Turnover of protein and nucleic acid in soluble and ribosome fractions of non-growing Escherichia coli

Distinct Survival, Growth Lag, and rRNA Degradation Kinetics during Long-Term Starvation for Carbon or Phosphate

The stationary phase is the general term for the state a bacterial culture reaches when no further increase in cell mass occurs due to exhaustion of nutrients in the growth medium. Depending on the type of nutrient that is first depleted, the metabolic state of the stationary phase cells may vary greatly, and the subsistence strategies that best support cell survival may differ. As ribosomes play a central role in bacterial growth and energy expenditure, ribosome preservation is a key element of such strategies. To investigate the degree of ribosome preservation during long-term starvation, we compared the dynamics of rRNA levels of carbon-starved and phosphorus-starved Escherichia coli cultures for up to 28 days. The starved cultures' contents of full-length 16S and 23S rRNA decreased as the starvation proceeded in both cases, and phosphorus starvation resulted in much more rapid rRNA degradation than carbon starvation. Bacterial survival and regrowth kinetics were also quantified. Upon replenishment of the nutrient in question, carbon-starved cells resumed growth faster than cells starved for phosphate for the equivalent amount of time, and for both conditions, the lag time increased with the starvation time. While these results are in accordance with the hypothesis that cells with a larger ribosome pool recover more readily upon replenishment of nutrients, we also observed that the lag time kept increasing with increasing starvation time, also when the amount of rRNA per viable cell remained constant, highlighting that lag time is not a simple function of ribosome content under long-term starvation conditions. IMPORTANCE The exponential growth of bacterial populations is punctuated by long or short periods of starvation lasting from the point of nutrient exhaustion until nutrients are replenished. To understand the consequences of long-term starvation for Escherichia coli cells, we performed month-long carbon and phosphorus starvation experiments and measured three key phenotypes of the cultures, namely, the survival of the cells, the time needed for them to resume growth after nutrient replenishment, and the levels of intact rRNA preserved in the cultures. The starved cultures' concentration of rRNA dropped with starvation time, as did cell survival, while the lag time needed for regrowth increased. While all three phenotypes were more severely affected during starvation for phosphorus than for carbon, our results demonstrate that neither survival nor lag time is correlated with ribosome content in a straightforward manner.

Bacterial Longevity Requires Protein Synthesis and a Stringent Response

Gram-negative bacteria in infections, biofilms, and industrial settings often stop growing due to nutrient depletion, immune responses, or environmental stresses. Bacteria in this state tend to be tolerant to antibiotics and are often referred to as dormant. Rhodopseudomonas palustris, a phototrophic alphaproteobacterium, can remain fully viable for more than 4 months when its growth is arrested. Here, we show that protein synthesis, specific proteins involved in translation, and a stringent response are required for this remarkable longevity. Because it can generate ATP from light during growth arrest, R. palustris is an extreme example of a bacterial species that will stay alive for long periods of time as a relatively homogeneous population of cells and it is thus an excellent model organism for studies of bacterial longevity. There is evidence that other Gram-negative species also continue to synthesize proteins during growth arrest and that a stringent response is required for their longevity as well. Our observations challenge the notion that growth-arrested cells are necessarily dormant and metabolically inactive and suggest that such bacteria may have a level of metabolic activity that is higher than many would have assumed. Our results also expand our mechanistic understanding of a crucial but understudied phase of the bacterial life cycle.IMPORTANCE We are surrounded by bacteria, but they do not completely dominate our planet despite the ability of many to grow extremely rapidly in the laboratory. This has been interpreted to mean that bacteria in nature are often in a dormant state. We investigated life in growth arrest of Rhodopseudomonas palustris, a proteobacterium that stays alive for months when it is not growing. We found that cells were metabolically active, and they continued to synthesize proteins and mounted a stringent response, both of which were required for their longevity. Our results suggest that long-lived bacteria are not necessarily inactive but have an active metabolism that is well adjusted to life without growth.

