REGULATION AND COORDINATION OF PURINE AND PYRIMIDINE BIOSYNTHESES IN YEAST. I. REGULATION OF PURINE BIOSYNTHESIS AND ITS RELATION TO TRANSIENT CHANGES IN INTRACELLULAR NUCLEOTIDE LEVELS

The control of purine biosynthesis in a yeast mutant deficient for uracil, adenine, and histidine has been studied in vivo. The adenine mutation causes accumulation of aminoimidazole ribotide in the cells. The control curve relating steady-state purine nucleotide level in the cell to rate of synthesis in the de novo purine synthetic pathway has been determined. Control in the cell depends on a feedback mechanism involving end-product inhibition. The transient responses of the purine nucleotide pool to changes in adenine input have been studied. Under certain conditions the pool overshoots when shifting from one steady-state to another. Transient changes in nucleotide levels are followed by inverse changes in the rate of attempted de novo purine synthesis. A study of the transient responses of specific intracellular nucleotides suggests that inosinic acid controls the rate of attempted purine synthesis. The transient response of nucleic acid synthesis rate to changes in nucleotide levels was studied and the implications for regulation of nucleic acid synthesis discussed.

THE RELATION BETWEEN THE INTRACELLULAR RIBONUCLEIC ACID DISTRIBUTION AND AMINO ACID INCORPORATION IN THE LIVER OF THE DEVELOPING CHICK EMBRYO

The RNA-P and DNA-P content of the nucleus and the RNA-P content of the whole cell of the livers of 8- to 20-day chick embryos and of adult fowls have been determined. The DNA-P content of the liver nuclei was slightly higher in the 8- and 10-day embryo than in all the other stages examined. A significant decrease in the RNA content of the cell occurred during embryonic development. The RNA content of the adult cell was the same as that of the 14- to 16-day embryo. The proportion of the cellular RNA contributed by the nucleus also decreased during development. In respect to both nuclear RNA content and distribution of RNA between nucleus and cytoplasm, the adult resembled the 8- to 12-day embryo. Examination of the fine structure of the cell showed that, as development progressed, free ribosomes decreased in number and the rough membranes increased. Slices of 8-, 14-, and 20-day embryonic livers and of adult livers were incubated with ¹⁴C-leucine, and the amount of labeled amino acid incorporated into whole tissue protein and into the proteins of the subcellular fractions was measured. Embryonic liver incorporated ¹⁴C-leucine 15 to 30 times more rapidly than adult liver. The microsomal protein was always more highly labelled than the protein in any other subcellular fraction; however, in the 8-day embryonic and the adult liver the proportion of total counts found in the nuclear fraction was considerably higher than in the 14- or 20-day embryonic liver. The significance of an apparent correlation between the proportion of the cell's RNA contributed by the nucleus and the proportion of total counts in the nuclear fraction is discussed.

COLLAGEN METABOLISM IN THE NORMAL AND LATHYRITIC CHICK

1. Radioisotope incorporation studies of normal and lathyritic chick embryo bone collagen do not demonstrate any interference by lathyrism with collagen synthesis or fibril formation. 2. The results indicate that a portion of the extractable collagen from lathyritic chick embryo bone represents newly synthesized protein. Evidence from a double labeling experiment and from analysis of isotope flow between the extractable and non-extractable pools suggests the extractable lathyritic collagen is heterogeneous. We propose that the lathyritic process affects collagen in all states of aggregation, probably in varying degree. 3. Puromycin, administered intravenously, reduces the amount of extractable collagen in both normal and lathyritic chick embryo bone, and diminishes the incorporation of labeled proline into collagen. 4. Marked fluctuations in incorporation of labeled amino acids into chick embryo bone collagen suggests the occurrence of wide fluctuations in metabolism of this protein.

ESTIMATION OF THE LIFE SPAN OF RED BLOOD CELLS

A modified method for fitting the model of Shemin and Rittenberg to study the life span of red cells is described. In this modification the assumption that incorporation of tag is complete before any cells die is relaxed. A recursion formula useful for fitting the model on the digital computer is given. Good results were obtained with data from the sheep, rat, cat, rabbit, and the dog, whereas in several of these cases the method of Shemin and Rittenberg was not satisfactory. By varying critical parameters of the model a continuum of curves is possible. Curves which formerly were explained on the basis of random death can be obtained without this assumption. Limitations and implications of the method are discussed.

