Dimer excision in Escherichia coli in the presence of caffeine

The effect of non-mutagenic stress on liquid holding recovery in Escherichia coli AB2463

Previous work has shown that exposure to many non-mutagenic stresses causes greater UV resistance in Escherichia coli K12 via an error-free, excision repair-dependent process. Induction of the latter should enhance liquid holding recovery in the bacteria. The results in this paper show that this is the case and that the increased UV-resistance is due entirely to an increase in the capacity of the cells for DNA excision repair. The latter arises wholly or in part from an increase in the intracellular level of the key enzyme of the pathway, UvrABC endonuclease.

The repair patch of E. coli (A)BC excinuclease

The size of repair patch made by E. coli DNA polymerase I (Poll) following the removal of a thymine-psoralen monoadduct by E. coli (A)BC excinuclease was determined by using an M13mp19 DNA with a single psoralen monoadduct at the polylinker region. Incubation of this substrate with (A)BC excinuclease, Poll and a combination of 3 dnTP plus 1 dNTP(alpha S) for each nucleotide, and DNA ligase resulted in a repair patch with phosphorothioate linkages. The preferential hydrolysis of phosphorothioate bonds by heating in iodoethanol revealed a patch size--with minimal nick translation--equal in length to the 12 nucleotide gap generated by this excision nuclease.

Long repair replication patches are produced by the short-patch pathway in a uvrD252 (recL152) mutant of Escherichia coli K-12

The uvrD252 mutation leads to increased UV sensitivity, diminished dimer excision and host cell reactivation capacity, and an increase in the average patch size after repair replication. A recA56 uvrD252 double mutant was far more resistant to UV than was a recA56 uvrB5 double mutant. Its host cell reactivation capacity was identical to that of uvrD252 single mutant and was far greater than that of the uvrB5 single mutant. The strain showed no Weigle reactivation. From these results, we concluded that the double mutant has no inducible DNA repair (including long-patch excision repair) but retains dimer excision capabilities comparable to the uvrD252 single mutant. It appears, therefore, that the long patches detected in the uvrD mutant were not identical to the recA-dependent patches seen in wild-type cells.

UV light induction of proteins in Bacteroides fragilis under anaerobic conditions

Far-UV irradiation of Bacteroides fragilis cells under anaerobic conditions resulted in the induction of a new 95,000-molecular-weight protein and the increased synthesis of two proteins with molecular weights of 90,000 and 70,000. The latter two proteins were synthesized in small amounts in unirradiated cells. The induction of a 37,000- to 40,000-molecular-weight protein was not observed in irradiated B. fragilis cells. Caffeine, which affected the survival of irradiated B. fragilis cells and reduced host cell-mediated UV reactivation, specifically inhibited the induction of the 95,000-, 90,000-, and 70,000-molecular-weight proteins. Sodium arsenite did not affect the induction of the three inducible proteins or the survival of irradiated B. fragilis cells.

Excision repair and patch size in UV-irradiated bacteriophage T4

We determined the average size of excision repair patches in repair of UV lesions in bacteriophage T4 by measuring the photolysis of bromodeoxyuridine incorporated during repair. The average patch was small, approximately four nucleotides long. In control experiments with the denV1 excision-deficient mutant, we encountered an artifact, a protein(s) which remained bound to phenol-extracted DNA and prevented nicking by the UV-specific endonucleases of Micrococcus luteus and bacteriophage T4.