Geoffrey Turnock

Patterns of Nucleic Acid Synthesis in Physarum polycephaium

Publisher Summary This chapter discusses the synthesis of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) during the cycle, correlating wherever possible both in vivo and in vitro experiments. It is the plasmodia1 form that is the unique feature of the myxomycetes and that has largely stimulated interest in the study of P. polycephalum and related organisms. This is so, because a plasmodium, which lacks a cell wall and is indefinite in form, grows without cell division and the nuclei in the common-cytoplasm divide with a high degree of synchrony. That this synchrony is natural in origin and does not require the chemical or physical perturbation of the organism makes the plasmodia1 form of phlysarum an ideal system, in which to analyze the biochemical processes in relation to the nuclear division cycle. The particular attraction of physarum is the high degree of natural synchrony of nuclear division in the plasmodia1 phase of the life cycle, and this chapter has surveyed some of the ways, in which this has been exploited, in the context of nucleic acid synthesis and function. The chapter also discusses the various patterns of synthesis of the major classes of nucleic acids and total protein, and considerable detail has been obtained about the organization of the replication of DNA during the S phase.

Coordination of macromolecular synthesis in the slime mould Physarum polycephalum

SummaryMicroplasmodia of P. polycephalum were grown either in batch culture, in both complex and defined media to give a 3–4 fold variation in growth rate, or in a chemostat. The protein/DNA ratio of batch cultures was almost invariant, whilst the RNA/DNA ratio increased as a non-linear function of growth rate. The amount of ribosomal RNA, expressed as a fraction of total RNA, showed little variation and this was also true for the proportion of ribosomes found in polyribosomes. Calculation of the rate of protein synthesis per ribosome shows that this parameter increases by approximately 50% over the range of growth rates studied, although it should be emphasized that the effect of protein turnover has not yet been taken into account. Enrichment of batch cultures growing in a defined medium produced an increase in the rate of RNA synthesis prior to an effect on protein synthesis.Data obtained with chemostat cultures differed in several respects from those described above for batch cultures, especially at low growth rates, and are discussed in relation to the early stages of differentiation of microplasmodia to spherules.

Glycoproteins from murine C-type virus are more acidic in virus derived from transformed cells than from nontransformed cells.

Abstract Viral proteins from Moloney-MSV(MuLV) and M-MuLV were compared by two-dimensional gel electrophoresis, which demonstrates that several of the viral proteins were present in multiple forms differing in charge but not in size. Except for the glycoprotein gp70, the isomers were found to be identical in virus from transformed and nontransformed cells. gp70 from both types of virus showed 10 to 15 spots, indicating great charge heterogeneity. Furthermore, gp70 from virus from transformed cultures had isoelectric points that were on the average one pH unit more acidic than their counterparts from nontransformed cells, and their apparent molecular weight was slightly increased. That the transformed state of the culture is important in this respect was substantiated by using a mutant M-MSV(MuLV)-CP27 which is temperature sensitive for maintenance of transformation.

A Mutant of Escherichia coli with a Defect in Energy Metabolism

SUMMARY: A mutant of Escherichia coli with a decreased growth efficiency has been investigated. The results of growth studies with different substrates and of measurement of P/O ratios in membrane preparations suggest that the strain is defective in the ability to couple synthesis of ATP to electron transport.

The direct photo-oxidative decarboxylation of α-oxo-carboxylic acids

Abstract α-Oxo-carboxylic acids undergo photo-oxidative decarboxylation from both C-1 and C-2 positions, and reaction does not involve singlet oxygen: a mechanism involving an electron transfer reaction is postulated.

Synthesis of protein during the nuclear division cycle in Physarum polycephalum

In a plasmodium of the Myxomycete Physarum polycephalum the nuclei divide synchronously, and the organism is therefore ideal for studying biochemical processes in relation to the mitotic cycle. However, its form of growth, as a single, large cell, creates difficulties in taking samples to measure growth or the synthesis of cellular components. Thus, experiments with plasmodia have usually involved taking portions of a fixed area from a single culture (for, say, pulse-labelling with a radioactive precursor) or sacrificing individual plasmodia from a replicate series as a function of time during the mitotic cycle. Such procedures cannot give very good precision in defining patterns of synthesis of macromolecules. We have developed an isotope-dilution method [ 1 ] which can be used to study the synthesis of any macromolecule that is metabolically stable and which does not involve the difficulties of interpretation of pulse-labelling experiments. Another important advantage of the method is that it does not require all the samples taken in one experiment to be identical in size. The procedure has been used to derive the patterns of synthesis of DNA and rRNA [1] and tRNA [2] during the nuclear division cycle. Determination of the accumulation curve for total protein by direct application of the isotope-dilution method, prelabelling the inoculum with a radioactive amino acid and then measuring the decrease in the specific activity of plasmodial protein, would not be acceptable because many proteins probably have halflives that are significantly short in relation to the growth rate of a plasmodium [3]. However, the isotope-dilution method can be applied indirectly by prelabelling the metabolically stable nucleic acids and then, in samples from a surface plasmodium, determining both the specific activity of the nucleic acid (S) and the amounts of nucleic acid (iV) and protein (P). Since S is proportional to N -1 [1], the relative accumulation curve for protein is given by S • N -1 • P. This paper describes the results obtained for the synthesis of protein during the mitotic cycle using this method.

A genetic analysis of a mutant of Escherichia coli with a defect in the assembly of ribosomes

SummaryThe complex phenotype of a mutant of Escherichia coli with a defect in the assembly of ribosomes has been shown to be due to the presence of two mutations, both of which are closely linked to the gene (xyl) that determines the ability to ferment xylose. One of the mutations is responsible for the defect in ribosome synthesis, whilst the other is associated with an altered degree of resistance to streptomycin and a reduced efficiency in the utilisation of the growth substrate.

Determination of the molecular weights of RNAs by low-speed sedimentation equilibrium: 16 S ribosomal RNA as a model compound

Problems associated with the determination of molecular weights of ribonucleic acids by sedimentation equilibrium have been investigated, using the sodium salt of 16 S ribosomal RNA from Escherichia coli as a model system, for which a chemical molecular weight has been computed from the sequence data (Fellner et al. , 1972; Fellner & Zimmermann, 1974) . It is shown that 16 S rRNA solutions exhibit significant interaction, probably in the form of a reversible dimerization. The sedimentation equilibrium procedure of Rowe & Rowe (1970 a,b ) is used to remove both interaction and non-ideality terms resulting in an estimated ideal molecular weight of 5·21 ± 0·11 × 10 5 for the monomeric species, based upon a partial specific volume &-v = 0·523 cm 3 ·g −1 determined using a Digital Density meter. This molecular weight value agrees well with the computed formula weight (5·33 ± 0·05 × 10 5 ), and implies that even for a polyelectrolyte such as a nucleic acid, &-v , measured conventionally, is a satisfactory parameter so far as hydrodynamic measurements are concerned. The partial specific volumes at 5°C of the sodium salts of poly(U), poly(A) and poly(U)·poly(A) have been measured and found to be closely similar (0·527, 0·531 and 0·529 cm 3 ·g −1 , respectively), and do not differ significantly from that of 16 S rRNA; suggesting that variation in base composition and secondary structure of RNAs has little effect on &-v , under defined conditions of temperature, ionic strength and counter-ion composition. A new method for measuring the absolute fringe displacement in the interference patterns obtained at sedimentation equilibrium has been developed and applied in this study (see Appendix).