Transfer RNA instability as a stress response in Escherichia coli: Rapid dynamics of the tRNA pool as a function of demand

Production of the translation apparatus of E. coli is carefully matched to the demand for protein synthesis posed by a given growth condition. For example, the fraction of RNA polymerases that transcribe rRNA and tRNA drops from 80% during rapid growth to 24% within minutes of a sudden amino acid starvation. We recently reported in Nucleic Acids Research that the tRNA pool is more dynamically regulated than previously thought. In addition to the regulation at the level of synthesis, we found that tRNAs are subject to demand-based regulation at the level of their degradation. In this point-of-view article we address the question of why this phenomenon has not previously been described. We also present data that expands on the mechanism of tRNA degradation, and we discuss the possible implications of tRNA instability for the ability of E. coli to cope with stresses that affect the translation process.

Transfer RNA is highly unstable during early amino acid starvation in Escherichia coli

Due to its long half-life compared to messenger RNA, bacterial transfer RNA is known as stable RNA. Here, we show that tRNAs become highly unstable as part of Escherichia coli's response to amino acid starvation. Degradation of the majority of cellular tRNA occurs within twenty minutes of the onset of starvation for each of several amino acids. Both the non-cognate and cognate tRNA for the amino acid that the cell is starving for are degraded, and both charged and uncharged tRNA species are affected. The alarmone ppGpp orchestrates the stringent response to amino acid starvation. However, tRNA degradation occurs in a ppGpp-independent manner, as it occurs with similar kinetics in a relaxed mutant. Further, we also observe rapid tRNA degradation in response to rifampicin treatment, which does not induce the stringent response. We propose a unifying model for these observations, in which the surplus tRNA is degraded whenever the demand for protein synthesis is reduced. Thus, the tRNA pool is a highly regulated, dynamic entity. We propose that degradation of surplus tRNA could function to reduce mistranslation in the stressed cell, because it would reduce competition between cognate and near-cognate charged tRNAs at the ribosomal A-site.

Heavy water and (15) N labelling with NanoSIMS analysis reveals growth rate-dependent metabolic heterogeneity in chemostats

To measure single-cell microbial activity and substrate utilization patterns in environmental systems, we employ a new technique using stable isotope labelling of microbial populations with heavy water (a passive tracer) and (15) N ammonium in combination with multi-isotope imaging mass spectrometry. We demonstrate simultaneous NanoSIMS analysis of hydrogen, carbon and nitrogen at high spatial and mass resolution, and report calibration data linking single-cell isotopic compositions to the corresponding bulk isotopic equivalents for Pseudomonas aeruginosa and Staphylococcus aureus. Our results show that heavy water is capable of quantifying in situ single-cell microbial activities ranging from generational time scales of minutes to years, with only light isotopic incorporation (∼0.1 atom % (2) H). Applying this approach to study the rates of fatty acid biosynthesis by single cells of S. aureus growing at different rates in chemostat culture (∼6 h, 1 day and 2 week generation times), we observe the greatest anabolic activity diversity in the slowest growing populations. By using heavy water to constrain cellular growth activity, we can further infer the relative contributions of ammonium versus amino acid assimilation to the cellular nitrogen pool. The approach described here can be applied to disentangle individual cell activities even in nutritionally complex environments.

Evaluating rRNA as an indicator of microbial activity in environmental communities: limitations and uses

Microbes exist in a range of metabolic states (for example, dormant, active and growing) and analysis of ribosomal RNA (rRNA) is frequently employed to identify the 'active' fraction of microbes in environmental samples. While rRNA analyses are no longer commonly used to quantify a population's growth rate in mixed communities, due to rRNA concentration not scaling linearly with growth rate uniformly across taxa, rRNA analyses are still frequently used toward the more conservative goal of identifying populations that are currently active in a mixed community. Yet, evidence indicates that the general use of rRNA as a reliable indicator of metabolic state in microbial assemblages has serious limitations. This report highlights the complex and often contradictory relationships between rRNA, growth and activity. Potential mechanisms for confounding rRNA patterns are discussed, including differences in life histories, life strategies and non-growth activities. Ways in which rRNA data can be used for useful characterization of microbial assemblages are presented, along with questions to be addressed in future studies.

