Thymine Groups Research Articles

Theoretical studies on the interaction of proteins and nucleic acid. II. The binding of alpha-helix to B-DNA.

Interactions between B-DNA and homopolymeric alpha-helices of glycine, alanine, serine, asparagine and aspartic acid have been studied theoretically. The complexation energy has been minimised taking into account the interactions between DNA and the polypeptides as well as the internal energy of the alpha-helix and the interaction energy of counterions with the complex. The results obtained indicate the important role of strong hydrogen bonds between the peptide side chains and nucleic acid phosphate groups, these bonds being much stronger than specific interactions with the base-pairs. The formation of these structural bonds depends on the size of the alpha-helix, which in turn determines whether bridging across the major groove is possible. The steric role of the methyl group of thymine in orienting the peptide helix and the role of DNA screening cations in complex stabilization are also significant.

Synthesis of polyamide containing optically active thymine derivative as a grafted pendant

AbstractA new route to polyamides containing optically active thymine groups as pendants has been established. The method is based on the grafting of (–) and (±)‐2‐(thymin‐1‐yl)propionic acid [(–) and (±) TPA] onto a polyamide containing hydroxyl groups. The hydroxy polyamide was prepared by selective N‐acylation of an active diester of N‐hydroxy‐5‐norborene‐2,3‐dicarboxamide (HONB), N,N'‐(isophthaloyl‐dioxy)‐bis(5‐norbornene‐2,3‐dicarboximide) (IPBONB), with 1,3‐diamino‐2‐hydroxypropane (AHP). Model compounds (−) and (±)‐(1,3‐dibenzoylamino‐2‐propyl)2‐(thymin‐1‐yl)propionate[(−) and (±) (BAPTP)] were prepared by direct, low‐temperature esterification before synthesizing the polymer.

Ionizing radiation and tritium transmutation both cause formation of 5-hydroxymethyl-2'-deoxyuridine in cellular DNA.

HeLa cells grown in the presence of [methyl-3H]thymidine contained large amounts of 5-hydroxymethyl-2'-deoxyuridine (HMdU) in their DNA. When the cells were grown in [6-3H]thymidine and their DNA was labeled to the same specific activity, no HMdU was present. When such [6-3H]thymidine-labeled cells were exposed to increasing amounts of gamma-radiation, small but increasing amounts of HMdU were formed in their DNA. This indicates that HMdU can be formed in DNA by two distinct mechanisms. The first is the result of the transmutation of 3H to 3He (beta decay) in the methyl group of thymidine, leading to formation of a carbocation. This short-lived ion reacts with hydroxide ions of water, yielding the hydroxymethyl group. HMdU that is formed by this mechanism is formed at the rate of beta decay of 3H. It appears only in [methyl-3H]thymidine residues and is present in the DNA of both nonirradiated and gamma-irradiated cells. The second mechanism is the result of the radiolysis of water caused by ionizing radiation. The resultant radical species, particularly hydroxyl radicals, may react with many sites on DNA. When the methyl group of thymine is attacked by hydroxyl radicals, the hydroxymethyl group is formed. The formation of HMdU by this mechanism was detected only when [6-3H]thymidine-labeled cells were used, since transmutation of 3H in position 6 of thymine cannot yield HMdU.

Hydrophobic hydration of thymine

The solubilities of adenine in water, thymine in methanol, and uracil in methanol were determined over the temperature range 15–40°C. From these and previously reported data the thermodynamic functions for the transfer of the solute from water to methanol were calculated. The differences in transfer functions for uracil and thymine are due to hydrophobic hydration of the methyl group of thymine.

Formation of an adenine-thymine photoadduct in the deoxydinucleoside monophosphate d(TpA) and in DNA.

A photoadduct is formed between the adenine (A) and thymine (T) bases of the deoxydinucleoside monophosphate d(TpA) when it is irradiated at 254 nanometers in aqueous solution. Treatment of the photoadduct with acid converts it specifically into a fluorescent hydrolysis product, C7H7N3O, incorporating the position-8 carbon of adenine and the methyl group of thymine. Isolation of the fluorescent hydrolysis product from acid hydrolyzates of oligo- and polydeoxyribonucleotides has shown that the photoadduct is formed by ultraviolet irradiation of d(pTpA), d(TpApT), d(TpApTpA), poly(dA-dT), and both single- and double-stranded DNA.

