Production of additional sites of deoxyribonucleic acid breakdown in bromouracil containing Escherichia coli exposed to ultra-violet light

Selective breakage of DNA alongside 5-bromodeoxyuridine nucleotide residues by high temperature hydrolysis

The substitution of thymine mucleotides (pT) in oligodeoxynucleotides by bromouracil nucleotides (pBU) changes the properties of the oligonucleotides in two ways: (1) It alters their mobility during DEAE-Cellulose homochromatography1. (2) It substantially enhances their sensitivity to high temperature hydrolysis under mildly alkaline conditions (pH 8.9). The resultant breaks occur adjacent to pBU residues and leave terminal phosphates on the breakage products. With more extreme conditions some loss of terminal phosphates can occur. Heating at 100 degrees for 16 hr at pH 8.9 produces cleavage at about half of the pBU residues with minimal loss of terminal phosphates. The properties described here may explain the thermal sensitivity of bacteria grown in 5BU2 and may have a use in DNA sequencing technology.

Repair replication and degradation of bromouracil-substituted DNA in mammalian cells after irradiation with ultraviolet light

Ultraviolet (UV) light irradiation of HeLa cells in which bromouracil (BU) is substituted for thymine in one strand of the DNA, elicits a number of responses that occur predominantly in the BU strand. A small amount of degradation of both strands occurs, but the BU strand is degraded to a greater extent than the normal strand. Large UV doses (1000 erg/mm(2)) induce degradation of about 1.7% of the DNA within 6 hr of irradiation of unsubstituted cells; in BU-substituted cells under these conditions about 1.9% of the normal strand is degraded but 17.5% of the BU strand. After irradiation fresh bases are inserted into the BU strands at infrequent intervals throughout the DNA and this is presumed to represent repair of UV damage in the BU strands. After 1000 erg/mm(2) the majority (70%) of the thymidine incorporated enters the BU strand. Inhibitors of normal DNA synthesis, hydroxyurea and arabinosyl cytosine, do not appear to inhibit the repair of DNA. The increased sensitivity of mammalian cells that contain BU to irradiation may consequently be due to damage of the BU strand. A specific interference between BU and repair of DNA which leads to large amounts of DNA degradation in bacteria, does not seem to be important in the sensitization of mammalian cells with BU.