Control of ribosome turnover during growth of the haloalkaliphilic archaeon Natronococcus occultus

The metabolism of ribosomes during growth of the haloalkaliphilic archaeon Natronococcus occultus was examined. The ribosome content was higher during exponential growth and diminished to 35% of the maximum in the stationary stage. The incorporation of H3-orotic acid and C14-uracil into rRNA was higher during exponential growth. After that, it decreased to 39% of the maximum in the stationary stage. The labeling of non-ribosomal RNA took place almost exclusively in the exponential stage. From loss of radioactivity, the half-life of rRNA was 11.43, 14.85, 5.28 and 7.14 h during the initial, exponential, late exponential and stationary growth stages, respectively. These results suggested that increased synthesis combined with diminished degradation were responsible for the high ribosome content displayed by Ncc. occultus during exponential growth. In contrast, diminished synthesis together with increased degradation provoked its posterior loss.

Evaluation of 16s rRNA and cellular fatty acid profiles as markers of human intestinal bacterial growth in the chemostat

Chemostats were used to study the effects of carbon and nitrogen limitation and specific growth rate on 16S rRNA synthesis and cellular fatty acid (CFA) profiles in four human intestinal bacteria (Bacteroides thetaiotaomicron, Bifidobacterium adolescentis, Clostridium bifermentans and Cl. difficile). Cellular fatty acid synthesis varied with dilution rate and nutrient availability in different species, but these cellular constituents were relatively stable phenotypic characteristics in Bact. thetaiotaomicron, where branched chain and hydroxy CFA were good taxonomic markers. Conversely, CFA in the Gram-positive bacteria varied markedly with changes in growth environment. For example, in chemostats, cyclopropane CFA were only synthesized in Cl. bifermentans and Cl. difficile under N-limited conditions. Similarly, Dimethyl acetal (DMA) fatty acids in Bif. adolescentis were primarily produced during N-limited growth, and this was inversely related to dilution rate. At low growth rates, 16S rRNA concentrations (microg rRNA per ml culture) correlated with viable bacterial counts, but were more closely related to specific growth rate when expressed as a function of cell mass (microg rRNA per mg dry weight bacteria). However, this did not reveal differences in bacterial population size and rRNA concentration in C-limited cultures. Thus, at low dilution rates, C limitation strongly reduced rRNA synthesis in Cl. bifermentans, despite viable cell counts being similar to those in N-limited cultures. These results indicate that, while 16S rRNA is a useful indicator of microbial activity, cell growth rate does not necessarily relate to rRNA concentration under all nutritional conditions. Consequently, bowel habit and diet will affect both CFA and rRNA content in bacteria isolated from intestinal samples, and this should be taken into consideration when interpreting such data measurements.

Adaptation of the CAS test system and synthetic sewage for biological nutrient removal. Part I: development of a new synthetic sewage

A new synthetic medium has been developed for routine use in laboratory-scale sewage treatment simulation and biodegradation tests, such as OECD guideline 302A & 303A or ISO method 11733. The new medium, Syntho, was designed to meet the following objectives: 1) to be more representative of real sewage than the existing standard OECD synthetic sewage, 2) the COD:N:P ratio and mineral composition must allow a good degree of biological nutrient (N, P) removal, and 3) the medium should result in stable unit operation, including good sludge settling and minimal need for control actions. The IAWQ Activated Sludge Model No. 2 (ASM2,) was used to help design the medium and predict reactor performance for different possible media compositions. The results obtained with Syntho indicate that Continuous Activated Sludge (CAS) units with or without nutrient removal can be operated routinely on this feed. The new medium was also characterized by means of a respiration test. The different influent fractions applied in the model were validated, and a respiration profile indicated that Syntho is a close approximation of real sewage.

