Thymine-thymine adduct as a photoproduct of thymine

A product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution undergoes dehydration on heating with acids. As judged by elemental analysis, mass, ultraviolet, infrared, and nuclear magnetic resonance spectra, the most probable structures for this compound and its dehydration product, respectively, are 5-hydroxy-6-4'-[5'-methylpyrimidin-2'-one]-dihydrothymine and 6-4'-[5'-methylpyrimidin-2'-one]-thymine. Apparently, this compound is a thymine-thymine adduct and presumably is formed through the rearrangement of an initial photoproduct. Both compounds are closely related to 6-4'-[pyrimidin-2'-one]-thymine which has been isolated from acid hydrolyzates of ultraviolet-irradiated DNA and supposedly is derived from cytosine-thymine adduct. Formation of such adducts between pyrimidine bases is apparently a common photoreaction and may be important to the study of the photochemistry and photobiology of nucleic acids.     

Crystal and molecular structure of a thymine-thymine adduct

Thymine-thymine adduct is a product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution. This compound is presumably formed through the rearrangement of an initial photoproduct. Single crystal x-ray diffraction analysis has confirmed the molecular formula of the adduct, 5-hydroxy-6-4'-(5'-methylpyrimid-2'-one)-dihydrothymine, except for the possibility of a hydrogen atom on the 3' nitrogren rather than the 1' nitrogen, and has established the stereoconfiguration of the molecule.     

5-Ethyldeoxyuridine, a thymidine analog: photochemical transformation

Irradiation at 254 millimicrons transforms 5-ethyldeoxyuridine to deoxyuridine by way of photohydration of the 5,6 bond and elimination of ethanol. At wavelengths to the red side of 265 millimicrons, photodimerization is the principal reaction, with a pronounced oxygen effect. The results are related to the photochemistry of thymidine and of bacteriophages containing incorporated 5-ethyluracil in place of thymine.     

Crystal and molecular structure of a thymine phototrimer

Thymine trimer was isolated from a frozen aqueous solution of thymine which was irradiated with ultraviolet light and was presumably formed through the rearrangement of an initial oxetane photoproduct. X-ray diffraction analysis of a single crystal has confirmed the trimeric diol structure and has established the stereoconfiguration of the molecule. The possible importance of the diol structure in photobiology is pointed out.     

Ultraviolet damage to bacteria and bacteriophage at low temperatures

The survival of Escherichia coli B/r WP2 (tryptophan-requiring) from ultraviolet irradiation when suspended in 0.067M phosphate buffer (pH 7) has been studied over the temperature range 22 degrees to -269 degrees C. In unfrozen suspensions there was no appreciable change in sensitivity between 22 degrees and -10 degrees C. The sensitivity in the presence of ice progressively increased by a factor of 7 when the temperature was lowered to -79 degrees C. Between -79 degrees and -196 degrees C the sensitivity decreased to less than four times the sensitivity at 22 degrees C and was not appreciably different at -269 degrees C. Evidence from experiments with bacteriophage T1 and E. coli WP2 HCR(-) (a strain unable to excise thymine dimers) indicates that a new, qualitatively different lesion, less amenable to repair, may replace the thymine dimer in E. coli irradiated at -79 degrees C.     

Microspectrophotometry of photoreceptor organelles from eyes of the prawn Palaemonetes

Microspectrophotometric measurements of individual dark-adapted rhabdoms of the prawn Palaemonetes vulgaris reveal the presence of two light-sensitive pigments. A pigment with maximum absorbancy at 555 nanometers is converted by light to a long-lived intermediate with wavelength of maximum absorbancy at 496 nanometers. A second pigment with wavelength of maximum absorbancy at 496 nanometers bleaches in the light, seemingly without forming detectable products at wavelengths longer than 375 nanometers. Both pigments occur in each layer of microvilli.     

Uracil photoproducts from uracil irradiated in ice

Two new products were isolated from uracil irradiated with ultraviolet light in frozen aqueous solution. As judged by mass, nuclear magnetic resonance, and ultraviolet and infrared spectra, one is a photopolymer, U(3) and the structure of the other is probably 6-4'-[ pyrimidin-2'-one]-uracil. Formations of these products between pyrimidine bases are apparently common photoreactions, and may be important to the study of the photochemistry and photobiology of nucleic acids.     

