Turnover of murein in a diaminopimelic acid dependent mutant ofEscherichia coli

Escherichia coli 173-25, whose cell wall was labelled with¹⁴C-diaminopimelic acid, was found to lose about 15% radioactivity during growth in a fresh medium, two thirds or more being lost during the first two generations. Degradation products of the cell wall were mostly of low-molecular type. About 5% of the cells lyzed as a result of transfer associated with filtration, washing and resuspension of the bacterial population in a diaminopimelic acid (DAP) deficient medium. The degradation was very low during the first 20 min. The amount of wall material released from the cells increased between 20–30 min and a sudden decrease of viability of the population was observed. The degradation of murein triggered by starvation for DAP continued when supplementing the deficient medium with DAP and when growth was resumed. About one-half of the cell wall material released into the medium under these conditions was macromolecular. However, lysis of the cells and release of proteins into the medium were rapidly interrupted after DAP was added to the starving culture and the differential rate of synthesis of the cell wall increased. Turnover of murein was not associated with protein turnover.     

The action of 5-amino uracil on log growth and division-synchronized Tetrahymena

The influence of 5-amino uracil (5-AU) was investigated on the cell cycle of log growth and division-synchronized Tetrahymena pyriformis GL. The division index of log growth phase Tetrahymena was suppressed by 50% after 40 min in 8 mM 5-AU. Cells division-synthronized by one heat shock per generation were also treated with 5-AU. Cells treated either prior to the first synchronous division (80 min EH) or up to 25 min prior to the second synchronous division (after 160 min EH) were not delayed in their progress through the cell cycle. Cells treated during the S phase of the first free running cell cycle, however, were delayed 5-30 min from reaching the second synchronous division. The effect of 5-AU on DNA and RNA synthesis was also examined. Incorporation of [3H]thymidine into acid-precipitable material was reduced in the presence of 5-AU; the rate of DNA synthesis was also reduced. The depression in the rate of DNA synthesis was greater at the beginning of S than at the end of S. The size of the thymidine pool (nucleosides + nucleotides) did not change during 5-AU treatment; however, an accumulation of thymidine tri-phosphate and a decrease in the amount of thymidine nucleoside was observed. A suppression of [14C]uridine incorporation resulting from 5-AU treatment was observed throughout the cell cycle. The rate of RNA synthesis as monitored by [14C]uridine incorporation into acid precipitable material was also reduced during 5-AU treatment. No change in either the size or the composition of the pool of uridine (nucleoside + nucleotide) was detected in 5-AU treated cells as compared to controls.     

Interactions of Escherichia coli membrane lipoproteins with the murein sacculus.

Bifunctional cross-linking reagents were used to identify cell envelope proteins that interacted with the murein sacculus. This revealed that a number of [3H]leucine-labeled proteins and [3H]palmitate-labeled lipoproteins were reproducibly cross-linked to the sacculus in plasmolyzed cells. The results suggested that most of the cell envelope lipoproteins, and not only the murein lipoprotein, mediate interactions between the murein sacculus and the inner and/or outer membrane of the cell.     

In vivo metabolism of 2,2'-diaminopimelic acid from gram-positive and gram-negative bacterial cells by ruminal microorganisms and ruminants and its use as a marker of bacterial biomass.

