Adenylate kinase 1 knockout mice have normal thiamine triphosphate levels

Thiamine triphosphate (ThTP) is found at low concentrations in most animal tissues and it may act as a phosphate donor for the phosphorylation of proteins, suggesting a potential role in cell signaling. Two mechanisms have been proposed for the enzymatic synthesis of ThTP. A thiamine diphosphate (ThDP) kinase (ThDP+ATP if ThTP+ADP) has been purified from brewer's yeast and shown to exist in rat liver. However, other data suggest that, at least in skeletal muscle, adenylate kinase 1 (AK1) is responsible for ThTP synthesis. In this study, we show that AK1 knockout mice have normal ThTP levels in skeletal muscle, heart, brain, liver and kidney, demonstrating that AK1 is not responsible for ThTP synthesis in those tissues. We predict that the high ThTP content of particular tissues like the Electrophorus electricus electric organ, or pig and chicken skeletal muscle is more tightly correlated with high ThDP kinase activity or low soluble ThTPase activity than with non-stringent substrate specificity and high activity of adenylate kinase.     

Stimulation of mutagenesis by proportional deoxyribonucleoside triphosphate accumulation in Escherichia coli

Intracellular pool sizes of deoxyribonucleoside triphosphates (dNTPs) are highly regulated. Unbalanced dNTP pools, created by abnormal accumulation or deficiency of one nucleotide, are known to be mutagenic and to have other genotoxic consequences. Recent studies in our laboratory on DNA replication in vitro suggested that balanced accumulation of dNTPs, in which all four pools increase proportionately, also stimulates mutagenesis. In this paper, we ask whether proportional dNTP pool increases are mutagenic also in living cells. Escherichia coli was transformed with recombinant plasmids that overexpress E. coli genes nrdA and nrdB, which encode the two protein subunits of aerobic ribonucleotide reductase. Roughly proportional dNTP pool expansion, by factors of 2- to 6-fold in different experiments, was accompanied by increases in spontaneous mutation frequency of up to 40-fold. Expression of a catalytically inactive ribonucleotide reductase had no effect on either dNTP pools or mutagenesis, suggesting that accumulation of dNTPs is responsible for the increased mutagenesis. Preliminary experiments with strains defective in SOS regulon induction suggest a requirement for one or more SOS functions in the dNTP-enhanced mutagenesis. Because a replisome extending from correctly matched 3'-terminal nucleotides is almost certainly saturated with dNTP substrates in vivo, whereas chain extension from mismatched nucleotides almost certainly proceeds at sub-saturating rates, we propose that the mutagenic effect of proportional dNTP pool expansion is preferential stimulation of chain extension from mismatches as a result of increases in intracellular dNTP concentrations.     

Adenylate kinases from thermosensitive Escherichia coli strains

The adk genes from several thermosensitive (ts) mutants of Escherichia coli were cloned and sequenced. The mutations responsible for the thermolability of the gene product, the enzyme adenylate kinase, were established. From five independently isolated strains analysed, two contain a CCG to TCG transition changing proline 87 to serine (P87S), another two have a TCT to TTT transition that mutates serine 129 to phenylalanine (S129F), and the last one was found not to contain a mutation in the adk gene. Overproducing strains were constructed that contain ts genes in the genome as well as in the plasmids. These strains grow at high temperature, although much slower than wild-type. Most probably, the high rate of synthesis of adenylate kinase compensates for the destruction of the thermolabile protein by the elevated temperature. Mutated proteins were purified. The P87S but not the S129F mutation was found to cause thermosensitivity of the adenylate kinase reaction. Revertants of thermosensitivity were isolated and the nature of the mutation was determined by the RNase digestion method of RNA-DNA hybrids and by DNA sequencing. The revertants of the P87S mutation regained the wild-type sequence, whereas the revertants of the S129F strain retained the original mutation in the adenylate kinase gene. These results are discussed in the light of the three-dimensional structure of the enzyme and the possible role of adenylate kinase in phospholipid synthesis.     