Nucleic acid guanine: reaction with the carcinogen N-acetoxy-2-acetylaminofluorene

Reaction of N-acetoxy-2-acetylaminofluorene with DNA or RNA at pH 7 causes marked increase in absorption at 280 to 320 millimicrons and marked decrease in guanine content. Reaction with guanosine-8-(14)C yields a radioactive fluorescent derivative. The data suggest that metabolic esters of N-hydroxy-2-acetylaminofluorene may be intermediates in the binding of this proximate carcinogen to nucleic acids in vivo.     

Absorption and Metabolism of Iron

In the article "Absorption and metabolism of iron" [Science 123, 87 (20 Jan. 1955)], the wavelength of maximum absorption of the iron-siderophilin complex was incorrectly given as 520 millimicrons. On page 89, the first sentence in the second paragraph under the heading "Iron transport" should read "The Fe+++-B(1)-globulin complex has a characteristic salmon-pink color with a maximum light absorption at 460 millimicrons...."     

Volatilization of high molecular weight DNA by pulsed laser ablation of frozen aqueous solutions

DNA has been volatilized by pulsed laser ablation of a thin film of a frozen aqueous DNA solution. The target film was irradiated in vacuum by a pulsed laser at power densities sufficient to ablate the ice matrix. The expanding ablated water vapor entrained embedded DNA molecules and expelled them into the gas phase. Ejection of DNA molecules as large as 410 kilodaltons was verified by collection of the ablation products and subsequent mass analysis by polyacrylamide gel electrophoresis with autoradiographic detection.     

Quantum yield of oxygen evolution and the emerson enhancement effect in deuterated chlorella

The maximum quantum yield of oxygen evolution in deuterated chlorella is found to be 0.075, while normal chlorella showed, in parallel experiments, a value of 0.10. Deuterated Chlorella vulgaris showed a decline in the quantum yield of oxygen evolution ("red drop") beginning at 680 millimicrons and a clear Emerson enhancement effect qualitatively similar to that obtained in normal chlorella cells. However, the ratio of quantumn yield at 680 to that at 710 millimicrons was about 1.5 times higher in normal than in deuterated chlorella cells. Action spectra of the Emerson enhancement effect in deuterated chlorella also are qualitatively similar to those of normal cells.     

Antibodies to photoproducts of deoxyribonucleic acids irradiated with ultraviolet light

A rabbit immunized with complexes of methylated bovine serum albumin and ultraviolet-irradiated DNA from calf thymus produced antibodies directed toward the photoproducts in the DNA. Serologic activity appeared after irradiation of DNA at 270 mmicro and decreased upon irradiation at 235 mmicro. The antigenic determinants of the ultraviolet-treated DNA appear to be photoproducts associated primarily with thymine, as measured by direct dependence of serologic activity on the adenine-thymine content of the DNA, and by inhibition of the Serlolgic reaction by the irradiated di-,tri-,and tetra-(thymidine-5'-phosphate nucleotides.     

Photoinduced DNA-protein cross-links and bacterial killing: a correlation at low temperatures

The increased sensitivity of Escherichia coli to killing by ultraviolet irradiation when frozen and the variation in this sensitivity as a function of the temperature during irradiation have been correlated with changes in the amount of DNA that was cross-linked to protein by ultraviolet light. These variations in sensitivity to killing do not correlate with the production of thymine dimers.     

Echinomycin binding sites on DNA

The preferred binding sites of echinomycin on DNA can be determined by a method called "footprinting." A 32P end-labeled restriction fragment from pBR322 DNA is protected by binding to echinomycin, and cleaved by a synthetic DNA cleaving reagent, methidiumpropyl--EDTA . Fe(II); the DNA cleavage products are then subjected to high-resolution gel analyses. This method reveals that echinomycin has a binding site size of four base pairs. The strong binding sites for echinomycin contain the central two-base-pair sequence 5'-CG-3'. From an analysis of 15 echinomycin sites on 210 base pairs of DNA, key recognition elements for echinomycin are contained in the sequences (5'-3') ACGT and TCGT (A, adenine; C, cytosine; G, guanine; T, thymine).     

