Conditions that trigger guanosine tetraphosphate accumulation in Caulobacter crescentus.

Caulobacter crescentus accumulated guanosine tetraphosphate in response to nitrogen starvation but not in response to amino acid starvation. Nitrogen starvation also acted specifically to inhibit certain transitions in the C. crescentus life cycle, and guanosine tetraphosphate may act as an intracellular regulator of cell cycle events.     

Purification of native forms of eel acetylcholinesterase: active site determination

The 14 and 18 S forms of acetylcholinesterase from the electric organ of Electrophorus electricus were purified by chromatography on an N-methyl-3-aminopyridinium derivative of Affi-Gel 202. a further increase in purity was seen when these forms were separated by density gradient sedimentation subsequent to the affinity step. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate demonstrated that the 14 and 18 S forms were highly purified following these procedures. Using [³H]diisopropyl fluorophosphate labeling and separation of labeled enzyme from unreacted [³H]diisopropyl fluorophosphate by gel filtration, active site numbers of 8.3 and 11.4 were determined for the 14 and 18 S forms, respectively. These numbers compare to 4.2 active sites determined for the 11.8 S globular form of acetylcholinesterase. These results are in accord with a proposed model of two and three tetrameric structures comprising the head groups of the 14 and 18 S forms of electric tissue acetylcholinesterase.     

Threshold results in the study of schistosomiasis

In a four-population model for schistosomiasis, the effect of the introduction of a trace of infection is considered. In particular, we are interested in whether the disease establishes itself or dies out quickly. Threshold results are presented for both the deterministic and the stochastic treatment of the model.     

Investigations on the biosynthesis of steroids and terpenoids: XI. The 24-methylene sterol 24(28)-reductase of Saccharomyces cerevisiae

The microsomal fraction of Saccharomyces cerevisiae has been shown to catalyse the NADPH-dependent reduction of ergosta-5,7,22,24(28)-tetraen-3β-ol to ergosterol. This cell-free system together with whole-cell cultures of polyene-resistant mutants has been used to compare the rates of reduction of other 24-methylene sterols. The results indicate that the enzyme involved exhibits a marked specificity for ergosta-5,7,22,24(28)-tetraen-3β-ol and support the concept of a major terminal step in ergosterol biosynthesis.     

Mode of formation of cholesta-5,7-dien-3β-ol from cholest-5-en-3β-ol by larvae of Calliphora erythrocephala

1. The conversion of cholest-5-en-3beta-ol (cholesterol) into cholesta-5,7-dien-3beta-ol by axenic Calliphora erythrocephala larvae was demonstrated. 2. The transformation is probably direct (Delta(5)-->Delta(5,7)) and does not involve a Delta(0) intermediate (Delta(5)-->Delta(0)-->Delta(7)--> Delta(5,7)). 3. Delta(7)-bond formation involves the stereospecific elimination of the 7beta hydrogen atom. 4. The relative amounts of free and esterified sterols were determined in larvae grown on cholesterol as sole sterol source and on 5alpha-cholestan-3beta-ol supplemented with minimal amounts of cholesterol. 5. The significance of the results is assessed in relation to the probable role of cholesta-5,7-dien-3beta-ol as an intermediate in the biosynthesis of ecdysones.     

The maximum non-unit eigenvalue for the Wright-Fisher model of random genetic drift incorporating selection

The largest non-unit eigenvalue λ of the transition matrix for the Wright-Fisher Markov chain model of random genetic drift is found numerically with selective advantages of genotypes taken into account. Polynomials in the selection coefficients are fitted to λ in order to summarize the behaviour of λ with varying selection. Also found are the values of the selective advantages which give rise to an acceleration to the rate of fixation of alleles. These values are compared to results for the diffusion approximation to the Wright-Fisher model.     

Purine nucleoside phosphorylase (Np) in the mouse: Linkage relationship of Np-2 to esterase-10 (Es-10) and Np-1 on chromosome 14

An improved method for detecting four Np-1 (purine nucleoside phosphorylase) alleles in mouse erythrocytes by cellulose acetate electrophoresis is described. The previous linkage of Np-1 and Es-10 (esterase-10) was confirmed, with a map distance of 13.0±2.6 cM. Np-2 was detected by either specific activity assay or starch gel electrophoresis and shown to be linked to Es-10, 15.9 ± 3.1 cM, on chromosome 14. No recombinants between Np-1 and Np-2 were observed in 52 offspring, indicating either that these loci are either closely associated or that Np-2 represents simply a property of existing allelic products of the Np-1 locus.This research was supported by Medical Research Council of Canada grants to F.G.B. and F.F.S.