Adenylate energy charge in Escherichia coli CR341T28 and properties of heat-sensitive adenylate kinase

Escherichia coli strain CR341T28 will not grow at temperatures above 34 degrees C in liquid medium, and the adenylate kinase of this strain is heat sensitive. When a culture was shifted from a permissive (30 degrees C) to a nonpermissive (36 degrees C) temperature, the adenylate energy charge fell from 0.9 to 0.2, with a concurrent decrease in the number of viable cells and in the specific activity of adenylate kinase. When cultures of the temperature-sensitive strain were grown at temperatures above 30 degrees C, the adenylate energy charge, the specific activity of adenylate kinase, and the growth rate were lower than the corresponding parameters for the parental strain. By isotopic labeling of the adenine nucleotides in vivo, it was determined that increasing growth temperatures between 30 and 34 degrees C for the heat-sensitive strain resulted in a decrease in the adenosine triphosphate-to-adenosine monophosphate and adenosine triphosphate-to-adenosine diphosphate ratios. Between 26 and 30 degrees C the adenosine triphosphate-to-adenosine diphosphate ratio was essentially normal in the temperature-sensitive strain, but the adenosine triphosphate-to-adenosine diphosphate ratio was decreased. The adenylate ratios in the parental strain did not change between 30 and 34 degrees C. The adenylate kinase mass action ratio for each strain was essentially constant under all growth conditions. When assayed at 30 degrees C, the affinities of the enzyme from the mutant strain were somewhat lower than those of the parent adenylate kinase. The mutant enzyme also did not exhibit the substrate inhibition that was observed at high adenosine monophosphate concentrations with the parental enzyme. An increase in the assay temperature from 30 degrees to 40 degrees C had little or no effect on the Km values determined for the parental adenylate kinase, but caused the Km values determined for the mutant adenylate kinase to increase by a factor of two or more.     

Functional capacities and the adenylate energy charge in Escherichia coli under conditions of nutritional stress.

Functional capacities in Escherichia coli cells starved for glucose were examined by comparing protein synthesis, utilization of new substrates, and maintenance of viability with the adenylate energy charge of the culture. When growth ceased because of glucose exhaustion in an E. coli culture, the energy charge dropped from 0.90 to about 0.80. During this time, the viable-cell count and the capacity for protein synthesis and for induction of new enzymes were maintained only if other substrates were available in the medium. The culture could be maintained for many hours without growth or death if glucose was added slowly; the energy charge in this case stabilized at about 0.80. A consistent transient decrease in the energy charge to around 0.80, accompanied by a decrease in protein synthesis, was also observed during the adaptation from glucose to other substrates during diauxic growth on glucose and glycerol or lactose.     

Biosynthesis of thiazole from thiamine in Escherichia coli]

The incorporation into the thiazole moiety of thiamine of several labeled compounds has been studied on short time incubations of washed-cells suspensions. No incorporation of radioactivity from [G-14C] methionine was found in a mutant auxotrophic for methionine. No radioactivity was incorporated from [U-14C] aspartate or from [U-14C] serine. The incorporation of 35S from sulphate was lowered by cysteine or glutathione but was unaffected by methionine or homocystine. Although the synthesis of thiazole is dependent on methionine, neither the sulphur atom nor the carbon chain of thiazole originate from methonine in E. coli. No carbon originates from cysteine which is the likely direct donor of sulphur.     

Adenylate Energy Charge in Escherichia coli During Growth and Starvation

The value of the adenylate energy charge, [(adenosine triphosphate) + (1/2) (adenosine diphosphate)]/[(adenosine triphosphate) + (adenosine diphosphate) + (adenosine monophosphate)], in Escherichia coli cells during growth is about 0.8. During the stationary phase after cessation of growth, or during starvation in carbon-limited cultures, the energy charge declines slowly to a value of about 0.5, and then falls more rapidly. During the slow decline in energy charge, all the cells are capable of forming colonies, but a rapid fall in viability coincides with the steep drop in energy charge. These results suggest that growth can occur only at energy charge values above about 0.8, that viability is maintained at values between 0.8 and 0.5, and that cells die at values below 0.5. Tabulation of adenylate concentrations previously reported for various organisms and tissues supports the prediction, based on enzyme kinetic observations in vitro, that the energy charge is stabilized near 0.85 in intact metabolizing cells of a wide variety of types.     