Interaction of the HpaI endonuclease with synthetic oligonucleotides.

To determine which functional groups of bases within the grooves of double-helical DNA interact with the HpaI endonuclease, we have employed chemically synthesized octanucleotides containing base analogues. The 5-methyl group of thymine was probed as a contact between the HpaI endonuclease and its recognition sequence by using the oligonucleotides d(G-G-T-T-A-A-C-C), d(G-G-T-U-A-A-C-C), and d(G-G-T-U(Br)-A-A-C-C). The 2-amino group of guanine was probed as a contact for the HpaI endonuclease by using the octanucleotide d(G-I-T-T-A-A-C-C). The HpaI endonuclease cleaves octanucleotides d(G-G-T-T-A-A-C-C) and d(G-G-T-B-A-A-C-C) according to Michaelis-Menten kinetics. However, both the Km and turnover number for d(G-G-T-B-A-A-C-C) were severalfold lower than those for cleavage of d(G-G-T-T-A-A-C-C). In addition, d(G-G-T-U-A-A-C-C) was not cleaved by HpaI endonuclease, suggesting that the 5-methyl group of thymine is a contact between the HpaI endonuclease and its recognition sequence. d(G-I-T-T-A-A-C-C) was not cleaved by the HpaI endonuclease which may be due in part to the low thermal stability of the duplex. Nevertheless, our results suggest that the 2-amino group of guanine is a contact for the HpaI endonuclease. A phosphate group 5' external to the HpaI recognition sequence has been identified as a contact between the HpaI endonuclease and DNA. The HpaI endonuclease cleaved 5'-phosphorylated octanucleotide 30-fold faster than unphosphorylated octanucleotide. In addition, the Km of the d(G-G-T-T-A-A-C-C) was 8000-fold higher than the Km of the phage f1 RFI DNA, suggesting that the octanucleotide is too short to take advantage of the entire DNA binding site of the enzyme.

Poly(dA-dT) complexes with histone H1 and pancreatic ribonuclease: specific base recognition evidenced by ultraviolet resonance raman spectroscopy.

AbstractThe conformational changes of poly(dA‐dT) from random coil to ordered structure with stacked bases produce important changes in the Raman line intensities (hypochromism) when the polymer is excited under the preresonance Raman conditions (λ excitation = 300 nm). Poly(dA‐dT)–RNase and poly(dA‐dT)–histone H1 interactions have been studied as models of mechanisms of destabilization and stabilization by proteins of the DNA secondary structure, respectively, following this intense preresonance Raman hypochromism. In addition, the specific variation of the intensity of the 1582‐cm−1 line of adenine is interpreted in terms of the interaction of the amino group with the RNase (thus involving the large groove). In the poly(dA‐dT)–H1 complex, the intensity of the 1665‐cm−1 line of thymine increases. This increase appears to involve the C2O group of thymine, located in the narrow groove.

A chemical model for the thymidylate synthetase catalysed methylation of deoxyuridine monophosphate

In a study of a model for the reaction catalysed by the enzyme thymidylate synthetase, a compound previously thought to be the putative intermediate 5-(uracil-5-ylmethyl)tetrahydropteroylglutamic acid (4b) has been prepared and shown to be the 5,10-bis(uracil-5-ylmethyl) adduct (4c). The tetrahydrofolic acid adduct (4d) and its [6-2H1] analogue have been prepared and have been shown to be thermally converted into thymine (5, R2= H). The reaction with deuteriated (4d) shows a high discrimination for the hydrogen at C-6 of the pteridine moiety which rearranges to the methyl group of the resultant thymine (5, R2= H). These findings parallel results found for the biochemical process very closely and possible mechanisms for the reaction are discussed. The compounds (4c) and (4d) are neither substrates nor inhibitors of thymidylate synthetase.

The low frequency vibrations of pyrimidine and purine bases

We report the far i.r. spectra of crystalline powders of uracil, cytosine, thymine, xanthine and hypoxanthine, from 25 to 1000 cm −1. The carbonyl bending modes of uracil and cytosine have been identified through the substitution of sulfur for oxygen at the C2 position. Wagging and torsional modes of the amino group of cytosine as well as the N1H out-of-plane bending mode have been identified by deuteration studies. Several ring vibrations below 800 cm −1 have also been identified. A Fermi resonance was observed in the case of 2-thiocytosine which involves the sulfur out-of-plane bending mode, the lowest lying ring bending mode and the wagging and torsional modes of the amino group. There also appears to be a Fermi resonance in cytosine involving the carbonyl modes and two external modes. Two modes at 285 and 320 cm −1 which involve the methyl group of thymine are tentatively identified. The temperature dependence of the spectra of hypoxanthine for frequencies <350 cm −1 indicates a large mixing between the internal and external vibrations in the 200–250 cm −1 region.