Non-random fragmentation of ribosomal RNA in Helicobacter pylori during conversion to the coccoid form

The integrity of DNA and ribosomal RNAs in exponentially growing (bacillary) and ageing stationary phase (coccoid) cultures of Helicobacter pylori type strain CCUG 17874 was investigated. Extensive non-random fragmentation of rRNAs was observed during the conversion to the coccoid form. Beside a small proportion of full-length 16S and 23S rRNA that was always present, the majority of both 16S and 23S rRNA molecules showed distinct highly specific fragmentation patterns. The 16S rRNA fragmentation was characterised in detail by means of Northern blot and primer extension analysis. One cleavage site was located within the highly conserved U5 region (position about 920). The results could not be attributed to the presence of intervening sequences in the 16S and 23S rRNA genes.

In situ analysis of nucleic acids in cold-induced nonculturable Vibrio vulnificus

Low-temperature-induced nonculturable cells of the human pathogenic bacterium Vibrio vulnificus retained significant amounts of nucleic acids for more than 5 months. Upon permeabilization of fixed cells, however, an increasing number of cold-incubated cells released the nucleic acids. This indicates substantial degradation of DNA and RNA in nonculturable cells prior to fixation. Treatment of permeabilized cells with DNase and RNase allowed differential staining of DNA and RNA with the nucleic acid dye 4',6-diamidino-2-phenylindole (DAPI). Epifluorescence microscopy revealed that the could-induced nonculturable populations of V. vulnificus are highly heterogeneous with regard to their nucleic acid content. The fraction of nonculturable cells which maintained DNA and RNA structures decreased gradually during cold incubation. After 5 months at 5 degrees C, less than 0.05% of the cells could be observed to retain DNA and RNA. In parallel with the loss of nucleic acids, an increase in the concentrations of UV-absorbing material in the culture supernatants was observed in nonculturable-cell suspensions. It is hypothesized that there are two phases of the formation of nonculturable cells of V. vulnificus: the first involves a loss of culturability with maintenance of cellular integrity and intact RNA and DNA (and thus possibly viability), and the second is typified by a gradual degradation of nucleic acids, the products of which partly remain inside the cells and partly diffuse into the extracellular space. A small number of nonculturable cells, however, retain DNA and RNA, and thus may be viable despite having reduced culturability.

SYNTHESIS OF RIBONUCLEIC ACID BY X-IRRADIATED BACTERIA

Frampton, E. W. (The University of Texas M. D. Anderson Hospital and Tumor Institute, Houston). Synthesis of ribonucleic acid by X-irradiated bacteria. J. Bacteriol. 87:1369-1376. 1964.-Postirradiation synthesis of total ribonucleic acid (RNA) and of RNA components was measured after exposure of Escherichia coli B/r to X rays. Net synthesis of RNA measured by the orcinol reaction and by the incorporation of uridine-2-C(14) was depressed in irradiated cells, but paralleled the period of postirradiation growth (30 to 40 min). Incorporation of uridine-2-C(14), added after net synthesis of RNA had ceased, detected an apparent turnover in a portion of the RNA. Irradiated cells retained their ability to adjust RNA synthesis to growth rate. After a shift-down in growth rate, irradiated cells incorporated radioactive uridine, while the net synthesis of RNA ceased-presumptive evidence for a continued synthesis of messenger RNA. Chloramphenicol addition (100 mug/ml) did not influence the total amount of RNA synthesized. Synthesis of ribosomes and transfer RNA preceded by 0, 5, 10, and 15 min of postirradiation incubation was observed by the resolution of cell-free extracts on sucrose density gradients. Little immediate influence of irradiation could be detected on the synthesis of 50S and 30S ribosomes. A decline was observed in the synthesis of 50S ribosomes with continued postirradiation incubation; 30S ribosomes, ribosomal precursors, and 4S RNA continued to be synthesized.