PROTEIN SYNTHESIS AND RNA SYNTHESIS DURING MITOSIS IN ANIMAL CELLS

Protein synthesis and RNA synthesis during mitosis were studied by autoradiography on mammalian tissue culture cells. Protein synthesis was followed by incubating hamster epithelial and human amnion cells for 10 or 15 minutes with phenylalanine-C(14). To study RNA synthesis the hamster cells were incubated for 10 minutes with uridine-C(14). Comparisons of the synthetic capacity of the interphase and mitotic cells were then made using whole cell grain counts. The rate of RNA synthesis decreased during prophase and reached a low of 13 to 16 per cent of the average interphase rate during metaphase-anaphase. Protein synthesis in the hamster cells showed a 42 per cent increase during prophase with a subsequent return to the average interphase value during metaphase-anaphase. The human amnion cells showed no significant change at prophase but there was a 52 to 56 per cent drop in phenylalanine incorporation at metaphase-anaphase as compared to the average interphase rate. Colcemide was used on the hamster cells to study the effect of a prolonged mitotic condition on protein and RNA synthesis. Under this condition, uridine incorporation was extremely low whereas phenylalanine incorporation was still relatively high. The drastic reduction of RNA synthesis observed under mitotic conditions is believed to be due to the coiled condition of the chromosomes. The lack of a comparable reduction in protein synthesis during mitosis is interpreted as evidence for the presence in these cells of a relatively stable messenger RNA.

STUDIES ON IN VITRO ANTIBODY PRODUCTION. 3. PRODUCTION OF COMPLEMENT

Bone marrow, spleen, liver, and thymus cells of the guinea pig were incubated in Eagle"s minimal essential medium which contained glycine-2-C14. Flasks were removed from the water bath at 0, 6, 18, 24, and 48 hours and their contents were homogenized, centrifuged, and concentrated. Experiments were carried out with the concentrated supernatant samples to determine whether or not they contained proteins with properties similar to those of complement (C′) proteins. Titrations for hemolytic activity revealed that bone marrow cells produced C′2, C′3, and C′4; spleen cells synthesized C′2 and liver slices produced C′2. Liver supernatant samples also contained C′3 activity, but proteins associated with C′3 actiyity seemed to be released rather than synthesized by liver slices. We were unable to demonstrate C activity with thymus cell supernatants. Heat inactivation (56 °C for 30 minutes), as well as EDTA inhibited the hemolytic activity of supernatant samples.Further experiments showed that these supernatant proteins were fixed to antigen–antibody complexes and to aggregated gamma-globulin. The fixation was inhibited by guinea pig serum. Immunoelectrophoresis and gel-diffusion showed that proteins synthesized in vitro migrated as beta-globulins and were antigenically similar to C′ proteins.

PHYSIOLOGICAL STATE OF ESCHERICHIA COLI AND THE INHIBITION OF DEOXYRIBONUCLEIC ACID SYNTHESIS BY PHENETHYL ALCOHOL

Treick, R. W. (Indiana University, Bloomington), and W. A. Konetzka. Physiological state of Escherichia coli and the inhibition of deoxyribonucleic acid synthesis by phenethyl alcohol. J. Bacteriol. 88:1580-1584. 1964.-The effects of concentration of phenethyl alcohol (PEA) and the physiological state of the cells on inhibition of macromolecular synthesis in Escherichia coli were investigated. Deoxyribonucleic acid (DNA) synthesis by cells of E. coli from the maximum stationary phase is completely inhibited by 0.32% (v/v) PEA immediately upon addition of the inhibitor, although there is a net increase in the synthesis of ribonucleic acid (RNA) and protein. However, DNA synthesis in cells from the exponential phase is inhibited only after an increase which corresponds to 1.4 to 1.6 times the amount of DNA present at the time of PEA addition. In a randomly dividing culture, this increment of DNA synthesis presumably represents completion of the DNA replication cycle initiated at the time of PEA addition. By programming the addition and removal of PEA, DNA synthesis can be made to proceed in stepwise increments corresponding to doublings of the DNA. The data indicate that the DNA being replicated at the time of PEA addition completes the replication cycle and, although there is net synthesis of RNA and protein, no initiation of a second cycle of DNA replication occurs until the removal of the inhibitor.