Cells of Bacillus megaterium GW1 and Escherichia coli W7-M5 were specifically radiolabeled with 2,2'-diamino[G-3H]pimelic acid ([3H]DAP) as models of gram-positive and gram-negative bacteria, respectively. Two experiments were conducted to study the in vivo metabolism of 2,2'-diaminopimelic acid (DAP) in sheep. In experiment 1, cells of [3H]DAP-labeled B. megaterium GW1 were infused into the rumen of one sheep and the radiolabel was traced within microbial samples, digesta, and the whole animal. Bacterially bound [3H]DAP was extensively metabolized, primarily (up to 70% after 8 h) via decarboxylation to [3H]lysine by both ruminal protozoa and ruminal bacteria. Recovery of infused radiolabel in urine and feces was low (42% after 96 h) and perhaps indicative of further metabolism by the host animal. In experiment 2, [3H]DAP-labeled B. megaterium GW1 was infused into the rumens of three sheep and [3H]DAP-labeled E. coli W7-M5 was infused into the rumen of another sheep. The radioactivity contents of these mutant bacteria were insufficient to use as tracers, but the metabolism of DAP was monitored in the total, free, and peptidyl forms. Free DAP, as a proportion of total DAP in duodenal digesta, varied from 0 to 9.5%, whereas peptidyl DAP accounted for 8.3 to 99.2%. These data reflect the extensive metabolism of bacterially bound DAP within the gastrointestinal tracts of ruminant animals and serve as a serious caution to the uncritical use of DAP as a marker of bacterial biomass in the digesta of these animals.     

Glucocorticoid induction of growth hormone synthesis in a strain of rat pituitary cells

Conditions were determined for measuring growth hormone synthesis by a clonal strain of rat pituitary cells grown in suspension culture. Incubation of the cells with [3H]leucine in either continuous labeling or pulse-chase experiments showed that secretion of newly synthesized growth hormone commences only after a lag of about 15 min. The pulse-chase experiments also demonstrated that there is no detectable degradation by the cells of growth hormone. Thus growth hormone synthesis could be measured, in the absence of complications arising either from secretion or degradation of growth hormone, by incubating the cells with [3H]leucine for 10 min. Exposure of cells grown under the usual culture conditions to dexamethasone (a synthetic glucocorticoid) led to an average stimulation of specific growth hormone synthesis (growth hormone synthesis/total cytoplasmic protein synthesis) of only 2.6-fold. However, two other growth conditions were found in which dexamethasone routinely yielded a 5- to 15-fold stimulation of specific growth hormone synthesis. One of these conditions, involving substitution of 10% fetal calf serum for the normal serum supplement, was employed in subsequent experiments. A stimulation of specific growth hormone synthesis could be observed at 10(-9) M dexamethasone, and the maximum stimulation was observed at dexamethasone concentrations of about 10(-8) to 10(-7) M. There was a lag of about 6 h before a stimulation by dexamethasone of specific growth hormone synthesis was detected. Thereafter, the stimulation increased in a nearly linear fashion until maximum stimulation was reached at about 48 h.     

Evidence for multisite growth of Escherichia coli murein involving concomitant endopeptidase and transpeptidase activities.

During diaminopimelic acid starvation of Escherichia coli W7, a large fraction of the preexisting murein cross-links are opened by murein endopeptidase and the resulting uncross-linked material is degraded. This is reflected morphologically in a general loss of rigidity of the murein sacculus long before lysis occurs. In growing cells, a dynamic situation is demonstrable. When cells whose murein sacculi are uniformly labeled with [14C]diaminopimelic acid were chased with unlabeled DAP, a significant, rapid shift of [14C]diaminopimelic acid from the donor to the acceptor half of dimers was observed. The shift can be explained by the presence of about 100 separate sites where new murein strands were being inserted between old radioactive strands of murein. Thus, the gradual loss of rigidity of the murein sacculus as endopeptidase continues to function during starvation of E. coli W7 suggests an even distribution of the active endopeptidases. This is consistent with the kinetic data which suggest that endopeptidase, along with murein synthetase and transpeptidase, acts at about 100 distinct sites to elongate the murein sacculus.     

Macromolecules released into the culture medium during the vegetative cell cycle of the unicellular green alga Chlamydomonas reinhardii.

