxa. These effects depend upon the concentration of the pesticide and the frequency of exposure. 2. While more research is needed to understand community‐level responses to pesticide exposure, testing the effects of multitudes of registered chemicals on ecologically relevant communities is overwhelming. Recent reviews suggest that contaminants with similar modes of action should produce comparable community‐level responses because they have similar direct effects and, as a result, similar indirect effects; this hypothesis remains largely untested. 3. We subjected pond communities [containing zooplankton, phytoplankton, periphyton and leopard frog tadpoles (Rana pipiens)] to several applications (single applications of medium or high concentrations or weekly applications of a lower concentration) of two acetylcholine esterase inhibiting insecticides, malathion and carbaryl that have comparable toxicity for aquatic organisms. 4. We found that both insecticides cause comparable trophic cascades that affect zooplankton and phytoplankton abundances; however, their effects on amphibians diverged, especially when exposed to higher concentrations of insecticides. Malathion caused a trophic cascade beginning with a decline in cladocerans followed by increases in phytoplankton. At a medium concentration, this cascade also caused a subsequent decrease in periphyton. Carbaryl caused a similar trophic cascade with the highest application, a weak trophic cascade with the medium application and no cascade with smallest application. Malathion directly reduced tadpole survival at all concentrations. Survivors in the two higher treatments were larger at metamorphosis while survivors in the lowest treatments were smaller and developed slowly. In contrast, carbaryl was not directly toxic to tadpoles, but indirectly reduced survival because slow growth and development prevented some tadpoles from metamorphosing before the mesocosms dried at medium and low applications. 5. These results suggest that these common pesticides, which share the same mode of action, have similar effects on zooplankton and algae, but differences in the strength and timing of their effects on tadpoles reduce the generality of responses at higher trophic levels. Overall, general predictive models of contaminant effects could be improved by incorporating the relative timing of direct and indirect effects of exposure.