Fluctuational opening of the double helix as revealed by theoretical and experimental study of DNA interaction with formaldehyde

It follows from the theory of helix-coil transition that local opening of the double helix due to thermal fluctuations must take place at temperatures well below the melting range. Provided that formaldehyde can not react with hydrogen-bonded bases and can react only with the exposed ones, such fluctuational opening of base-pairs may be probed by formaldehyde. To test the adequacy of the theory for describing the fluctuational opening of the double helix, the process of DNA interaction with formaldehyde has been simulated by the Monte Carlo method with the aid of a computer. On the basis of kinetic constants for the forward and reverse reactions of formaldehyde with all four nucleotides measured by McGhee & von Hippel (1975 a,b ), the kinetic curves of DNA unwinding by formaldehyde have been calculated theoretically without the use of any adjustable parameter. The calculations have demonstrated that the highly reversible but very fast reaction of formaldehyde with the imino group of thymine plays a very important role in the process of DNA unwinding by formaldehyde have been compared with experimental data obtained for bacteriophage T7 DNA for different values of pH, temperature and concentration of formaldehyde. This comparison leads to the conclusion that the theory offers a correct general description of fluctuational opening of the double helix. The main characteristics of this process calculated by the theory are as follows. At room temperatures only individual base-pairs are opened and the mean distance between adjacent unpaired bases is as great as 2×10 5 base-pairs. Each A·T pair is shown to be opened with frequency about 10 2 s −1 and each G·C pair with a frequency about 10 s −1 . At elevated temperatures, in the DNA premelting region, the probability of fluctuational opening, as well as the average number of base-pairs in an opened region, increases considerably. A possible role of the results obtained from an analysis of the processes of DNA function in the cell is discussed.

Preferential methylation of regulatory genes in hela cells

1. Introduction The genome of a cell is exposed to the action of two classes of DNA-modifying enzymes: (1) a DNA methyl- ase which transforms roughly l/l 5 of its cytosines to 5-methyl-cytosines, while (2) a mere I/ 1000 of these 5-methyl-cytosines are transformed by a DNA deami- nase to minor thymines [l-3]. Transformation from cytosine to 5-methylcytosine can take place in isolated nuclei of sea urchin eggs [4] and HeLa cells [5]. But, at least for HeLa cells, transformation from 5-methyl- cytosine to ‘minor’ thymine does not take place in isolated nuclei [5] and probably is under cytoplasmic control. This was suggested by investigation on syn- chronized HeLa cell suspension in which [’ 4C]methyl- Lmethionine was employed as the sole tracer for both methylation and synthesis of DNA [5 ] : The labelled C-atom of the methyl group of L-methio- nine does not enter the pyrimidine ring [2] : via methy- lation it is transferred to DNA 5-methylcytosine; via Cl -intermediates it enters the purine ring and the methyl group of thymine to participate in the DNA biosynthetic process. The two pathways of DNA methylation and DNA synthesis were then separated from each other during the HeLa cell cycle [5] : in the whole cell, DNA methylation parallels DNA syn- thesis during the S-phase; in the isolated nuclei, DNA methylation proceeds during the S-phase in absence of DNA synthesis. Therefore, completed DNA chains can be methylated (furthermore, Adams [6] has demonstrated that newly synthesized chains are not methylated); their methylation during S might represent a condition of DNA duplication or of gene expression [2,4]. S-CH3 ----+ SAM - DNA (Cytosine + 5-methyl-cytosine 4 Minor thymine) I Methylase Deaminase (CH,), t t H,N-CH-COOH -_---_ ---------__--_ Methylase pathway Nucleus i _ ________ _______----_---_- - ---- 4 J(Oxidation) Biosynthetic pathway - Cytoplasm 4 C 1 -chain + dUMP -+ dTMP + dTTP 4 J- Cytoplasmic + regulator Adenine-----+ dATP Guanine------+ dGTP

Potentiation of the teratogenic effects of 5-fluorouracil by natural pyrimidines. II. Biochemical aspects.