The culture medium of growing Chlamydomonas reinhardii cells contains hydroxyproline-rich glycoproteins, which are mainly liberated during release of the zoospores from the mother-cell wall. Pulse-labelling studies with [3H]proline and [35S]methionine have been performed in order to detect the protein components released by synchronously growing cells at different stages of the cell cycle. When either [3H]proline or [35S]methionine were applied during the phase of cell growth, radioactive label appeared in the released macromolecules after a lag period of 40 min, whereas incorporation into the insoluble part of the cell wall was delayed only by 20 min. When applied at the end of the growth phase, e.g. 13 h after beginning of the illumination period, the radioactive amino acids were incorporated into the cell wall, but radioactive labelling of macromolecules released into the culture medium could not be detected before the zoospores were liberated from the mother-cell wall. Maximal incorporation of [3H]proline and [35S]methionine into the insoluble part of the cell wall was observed during cell division, but essentially no radioactively-labelled macromolecules were released into the culture medium during this time period. Analysis of the macromolecules, which were liberated during cell enlargement, by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed distinct radioactive bands, which were differentially labelled with [3H]proline and [35S]methionine. Among the macromolecules released into the culture medium during cell growth, a component of an apparent Mr 35 000 was preferentially labelled with [3H]proline. This component was also detected after labelling with [35S]methionine, but components of an apparently higher Mr were more prominent after labelling with [35S]methionine. Macromolecules released during the cell-enlargement period of synchronously growing cultures in the presence of [3H]proline contained radioactively-labelled hydroxyproline in addition to proline. These results show that, during cell-wall growth, specific protein components are released into the culture medium and that at least one of these components contains large amounts of proline and hydroxyproline. At least some of these macromolecules seem to be constituents of the cell wall, because during pulse-chase experiments radioactively-labelled macromolecules appeared in the culture medium mainly during the time period when the specific radioactivity of the insoluble inner-cell-wall layer decreased.     

Absolute rates of sterol synthesis estimated from [3H]water for bovine lens epithelial cells in culture

All cells of the avascular ocular lens derive from a monolayer of epithelial cells located on only the anterior surface of this organ. The source of the cholesterol required for the growth and division of these cells was studied by using cultures of bovine lens epithelial cells. Cells were in active growth during the third to fourth day of subculture following seeding. Absolute rates of cholesterol synthesis were estimated for the cultured cells from incorporation of [3H]water. Rates were estimated on the assumption that 0.81 atoms of 3H of [3H]water were incorporated into cholesterol per carbon atom of cholesterol, a situation where all of the NADPH would be generated by oxidative enzymatic processes. We tested this assumption by measuring the changes in sterol mass per dish of cells grown in lipoprotein-deficient media over day 3 to 4 of subculture and by simultaneously measuring the rates of incorporation of [3H]water into sterols during this period. In this situation, the increases in sterol mass should be attributable solely to de novo sterol synthesis. We calculated that an average of 0.79 atoms of 3H of [3H]water were incorporated by these cells into cholesterol per carbon atom of cholesterol. Sterol synthesis was only modestly decreased (about 30%) when the cells were cultured in media prepared with whole calf serum. Growth rates of the cells were also little affected by the absence of lipoproteins. In spite of the capacity to furnish its sterol requirements by de novo synthesis, the lens epithelial cells readily degraded 125I-labeled bovine LDL, and LDL greatly decreased sterol synthesis when added to the media at low levels.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo studies on the metabolism of pyrimidine nucleosides (author's transl)]

3H-labelled metabolites were determined in the perchloric acid-soluble fraction of blood plasma and liver of adult male Wistar rats, following the application of [5 - 3H]uridine. Ten minutes after the injection of uridine, only 20% of the total 3H activity of the plasma could be attributed to [3H]uridine. The remaining radioactivity was found chiefly in [3H]uracil (40%) and 3H2O (20%). In the liver, at 10 min, [3H]-uridine and [3H]uracil together accounted for less than 0.5% of the total radioactivity; about 70% of the radioactivity was due to [3H]beta-alanine, and 15% to 3H2O. 45 min after the injection, 70% of the radioactivity in the plasma was due to 3H2O, whereas uridine and uracil represented about 4% and 6%, respectively. At this time, about 55% of the radioactivity in the liver was due to [3H]beta-alanine, about 40% to 3H2O, and about 5% to unidentified metabolites; [3H]uridine and [3H]uracil were not observed. A comparison of the rate of catabolism of [5-3H]-uridine, [5-3H]cytidine and [6-3H]thymidine showed that cytidine is degraded in the organism 25 times more slowly than uridine or thymidine. The biological half lives for the total degradation of the [3H]nucleosides to 3H2O, based on the values in the plasma, were: uridine 1.1 h; thymidine 1.3 h; cytidine 25 h. Furthermore, the turnover time of exogenous uridine in the plasma was found to be 9 min, which gives a half life of 6 min for the metabolism of exogenous uridine to uracil.     

