Rapid separation of thymine dimer and pyrimidines on ion-exchange paper

Formation of dimers in ultraviolet-irradiated DNA

Evidence has been obtained that in UV-irradiated native DNA, pyrimidine dimers are preferentially formed in long pyrimidine tracts. This effects is not the result of the existence of more dimerizable thymine in the long pyrimidine tracts. The preferential formation of dimers in long pyrimidine tracts is enhanced as the dose of irradiation is decreased. These results suggest that the formation of dimers in native DNA occurs by a cooperative mechanism. Quite likely, dimers are formed only in regions of native DNA that are locally denatured (breathing), in which the bases can be aligned. The formation of a dimer in such a breathing region would tend to lock it open and could lead to cooperative formation of dimers in such regions. If this mechanism for the cooperative formation of dimers is operative then it should be restricted to double-strand DNA. The pyrimidine tracts of irradiated single-stranded DNA all show virtually the same dimer content, irrespective of the length of the tract. These results are consistent with the hypothesis that dimers form cooperatively in breathing regions.

Conjugal deoxyribonucleic acid replication by Escherichia coli K-12: stimulation in dnaB(ts) donors by minicells

R64-11(+) donor cells that are thermosensitive for vegetative DNA replication will synthesize DNA at the restrictive temperature when recipient minicells are present. This is conjugal DNA replication because it is R64-11 DNA that is being synthesized and there is no DNA synthesis if minicells that cannot be recipients of R64-11 DNA are used. The plasmid DNA present in the donor cells before mating is transferred to recipient minicells within the first 20 min of mating, but additional copies of plasmid DNA synthesized during the mating continue to be transferred for at least 90 min. However, the transfer of R64-11 DNA to minicells is not continuous because the plasmid DNA in minicells is the size of one R64-11 molecule or smaller, and there are delays between the rounds of plasmid transfer. DNA is synthesized in minicells during conjugation, but this DNA has a molecular weight much smaller than that of R64-11. Thus, recipient minicells are defective and are not able to complete the synthesis of a DNA strand complementary to the single-stranded R64-11 DNA received from the donor cell.

Repair of ultraviolet light-induced damage to the deoxyribonucleic acid of Neurospora crassa

A method is described for labeling a specific pyrimidine in the deoxyribonucleic acid (DNA) of Neurospora crassa. In cells grown in the presence of [5-(3)H]-uridine, more than 97% of the radioactivity associated with the DNA had been incorporated into cytosine. The specific activity of the labeled DNA was approximately 3 x 10(3) counts per min per mug. The DNA was isolated by elution from hydroxyapatite columns with sodium phosphate buffer (0.40 m, pH 6.8). This procedure was used to demonstrate that in vegetative cells of N. crassa both photoreactivation and excision repair are operative, as measured by the removal of ultraviolet light-induced cytosine-containing dimers.

Minor components of the DNA of Physarum polycephalum. Cellular location and metabolism

DNA metabolism in the slime mold Physarum polycephalum was studied by centrifugation in CsCl of lysates of cultures labeled with radioactive thymidine at various times in the cell cycle. During the G(2) (premitotic) phase of the cell cycle, two components of the DNA are labeled. One component is lighter (buoyant density 1.686 g/cc) than the mean of the principal DNA (1.700 g/cc), and one is heavier (approximately 1.706 g/cc). The labeled light DNA was identified chemically by its denaturability, its susceptibility to DNase, and the recovery of its radioactivity in thymine. Cell fractionation studies showed that the heavy and the principal DNA components are located in the nucleus and that the light DNA is in the cytoplasm. The light DNA comprises approximately 10% of the DNA. About (1/3)-(1/2) of the light DNA is synthesized during the S period, and the remainder is synthesized throughout G(2) (there is no G(1) in Physarum). The light DNA is metabolically stable. A low, variable level of incorporation of radioactive thymidine into the principal, nuclear DNA component was observed during G(2).