AbstractThymine (Th), uracil (U), and the antimetabolite 5‐fluorouracil (5‐FU) are metabolized in vitro by rat liver enzyme systems present in the high‐speed supernatant soluble cell fraction. The data presented demonstrate that addition of either U or Th to such preparations or the use of livers from rats pretreated intraperitoneally with U or Th markedly decreases the ability of liver preparations to metabolize 5‐FU. These findings indicate that the decreased elimination from the body of a teratogenic dose of 5‐FU after treatment with an equimolar dose of U is due to the inhibition of metabolism of the antimetabolite in the maternal liver.The concomitant administration of the natural pyrimidines Th or U with 5‐FU resulted in prolonged high serum levels of the teratogen 5‐FU, which tended to enhance transfer to and uptake by the embryo. Since the embryo was unable to dispose of 5‐FU metabolically, prolonged high levels of 5‐FU would be expected to cause increased teratogenic effects. Studies with labeled 5‐FU revealed that most of the radioactivity found in the embryos was incorporated into RNA, with only about 30% in the TCA ex tract. Concomitant treatment of pregnant rats with 5‐FU and U had greater inhibitory effect on the incorporation of formate into the methyl group of DNA thymine in the embryo than treatment with 5‐FU alone. This treatment inhibited thymidylate synthetase more than an equimolar dose of 5‐FU alone. Neither treatment affected the high kinase activity of the embryo.

On the Radiation Chemistry of Thymine in Aqueous Solution

The reactions of thymine in aqueous solution with radiation-induced radicals OH, H, and e-aq were studied under various conditions. Competition studies using scavengers of OH radicals (methanol, ethanol, iodide) or of e-aq and/or H atoms (N2O, H+, O2) led to the conclusion that OH and H radicals destroy the chromophoric group of thymine, but e-aq does not. A trace of O2 proved to be necessary to obtain maximal destruction. Removal of the last traces of O2 resulted in a decrease of the destruction yield, possibly through restitution reactions. It was found that (1) alcohol radicals destroy thymine, even in the presence of O2; (2) the rate constant, k(OH + thymine) = 4.3 X 10(9) M(-1) sec(-1) (from competition with iodide); and (3) k(H + thymine) = 8 X 10(8) M(-1) sec(-1) (from competition with O2 in acid solution).

Die Erzeugung atomaren Wasserstoffs in γ-bestrahlten Bakteriophagen und in deren DNS

ESR-spectra of purified preparations of bacteriophage and of DNA extracted from these objects were found to contain the characteristic lines of atomic hydrogen after γ-irradiation at 77 °K. The experimental results demonstrate the production of atomic hydrogen by irradiation of organic materials in a direct way. The high stability of the trapped hydrogen atoms is indicated by the fact that heating to 200 °K for 10 min reduces the spectral amplitudes by a factor of two only. The yield of trapped atomic hydrogen varied between 0.05 and 0.6 while the total radical yield derived from the ESR-spectra was found between 1.0 and 3.0. Addition of moisture to vacuumdried samples did not enhance the yield of H-atoms. At 50% water content the yield was also reduced to about 50% other parameters remaining unchanged. The disappearance of hydrogen lines on heating is accompanied by an enhancement of the octet spectrum due to the addition of atomic hydrogen to thymine groups. The observation of H-stabilization in inorganic phosphates is taken to suggest trapping by phosphate groups in DNA. The efficiency of trapping is probably influenced by the presence of impurities, notably transition metals. The apparent g-factor of 2.014 ± 0.002 is explained by IS-coupling. The true g-factor obtained by application of the Breit - Rabi formula is equal to the free electron value within experimental limits of error.

?-Radiolysis of aqueous thymine solutions. Determination of relative reaction rates of OH radicals

The loss of absorption of the chromophoric group of thymine on irradiation in aqueous solution with γ-rays has been studied under a variety of conditions. The effects of thymine and oxygen concentrations, of pH and of the presence of N2O have been investigated. In solutions containing oxygen, the loss of absorption is the result of initial OH radical attack on the pyrimidine molecule; radiation-induced electron attachment to thymine does not lead to destruction of the chromophoric group if oxygen is present in the system. Aerated thymine solutions (8 × 10–5-2 × 10–4 M) have been used to determine, spectrophotometrically, the relative reactivities of solutes towards OH radicals; because of an unusual pH effect observed in neutral solutions, the competition experiments were carried out below pH 6. The reactivities of a number of organic solutes have been obtained. Similar rate constant ratios were obtained in solutions containing N2O, indicating that N2O– was not an oxidizing species under the conditions of the experiments.