Effect of different muscle shortening velocities during prolonged incremental cycling exercise on the plasma growth hormone, insulin, glucose, glucagon, cortisol, leptin and lactate concentrations.

BACKGROUND: Strenuous exercise was reported to involve the alteration in the release of some "stress" hormones such as growth hormone (GH), cortisol, catecholamines and appropriate adjustment of energy metabolism but the relative contribution of these hormones to metabolic response, to cycling exercise performed at different muscle shortening velocities, has not been clarified. AIMS: The purpose of this experiment was to assess the effect of applying different pedalling rates during a prolonged incremental cycling exercise test on the changes in the plasma levels of growth hormone, cortisol, insulin, glucagon and leptin in humans. Material and METHODS: Fifteen healthy non-smoking men (means +/- SD: age 22.9 +/- 2.4 years; body mass 71.9 +/- 8.2 kg; height 178 +/- 6 cm; with VO2max of 3.896 +/- 0.544 1 x min(-1), assessed in laboratory tests, were subjects in this study. The subjects performed in two different days a prolonged incremental exercise tests at two different pedalling rates, one of them at 60 and another at 120 rev x min(-1). During this tests the power output has increased by 30 W every 6 minutes. The tests were stopped when the subject reached about 70 % of the VO2max. RESULTS AND CONCLUSIONS: We have found that choosing slow or fast pedalling rates (60 or 120 rev min(-1)), while generating the same external mechanical power output, had no effect on the pattern of changes in plasma cortisol, insulin, glucagon, glucose and leptin concentrations. But, generation of the same external mechanical power output at 120 rev min(-1) causes more stepper increase (p < 0.01) in the plasma growth hormone concentration [GH]pl and plasma lactate concentrations [La]pl when compared to that observed during cycling at 60 rev x min(-1). We have also found that the onset of a significant increase in [GH]pl during cycling at 60 rev x min(-1) was not accompanied by significant increase in [La]pl. While during cycling at 120 rev x min(-1) the onset of a significant increase in [La]pl occurred without increase in [GH]pl, but with continuation of exercise when plasma [La]pl increased, there was also a parallel rise in plasma [GH]pl, as reported before. This results indicates that the increase in [GH]pl during exercise is not closely related to the increase in [La]pl.     

Uptake and subcellular distribution of [3H]arachidonic acid in murine fibrosarcoma cells measured by electron microscope autoradiography

We have used quantitative electron microscope autoradiography to study uptake and distribution of arachidonate in HSDM1C1 murine fibrosarcoma cells and in EPU-1B, a mutant HSDM1C1 line defective in high affinity arachidonate uptake. Cells were labeled with [3H]arachidonate for 15 min, 40 min, 2 h, or 24 h. Label was found almost exclusively in cellular phospholipids; 92-96% of incorporated radioactivity was retained in cells during fixation and tissue processing. All incorporated radioactivity was found to be associated with cellular membranes. Endoplasmic reticulum (ER) contained the bulk of [3H]arachidonate at all time points in both cell types, while mitochondria, which contain a large portion of cellular membrane, were labeled slowly and to substantially lower specific activity. Plasma membrane (PM) also labeled slowly, achieving a specific activity only one-sixth that of ER at 15 min in HSDM1C1 cells (6% of total label) and one-third of ER in EPU-1B (10% of total label). Nuclear membrane (NM) exhibited the highest specific activity of labeling at 15 min in HSDM1C1 cells (twice that of ER) but was not preferentially labeled in the mutant. Over 24 h, PM label intensity increased to that of ER in both cell lines. However, NM activity diminished in HSDM1C1 cells by 24 h to a small fraction of that in ER. In response to agonists, HSDM1C1 cells release labeled arachidonate for eicosanoid synthesis most readily when they have been labeled for short times. Our results therefore suggest that NM and ER, sites of cyclooxygenase in murine fibroblasts, are probably sources for release of [3H]arachidonate, whereas PM and mitochondria are unlikely to be major sources of eicosanoid precursors.     