NITROGENOUS SUBSTRATES OF ENDOGENOUS RESPIRATION IN PSEUDOMONAS AERUGINOSA

Gronlund, Audrey F. (University of British Columbia, Vancouver, Canada) and J. J. R. Campbell. Nitrogenous substrates of endogenous respiration in Pseudomonas aeruginosa. J. Bacteriol. 86:58-66. 1963.-The nature of the nitrogenous reserves of Pseudomonas aeruginosa that are oxidized during endogenous respiration was studied by following the changes in the chemical constituents and in the distribution of radioactivity of starving cells that had been grown on C(14)-labeled substrates. The total protein and nucleic acid of Warburg vessel contents decreased during starvation. Deoxyribonucleic acid increased slightly, whereas ribonucleic acid (RNA) decreased. C(14)O(2) was evolved from endogenously respiring cells specifically labeled in the nucleic acid fraction and from cells specifically labeled in the protein fraction. Chemical fractionation of C(14)-labeled cells showed a decrease in hot trichloroacetic acid-soluble and -insoluble compounds, indicating that the C(14)O(2) arose from the degradation of RNA and protein and not free pool compounds. A decrease in ribosomal RNA and protein was evident from physical fractionations of starved, labeled cells. An enzyme responsible for the initiation of ribosomal degradation was found to be associated with the ribosome fraction. It was concluded that oxidation of the ribonucleo-protein during endogenous respiration may be a general phenomenon in microorganisms.

Dynamics of Substrate Consumption and Enzyme Synthesis in Chelatobacter heintzii during Growth in Carbon-Limited Continuous Culture with Different Mixtures of Glucose and Nitrilotriacetate

Regulation of nitrilotriacetate (NTA) degradation and expression of NTA monooxygenase (NTA-MO) in the NTA-degrading strain Chelatobacter heintzii ATCC 29600 in continuous culture at a dilution rate of 0.06 h(sup-1) under transient growth conditions when the feed was switched between media containing NTA, glucose, or different mixtures thereof as the sole carbon and energy sources was investigated. A transition from NTA to glucose was accompanied by a rapid loss of NTA-MO. A transition from glucose to NTA resulted in a lag phase of some 25 h until NTA-MO expression started, and approximately 100 h was needed before a steady state for NTA-MO specific activity was reached. This transient lag phase was markedly shortened when mixtures of NTA plus glucose were supplied instead of NTA only; for example, when a mixture of 90% glucose and 10% NTA was used, induction of NTA-MO was detected after 30 min. This suggests a strong positive influence of alternative carbon substrates on the expression of other enzymes under natural environmental conditions. Regulation of NTA-MO expression and the fate of NTA-MO were also studied during starvation of both glucose-grown and NTA-grown cultures. Starvation of NTA-grown cells led to a loss of NTA-MO protein. No synthesis of NTA-MO (derepression) was observed when glucose-grown cells were starved.

Entry of Escherichia coli into stationary phase is indicated by endogenous and exogenous accumulation of nucleobases

Endogenous and exogenous accumulation of nucleobases was observed when Escherichia coli entered the stationary phase. The onset of the stationary phase was accompanied by excretion of uracil and xanthine. Except for uracil and xanthine, other nucleobases (except for minor amounts of hypoxanthine), nucleosides, and nucleotides (except for cyclic AMP) were not detected in significant amounts in the culture medium. In addition to exogenous accumulation of nucleobases, stationary-phase cells increased the endogenous concentrations of free nucleobases. In contrast to extracellular nucleobases, hypoxanthine was the dominating intracellular nucleobase and xanthine was present only in minor concentrations inside the cells. Excretion of nucleobases was always connected to declining growth rates. It was observed in response to entry into the stationary phase independent of the initial cause of the cessation of cell growth (e.g., starvation for essential nutrients). In addition, transient accumulation of exogenous nucleobases was observed during perturbations of balanced growth conditions such as energy source downshifts. The nucleobases uracil and xanthine are the final breakdown products of pyrimidine (uracil and cytosine) and purine (adenine and guanine) bases, respectively. Hypoxanthine is the primary degradation product of adenine, which is further oxidized to xanthine. The endogenous and exogenous accumulation of these nucleobases in response to entry into the stationary phase is attributed to degradation of rRNA.