Variation in the photochemical reactivity of thymine in the DNA of B. subtilis spores, vegetative cells and spores germinated in chloramphenicol.

Abstract— The chief photoproduct of thymine produced in u.v. irradiated (2537Å) vegetative cells of B. subtilis is the cyclobutane‐type dimer while in spores very little of this dimer is produced (maximum yield 2·6 per cent of thymine) but a new photoproduct is produced in high yield (maximum of 28·4 per cent of thymine). This difference in photochemical response appears to be due, at least in part, to a difference in uydration of the DNA. The photochemistry of thymine in isolated DNA irradiated in solution is similar to that of DNA in irradiated vegetative cells, but differs markedly from that of isolated DNA irradiated dry. The yield of cyclobutane‐type thymine dimer is much reduced in isolated DNA irradiated dry but a new photoproduct of thymine. is produced which is chromatographically similar to the spore photoproduct. The yield of this photoproduct, however, is never as great as that obtained in irradiated spores.The photochemistry of the DNA thymine of spores germinated in the presence of chloramphenicol is very similar to that of normal vegetative cells. Except for hydration, the physical state of the DNA is probably not otherwise altered by germination in the presence of chloramphenicol since DNA replication is prevented by the presence of chloramphenicol. These results are also consistent with the hypothesis that the unique photochemistry of spores is due, at least in part, to the hydration state of the DNA.The acid stability of the spore photoproduct is indicated by the fact that it is isolated from irradiated spores after hydrolysis in trifluoroacetic acid at 155°C for 60 min. It still contains the methyl group of thymine as judged by the fact that for a given dose of u.v. the same yield of photoproduct was obtained whether the spores were labeled with thymine‐2–C‐14 or ‐methyl‐C‐14. This photoproduct is stable to reirradiation (2537Å) in solution under condiditions where thymine dimers of the cyclobutane‐type are completely converted back to monomeric thymine. On a column of molecular sieve material (Sephadex‐G10), the spore photoproduct elutes in a region intermediate between the cyclobutanetype thymine dimers and monomeric thymine. Of the numerous compounds tested by paper chromatography, the spore photoproduct is most similar (but not identical) in several solvents to 5–hydroxyuracil and 5–hydroxymethyluracil. Our data do not allow us to decide if the product is a monomer or a dimer.Although the photochemistry of thymine in the DNA of spores differs markedly from that for vegetative cells, several lines of evidence make it seem doubtful that the enhanced resistance of spores to u.v. relative to that of vegetative cells can be explained solely on the basis of this difference in the photochemistry of DNA thymine.

Some aspects of thymine biosynthesis in Ochromonas malhamensis

The donor of the 5-methyl group of thymine in the protozoan, Ochromonas malhamensis, was found to be N 5,N 10-methylene-tetrahydrofolate. N 5-methyl-tetrahydrofolate was not a methyl donor for this reaction. The incorporation of label from N 5,N 10-methylene-C 14-tetrahydrofolate into thymine derivatives was found to proceed in a nutritional deficiency of vitamin B 12, but was greatly decreased in the presence of B 12-anilide, a vitamin B 12 antagonist. Dimethylbenzimidazolylcobamide coenzyme prevented a large part of the decrease in incorporation of label due to the anilide, and cyanocobalamin was slightly effective. Adenylcobamide coenzyme decreased thymine synthesis further. Evidence was obtained for an alternate pathway of thymine biosynthesis operating through a compound tentatively identified as a 5-methyl-cytosine derivative. This pathway was not inhibited by B 12-anilide, but extracts from B 12-deficient cells caused marked increases in incorporation of label on addition of dimethylbenzimidazolylcobamide coenzyme.

STIMULATION OF PROTEIN SYNTHESIS IN VITRO BY DENATURED DNA.