DNA synthesis rate changes during the S phase in mouse epidermis

The in vivo DNA synthesis rate throughout the S phase of mouse epidermal cells was investigated. Epidermal basal cells were isolated at various times of the day from normal animals injected with [3H]TdR 30 min before sacrifice, and from pulse-labelled animals with regenerating and growth-inhibited epidermis. The cells were analysed by DNA flow cytometry combined with cell sorting. Cells from successive fractions of the S phase were sorted on glass slides and subjected to quantitative [3H]TdR autoradiography. The results confirmed the presence of unlabelled (slowly replicating) cells in the S phase, the proportion of which was circadian stage-dependent with minimum values at midnight and in the early morning. The DNA synthesis rate throughout the S phase showed a general trend with high values in the mid-fractions, a pattern which was similar in normal and in growth perturbed epidermis. In the early morning the DNA synthesis rate pattern was bimodal with maxima both in the first and second half of the S phase, with a corresponding trough in mid-S. At this time of day the cell progression rate through S is at its maximum, indicating a relationship between the overall DNA synthesis rate and the rate distribution pattern through S.     

Phytohemagglutinin induces rapid degradation of phosphatidylinositol 4,5-bisphosphate and transient accumulation of phosphatidic acid and diacylglycerol in a human T lymphoblastoid cell line, CCRF-CEM

The human T lymphoblastoid cell line designated CCRF-CEM responds to phytohemagglutinin with a 3.7-fold enhancement of the 32PO4 incorporation into phosphatidylinositol. In myo-[2-3H]inositol-prelabeled CCRF-CEM cells, phytohemagglutinin induced a 3.3-fold accumulation of myo-[2-3H]inositol phosphate during 15 min incubation at 37 degrees C in the presence of 5 mM LiCl. Since Li+ is a potent inhibitor of myo-inositol-1-phosphatase, the results indicate that phytohemagglutinin induces the hydrolysis of inositol lipids in CCRF-CEM cells. In 32PO4-prelabeled CCRF-CEM cells, phytohemagglutinin induced a breakdown of 28% of [32P]phosphatidylinositol 4,5-bisphosphate 40-60 s after the stimulation. The decrease of [32P]phosphatidylinositol 4,5-bisphosphate was found as early as 10 s after the stimulation. This decrease was followed by an increased 32P-labeling of phosphatidic acid. In [2-3H]glycerol-prelabeled CCRF-CEM cells, phytohemagglutinin induced a transient accumulation of [3H]phosphatidic acid and [3H]diacylglycerol. The amount of [3H]phosphatidic acid in the stimulated cells was 3.7-times the control value at 2 min after the stimulation, whereas the amount of [3H]diacylglycerol in the stimulated cells was 1.5-times the control value at 5 min after the stimulation. In [3H8]arachidonate-prelabeled CCRF-CEM cells, phytohemagglutinin induced a transient accumulation of [3H]phosphatidic acid; the amount was 2.5-times the control value at 2 min after the stimulation. Quinacrine (1 mM) caused 41% reduction in the amount of [3H]phosphatidic acid accumulated by the stimulation in [2-3H]glycerol-prelabeled cells. Stimulation in a Ca2+-free saline containing 1 mM EGTA caused 53% reduction in the amount of [3H]phosphatidic acid accumulated by the stimulation. The results presented in this paper indicate that a human T lymphoblastoid cell line, CCRF-CEM, responds to phytohemagglutinin with a rapid turnover of inositol lipids.     

Kinetics of uptake and incorporation of meso-diaminopimelic acid in different Escherichia coli strains.