Purification and characterization of Escherichia coli RNase I. Comparisons with RNase M

The endoribonuclease, RNase I, was purified from the periplasm of Escherichia coli. Based on PAGE, it has molecular mass of approximately 27 kDa with a migration rate indistinguishable from that of the recently reported RNase M from E. coli. The amino acid sequence of the two enzymes must be very similar based on two-dimensional mapping of their tryptic peptides and suggests either a post-transcriptional modification to yield different proteins from the same gene or evolution of two genes by gene duplication. However, while RNase I could degrade each of the four ribonucleotide homopolymers, only poly(U) or poly(C) were good substrates for RNase M with possibly some hydrolysis of poly(A). The reaction rate for poly(C) hydrolysis with RNase M was about ten times faster than for poly(U), while for RNase I the rates were about equal. Besides differences in specificity, RNase M was only located in the spheroplasts while RNase I found in the periplasm of growing cells. In terms of function, RNase I is known to cause degradation of rRNA during periods of stress or non-growth, whereas it has been proposed that RNase M is the endonuclease for mRNA degradation in growing cells.

Role of ribosome degradation in the death of starved Escherichia coli cells

In Escherichia coli cultures limited for phosphate, the number of ribosomal particles was reduced to a small percentage of its earlier peak value by the time the viable cell count began to drop; the 30S subunits decreased more than the 50S subunits. Moreover, the ribosomal activity was reduced even more: these cells no longer synthesized protein, and their extracts could not translate phage RNA unless ribosomes were added. The translation initiation factors also disappeared, suggesting that they become less stable when released from their normal attachment to 30S subunits. In contrast, elongation factors, aminoacyl-tRNA synthetases, and tRNA persisted. During further incubation, until viability was reduced to 10(-5), the ribosomal particles disappeared altogether, while tRNA continued to be preserved. These results suggest that an excessive loss of ribosomes (and of initiation factors) may be a major cause of cell death during prolonged phosphate starvation.

Stringent control and protein synthesis in bacteria

Most bacteria have evolved a number of regulatory mechanisms which allow them to maintain a balanced and rather constant cellular composition in response to nutritional variations. In particular, when the availability of any aminoacyl-tRNA species becomes limiting (namely through amino acid starvation or inactivation of an aminoacyl-tRNA synthetase), several biochemically distinct physiological processes are significantly modified. This coordinate adjustment of cellular activity is termed the "stringent response". Under such conditions of aminoacyl-tRNA limitation, protein synthesis still proceeds, but various quantitative as well as qualitative changes in polypeptide metabolism can be observed. In this review, after a brief recall of the main characteristics of the stringent response, several aspects concerning protein synthesis in deprived bacteria have been presented. First, the rates of residual protein formation, peptide chain growth and protein degradation, and the molecular weight distribution of proteins newly synthesized have been analyzed. Then, the data relative to the biosynthetic regulation of non-ribosomal and ribosomal proteins have been summarized and compared to the results obtained from in vitro experiments using transcription-translation coupled systems. Finally, the problem of translational fidelity during deprivation has been discussed in connection with the metabolic behavior of polysomal structures which are still maintained in cells. The stringent dependence of cellular activity on aminoacyl-tRNA supply is known to be abolished by single-site mutations which confer to bacteria a phenotype referred to as "relaxed". These mutant strains provide an useful analytical tool in the scope of understanding the stringency phenomenon. Therefore, their proteosynthetic activity under aminoacyl-tRNA deprivation has also been studied here, in comparison to that of normal wild-type strains.