Following the recognition of a role for ribosomes in protein synthesis,l ald the evolution of a messenger RNA theory,2 it has been well established' that certain types of RNA and synthetic polyribonucleotides can serve a coding function in protein synthesis. Current evidence suggests that only one of the two strands of DNA serves as a template for synthesis of messenger RNA.4 It is obvious, therefore, that the other strand of DNA should exhibit a nucleotide sequence resembling that of messenger RNA. The present study was initiated to determine whether single-stranded DNA might be capable of directly programing protein synthesis on E. coli ribosomes in a manner analogous to messenger RNA action, or whether the 5-methyl group of thymine and the lack of a 2' hydroxyl group in deoxyribose imposes steric or configurational blocks to translation of single-stranded DNA. In the assay of Nirenberg and Leder,5 the 2' hydroxyl group is apparently indispensable for codeword recognition by sRNA's since oligodeoxynucleotides may not be substituted for the corresponding oligoribonucleotides. On the other hand, Takanami and Okamoto demonstrated the formation of polyribosome complexes by denatured DNA and E. coli ribosomes in vitro, and Szer and and Ochoa showed that polyribothymidylate promotes phenylalanine incorporation by E. coli ribosomes.6 Materials and Methods.-E. coli strain K12 was grown at 37? in Difco brain-heart infusion broth and harvested by centrifugation while still in the log phase of growth. Cells were suspended in 2 or 3 vol of the standard buffer of Matthaei and Nirenberg;7 0.01 M tris-HCl, pH 7.8; 0.01 M magnesium acetate; 0.06 M KC1 and 0.006 M mercaptoethanol. The frozen cells were disrupted in a French pressure cell at 10,000-13,000 psi. The supernatant solution, after removal of cell wall debris by centrifugation at 30,000 g for 15 min (S30), was dialyzed against the above standard buffer, modified by reduction of the magnesium acetate concentration to 10-4 M, for 24 hr at 4?. The dialyzed S30 fraction was placed into sealed tubes in small aliquots and stored frozen at -85?C or below until used. Deoxyribonuclease I was a crystalline preparation obtained from Worthington Biochemical Co., electrophoretically purified to remove traces of ribonuclease. Pancreatic ribonuclease was a crystalline preparation from Boehringer & Soehne, boiled before use to inactivate traces of deoxyribonuclease. Unlabeled amino acids, ATP, CTP, UTP, CTP, phosphoenolpyruvate Na salt (PEP), PEP kinase, and poly U, were obtained from Calbiochem, as were salmon sperm DNA and calf thymus DNA and the enzyme pronase. Puromycin was a generous gift from Lederle Laboratories, and chloramphenicol was obtained from Parke-Davis and Co. Actinomycin 1) was kindly provided by Merck, Sharp and Dohme Co. C14 yeast protein hydrolysate (850 Ac/mg) and C14 L-phenylalanine (141 mc/mM) were obtained from Schwarz, and C14 L-valine (200 mc/ mM) was obtained from New England Nuclear Corporation. Two methods were used for the extraction of DNA. Nuclei prepared from animal tissues or intact cells from tissue cultures were lysed in a solution containing 0.2% sodium dodecyl sulfate, 50 ug/ml pronase, 0.001 M EDTA, and 27% w/v sucrose8 by incubation at 37? for several hours. Following 2 extractions with equal volumes of phenol at 60?, DNA was precipitated several times from ethanol. The final precipitate was dried in a stream of nitrogen to remove ethanol, dissolved in a small volume of distilled water, and frozen until used. Alternatively, DNA was recovered from the phenol water interphase after applying the Scherrer and Darnell procedure for quantitative extraction of RNA9 and purified by an additional phenol extraction and multiple

DISTRIBUTION OF ADENINE RESIDUES IN DEOXYRIBONUCLEIC ACIDS.

IN a previous communication1 we described a method for the determination of the distribution of adenine residues in DNA. This involved the oxidation of DNA (I) with potassium permanganate to give ODNA (II) in which all the guanine, cytosine and thymine groups had been oxidized to ureido groups.

X-ray studies of two synthetic DNA copolymers

An X-ray diffraction study of the synthetic DNA copolymers dAT† and dA BU ¯ has demonstrated that the lithium salts exist in a B form with helix dimensions identical with those of lithium DNA. The intensity distribution of the dAT pattern is very similar to that of DNA, whereas the dA BU ¯ pattern shows a number of intensity changes resulting from the substitution of bromine for the methyl group of thymine. The sodium salt of dAT has on occasion given fibers with a crystalline A form pattern like that of natural DNA. However, some new fiber forms, including one quite unlike any observed with natural DNAs, have been found in fibers of the sodium and ammonium salts of dAT.