The rate at which the peptidoglycan precursor meso-diaminopimelic acid (DAP) is incorporated into the cell wall of Escherichia coli cells was determined by pulse-label experiments. For different E. coli strains, the incorporation rate was compared with the rate of uptake of DAP into the cell. With E. coli W7, a dap lys mutant generally used in this kind of studies, steady-state incorporation was reached only after about 0.75 of the doubling time. This lag period can be ascribed to the presence of a large internal DAP pool in the cells. An E. coli K-12 lysA strain was constructed which could be grown without DAP in its medium. Consequently, due to the higher specific activity of the added [3H]DAP, faster incorporation and higher levels of radioactivity in the peptidoglycan layer were observed in the K-12 lysA strain than in the W7 strain. In addition, uptake and incorporation were faster in steady state (within about 0.2 of the doubling time), indicating a smaller DAP pool. The lag period could be further diminished and the incorporation rate could be increased by feedback inhibition of the biosynthetic pathway to DAP with threonine and methionine. These results make MC4100 lysA a suitable strain for studies on peptidoglycan synthesis. To explain our observations, we suggest the existence of an expandable pool of DAP in E. coli which varies with the DAP concentration in the growth medium. With 2 microgram of DAP per ml, the size of the pool is severalfold the amount of DAP contained in the cell wall. This pool can be partly washed out of the cells. Grown without DAP, MC4100 lysA still has a small pool caused by endogenous synthesis, which accounts for the fact that steady-state [3H]DAP incorporation in the lysA strain still shows a lag period.     

Site of Inhibition of Peptidoglycan Biosynthesis During the Stringent Response in Escherichia coli

The site of inhibition of peptidoglycan synthesis during the stringent response in Escherichia coli was determined in strains which were auxotrophic for both lysine and diaminopimelic acid (DAP). Cells were labeled with [(3)H]DAP for 30 to 60 min in the presence and absence of required amino acids, and the cellular distribution of [(3)H]DAP was determined. In both stringent (rel(+)) and relaxed (relA) strains, amino acid deprivation did not inhibit the incorporation of [(3)H]DAP into the nucleotide precursor and lipid intermediate fractions. The amount of [(3)H]DAP incorporated into the peptidoglycan fraction by the amino acid-deprived relA strain was over 70% of the amount incorporated in the presence of required amino acids. In contrast, the amounts of labeled peptidoglycan in amino acid-deprived rel(+) strains were only 20 to 44% of the amounts synthesized in the presence of amino acids. These results indicate that a late step in peptidoglycan synthesis is inhibited during the stringent response. The components of the lipid intermediate fraction synthesized by rel(+) strains in the presence and absence of required amino acids were quantitated. Amino acid deprivation did not inhibit the synthesis of either the monosaccharide-pentapeptide or the disaccharide-pentapeptide derivatives of the lipid intermediate. Thus, the reaction which is most likely inhibited during the stringent response is the terminal one involving the incorporation of the disaccharide-pentapeptide into peptidoglycan.     

RNA and protein synthesis in sperm cells isolated from Zea mays L. pollen

Summary Sperm cells are thought to be quiescent in pollen and activated upon pollen germination. To test this hypothesis, protein, RNA and DNA synthesis were assessed in Zea mays sperm cells at different times after isolation from pollen. Protein synthesis changed with time; while some proteins were found to be constitutive in both 0 and 24 h cells, others were synthesized and some disappeared. Overall, the number of proteins detected at 24 h doubled compared with freshly isolated cells. Incorporation of [³H]leucine in 24 h cells was about 50 times that in freshly isolated cells, and that of [5, 6-³H]uridine, about 7 times. Very low incorporation of [6-³H]thymidine into the cells was detected; there was no difference between freshly isolated and 24 h cells. It is possible that the differences in synthetic activity between freshly isolated and 24-h-old cells might correspond to sperm cell activation during pollen tube growth. If so, these metabolic changes may play an important role in fertilization.Supported by funds from a Strategic Grant (D.D.C.) and an Operating Grant (D.J.G.) from the Natural Sciences and Engineering Research Council of Canada     