THYMINE DIMERS AND INHIBITION OF DNA SYNTHESIS BY ULTRAVIOLET IRRADIATION OF CELLS

Measurements of the inhibition of DNA synthesis in bacteria and the formation of thymine dimers by ultraviolet irradiation of the cells indicate that one thymine dimer per 350-IA strand of DNA acts as a block to further synthesis. In a sensitive strain of Escherichia coli the blocks are permanent. In a resistant strain the blocks are only temporary but recovery of synthesis is not the result of splitting dimers.     

"C"-type viral particles in plasma of cats with feline leukemia

Linear sucrose-density gradient was used to detect and isolate typical "C"-type viral particles in plasma of cats with spontaneous and experimentally induced leukemia. The density of the agent is similar to known murine leukemia virus (1.15 to 1.17 grams per cubic centimeter). In the electron microscope the virus showed typical "C"-type particle morphology with various maturation stages. The maximum diameter of the mature viral particles in plasma was 115 millimicrons, a diameter slightly larger than budding particles observed in tissue. Leukemia was transmitted with cellular and cell-free inoculum after a 5-week period of latency.     

Cyclobutane-type pyrimidine dimers in polynucleotides

The formation of cyclobutane-type dimers between adjacent pyrimidine residues in model polynucleotides or DNA may be represented by the general scheme See pdf 379.pdf Whereas the formation of all other known photoproducts follows the irreversible path See pdf 379.pdf Thus dimers are distinguished from other photoproducts by the fact that they can be monomerized, as well as formed, by ultraviolet irradiation. At large incident fluxes of photons the steady-state value of dimers depends on wavelength and pH, as well as on other characteristics of the surrounding medium. The number of dimers in an irradiated polynucleotide may be decreased by purely photochemical means, whereas this is not true for most other photoproducts, for which continued irradiation, irrespective of wavelength, always results in the formation of more photoproduct (37). The wavelength dependence of the steady-state for dimers is also reflected in the biological activity of irradiated transforming DNA. This experiment and the fact that photoreactivating enzyme plus visible light monomerizes dimers (and has not been demonstrated to have any effect on other photoproducts) are the strongest lines of experimental evidence that pyrimidine dimers of the cyclobutane type are biologically important lesions and can account for a large fraction of the effects of ultraviolet light on DNA in solution. Insofar as DNA is one of the more important biological structures, such dimers, when formed, account for a large part of the effects of ultraviolet radiation on biological systems.     

Studies of chlorophyll-lipid-water systems

Preliminary studies of the electronic absorption spectra of chlorophyll molecules in aqueous glycolipid and phospholipid dispersions are reported. The spectra show a shift for the red peak of chlorophyll of some 10 millimicrons to longer wavelength compared with the spectrum of chlorophyll in ether. Photochemical studies were carried out with cytochrome c incorporated into the aqueous layers separating the lipid bilayers.     

Enhanced CpG mutability and tumorigenesis in MBD4-deficient mice

The mammalian protein MBD4 contains a methyl-CpG binding domain and can enzymatically remove thymine (T) or uracil (U) from a mismatched CpG site in vitro. These properties suggest that MBD4 might function in vivo to minimize the mutability of 5-methylcytosine by removing its deamination product from DNA. We tested this hypothesis by analyzing Mbd4-/- mice and found that the frequency of of C --> T transitions at CpG sites was increased by a factor of three. On a cancer-susceptible Apc(Min/+) background, Mbd4-/- mice showed accelerated tumor formation with CpG --> TpG mutations in the Apc gene. Thus MBD4 suppresses CpG mutability and tumorigenesis in vivo.     

Visual pigment fluorescence

The fluorescence of cattle rod outer segments (dried) and of rhodopsin in solution lies in the range of 575 to 600 millimicrons with a quantum efficiency of 0.005 if excitation is in the visible band near 500 millimicrons. The emission is abolished by bleaching at -196 degrees C but can be reversibly regenerated by irradiation with light of longer wavelength (600 millimicrons). This behavior reflects the known interconversion of rhodopsin to prelumirhodopsin at this temperature.