Nucleoside triphosphate pools in minicells of Escherichia coli.

The nucleoside triphosphate pools of Escherichia coli minicells are different from those in parental cells. The growth phase in which minicells accumulate significantly affects the pool sizes.     

Characterization of RNA damage under oxidative stress in Escherichia coli

We have examined the level of 8-hydroxyguanosine (8-oxo-G), an oxidized form of guanosine, in RNA in Escherichia coli under normal and oxidative stress conditions. The level of 8-oxo-G in RNA rises rapidly and remains high for hours in response to hydrogen peroxide (H?O?) challenge in a dose-dependent manner. H?O? induced elevation of 8-oxo-G content is much higher in RNA than that of 8-hydroxydeoxyguanosine (8-oxo-dG) in DNA. Under normal conditions, the 8-oxo-G level is low in RNA isolated from the ribosome and it is nearly three times higher in non-ribosomal RNAs. In contrast, 8-oxo-G generated by a short exposure to H?O? is almost equally distributed in various RNA species, suggesting that although ribosomal RNAs are normally less oxidized, they are not protected against exogenous H?O?. Interestingly, highly folded RNA is not protected from oxidation because 8-oxo-G generated by H?O? treatment in vitro increases to approximately the same levels in tRNA and rRNA in both native and denatured forms. Lastly, increased RNA oxidation is closely associated with cell death by oxidative stress. Our data suggests that RNA is a primary target for reactive oxygen species and RNA oxidation is part of the paradox that cells have to deal with under oxidative stress.     

Cytidine triphosphate: phosphatidic acid cytidyltransferase in Escherichia coli

An enzyme has been found in particulate fractions of Escherichia coli that catalyzes the incorporation of cytidine triphosphate (CTP) into lipid in the presence of exogenous phosphatidic acid and Mg(++). The product has been identified enzymatically and by chromatography as cytidine diphosphate diglyceride. The reaction is optimal at a pH of 6.5 and Mg(++) concentration of 5-10 mm. The apparent K(m) for CTP is 7 x 10(-4)M and for phosphatidic acid, 2 x 10(-3)M. The reaction rate falls off rapidly with time and ceases entirely after 1 hr as the result of inactivation of the system by Mg(++).     

UV irradiation alters deoxynucleoside triphosphate pools in Escherichia coli

UV irradiation of exponentially growing Escherichia coli increased intracellular concentration of dATP and DTTP without significantly changing the concentrations of dGTP and dCTP. These selective increases in dATP and dTTP pools are seen in wild-type E. coli K12 and AB1157, as well as in recA and umuC strains, and are proportional to UV dose. The possible significance of these findings with respect to induction of the SOS response and nontargeted mutagenesis are discussed.     

Nucleoside-triphosphatase and hydrolysis of thiamin triphosphate in Escherichia coli

A membrane-bound nonspecific triphosphatase of E. coli was solubilized and purified to a homogeneous SDS-acrylamide gel electrophoresis band. It was found to be a single polypeptide of 16 kDa requiring no Mg2+, with an optimal pH at 6.5. The substrate specificity was broad and a nonspecific Mg2+-independent ribonucleoside-triphosphatase (NTPase) activity was expressed together with thiamin-triphosphatase activity. The molecular size and characteristics were clearly different from the known NTPase (EC 3.6.1.15). Using the purified thiamin-triphosphatase II, ATP:thiamin-diphosphate phosphoryl transferase (EC 2.7.4.15) activity was demonstrated with an optimal pH of approx. 5.3. Considering its kinetic parameters and other characteristics, however, the thiamin triphosphate synthesizing activity was not thought to take part in cellular thiamin triphosphate synthesis. The possibility that thiamin-triphosphatase II plays a part in the hydrolysis of thiamin triphosphate to control its cellular level is suggested.