Proliferative kinetics of human lymphocytes in culture measured by autoradiography and sister chromatid differential staining

A simple combination of autoradiography, to determine when a cell synthesized DNA, and sister chromatid differential staining, to determine how many times a cell has divided, was used to follow up the proliferating fate of human lymphocytes in culture. Cells were incubated continuously with 5-bromodeoxyuridine (BrdU) and pulse-labelled with 0.1 muCi/ml [3H]thymidine at various times after stimulation with phytohemagglutinin (PHA). The cells were then harvested at 4 h intervals up to 72 h, and the percentage of labelled mitoses was determined separately in first, second, or third division cells. The data showed that the cycling cells, whether they began cycling at earlier or later times after stimulation, had about the same generation times of 12--14 h. This confirms that the heterogeneity of cell generations seen in short-term lymphocyte cultures is in large part due to the difference in the times when cells began cell cycling in response to PHA.     

Movement of zymosterol, a precursor of cholesterol, among three membranes in human fibroblasts

Where examined, cholesterol is synthesized in the endoplasmic reticulum; however, its precursor, zymosterol, is found mostly in the plasma membrane. The novel implication of these disparate findings is that zymosterol circulates within the cell. In tracing its movements, we have now established the following: (a) in human fibroblasts, zymosterol is converted to cholesterol solely in the rough ER. (b) Little or no zymosterol or cholesterol accumulates in the rough ER in vivo. (c) Newly synthesized zymosterol moves to the plasma membrane without a detectable lag and with a half-time of 9 min, about twice as fast as cholesterol. (d) The pool of radiolabeled zymosterol in the plasma membrane turns over rapidly, faster than does intracellular cholesterol. Thus, plasma membrane zymosterol is not stagnant. (e) [3H]Zymosterol pulsed into intact cells is initially found in the plasma membrane. It is rapidly internalized and is then converted to [3H] cholesterol. Half of the [3H]cholesterol produced returns to the plasma membrane within 30 min of the initial [3H]zymosterol pulse. (f) Nascent zymosterol accumulates in a buoyant sterol-rich intracellular membrane before it reaches the plasma membrane. This membrane also acquires nascent cholesterol, exogenous [3H]zymosterol pulsed into intact cells, and [3H]cholesterol synthesized from the exogenous [3H] zymosterol. These results suggest that at least one sterol moves rapidly and in both directions among the rough endoplasmic reticulum, a sterol-rich intracellular membrane bearing nascent cholesterol, and the plasma membrane.     

Turnover of polyadenylated messenger RNA in fission yeast. Evidence for the control of protein synthesis at the translational level.

Polyadenylated RNA was isolated from fission yeast (Schizosaccharomyces pombe) total RNA using oligo(dT)-cellulose, and was studied as a model for messenger RNA. The half-life of poly adenylated RNA was measured by two independent methods. (a) The rate of labelling of polyadenylated RNA during incubation of cells with [5-3H]uridine was measured. A half-life of 40-45 min was found by comparing the experimental data with theoretical curves calculated for labelling of RNAs with various half-lives. The influence of precursor-pool specific activity on RNA labelling kinetics is considered. (b) Cells were labelled with [5-3H]uridine then further RNA synthesis was inhibited by addition of 8-hydroxyquinoline. The rate of loos of radioactivity from polyadenylated RNA indicated a half-life of 50 min. The half-life found by these two methods is about one-third of the cell doubling time, and is much longer than previous estimates by indirect methods of yeast messenger RNA half-life. Both experimental methods provided evidence for the existence of tas a half-life of 40-50 min; a much smaller population is probably turning over more rapidly. After inhibition of RNA synthesis by 8-hydroxyquinoline, the rate of total protein synthesis declined much more rapidly than the polyadenylated RNA content of the cells. However, 60 min after inhibition of RNA synthesis there was a small rise in the rate of portein synthesis. These data are interpreted as evidence for mechanisms controlling protein synthesis which operate at the level of messenger RNA translation.