Adenosine triphosphate pool levels and endogenous metabolism in Arthrobacter crystallopoietes during growth and starvation

The adenosine triphosphate (ATP) content of Arthrobacter crystallopoietes was measured during growth, starvation and recovery from starvation. During exponential growth of the cells as spheres in a glucose slats medium, the level of ATP per cell remained constant at 8.0 x 10(-10) micrograms/cell. Morphogenesis to rodshaped cells and an increased growth rate following addition of casein hydrolysate was accompanied by an almost two-fold increase in the ATP level. As division of the rod-shaped cells proceeded, the level of ATP declined. After growing as rods for 12-14 h the cells underwent fragmentation to spheres during which time the ATP level again increased to the original value of 8.0 x 10(-10) micrograms/cell. As the spherical cells resumed growth on the residual glucose, their ATP content declined for a short period and then remained relatively constant. During starvation of sphere or rod-shaped cells for one week, the ATP level declined by approximately 70% during the first 40-50 h and then remained constant. The endogenous metabolism rate of spherical cells declined during the first 10-20 h of starvation and then remained constant at approximately 0.02% of the cell carbon being utilized per h. Addition of glucose to spherical cells which had been starved for one week increased both the ATP content per cell and their rate of endogenous metabolism. The ATP content fluctuated and then remained at a level higher than maintained during starvation while endogenous metabolism quickly declined.

Adenylate nucleotide levels and energy charge in Arthrobacter crystallopoietes during growth and starvation

The adenylate nucleotide concentrations, based on internal water space, were determined in cells of Arthrobacter crystallopoietes during growth and starvation and the energy charge of the cells was calculated. The energy charge of spherical cells rose during the first 10 h of growth, then remained nearly constant for as long as 20 h into the stationary phase. The energy charge of rod-shaped cells rose during the first 4 h of growth, then remained constant during subsequent growth and decreased in the stationary growth phase. Both spherical and rod-shaped cells excreted adenosine monophosphate but not adenosine triphosphate or adenosine diphosphate during starvation. The intracellular energy charge of spherical cells declined during the initial 10 h and then remained constant for 1 week of starvation at a value of 0.78. The intracellular energy charge of rod-shaped cells declined during the first 24 h of starvation, remained constant for the next 80 h, then decreased to a value of 0.73 after a total of 168 h starvation. Both cell forms remained more than 90% viable during this time. Addition of a carbon and energy source to starving cells resulted in an increase in the ATP concentration and as a result the energy charge increased to the smae levels as found during growth.

Protein synthesis and degradation in a leucine auxotroph of Escherichia coli

The synthesis and degradation of the soluble and sodium dodecyl sulfate-(SDS)-solubilized protein fractions of Escherichia coli were studied in both growing and nongrowing cultures. When separated according to molecular weight on SDS-polyacrylamide gels, the proteins of both fractions of growing cells undergo no measureable differential synthesis or degradation during logarithmic growth. However, when a leucine auxotroph is suspended in medium containing 5.3 muM leucine (a level that will not sustain growth), the SDS-solubilized protein of such a nongrowing culture shows a rapid synthesis of two protein components (32,000 and 12,000 daltons) found only in the out membrane.

Synthesis of ribosomal ribonucleic acid during sporulation of Bacillus subtilis

The incorporation of radioactive uracil into 50s and 30s ribosomal subunits and ribosomal ribonucleic acid (rRNA) was studied during the growth cycle of different sporogenic and asporogenic strains of Bacillus subtilis. It was found that partially synchronized cultures of the strains examined incorporated labeled uracil into the two ribosomal subunit species and rRNA during sporulation and during the stationary phase of the asporogenic strains. Kinetic studies have shown that, compared to vegetative cells, the percentage of uracil incorporated into the ribosomal subunits of cells taken 30 min after the end of exponential growth was decreased by about 25 to 35%. This decrease, however, appeared to be a general characteristic of stationary-phase cells and seems to depend on the nature of the sporulation medium and to some extent on the nature of the strain but not on the sp(+) or sp(-) phenotype of the strain. Moreover, by use of actinomycin D it was shown that the labeled uracil incorporated, in the presence of the drug, during the sporulation period was located in the ribosomal subunits (stable RNA). Based on these results, we concluded that during sporulation ribosomal genes are transcribed and consequently rRNA continues to be synthesized, although to a lesser extent than during vegetative growth. These results are discussed in the light of those obtained by Hussey et al.