Photoreactivation of template activity of UV-irradiated DNA in an RNA-polymerase system. A rapid assay for photoreactivating enzyme

Mitochondrial DNA repair by photolyase

Photolyase genes of Saccharomyces cerevisiae and Escherichia coli were expressed in S. cerevisiae and photoreactivation in nuclei and mitochondria of the host cells was analyzed by determination of survival and petit rates. Yeast photolyase was able to repair mitochondrial DNA effectively, whereas E. coli photolyase could reduce only a small fraction of the petit rate produced by UV irradiation. Analysis using fusion between yeast photolyase and E. coli lacZ genes as well as a chimeric gene between yeast and E. coli photolyase genes suggests the importance of the protruding amino terminal region of the yeast photolyase for its transport into mitochondria. A significant similarity between the protruding amino termini of yeast photolyase and yeast uracil-DNA-glycosylase suggests a common functional importance of the terminal sequences for both DNA repair enzymes.

Existence and expression of photoreactivation repair genes in various yeast species

Photoreactivation repair (Phr) activities in cell extracts of 13 different yeast species were measured by the Haemophilus influenzae transformation assay. Five species including Schizosaccharomyces pombe showed no or low enzymatic activity. In contrast to the other species, chromosomal DNAs of these 5 species did not show detectable hybridization using a DNA fragment of the photolyase PHR1 gene of Saccharomyces cervisiae as a probe even at a low stringency condition. When the PHR1 gene was attached to the 5'-flanking sequence of the iso-1-cytochrome c (CYC-1) gene of S. cerevisiae and introduced into S. pombe cells, the transformants acquired a high Phr activity, indicating that the PHR1 gene alone can provide a Phr-negative species with this repair activity and the light-absorbing cofactor(s) must be present in S. pombe. Our results also demonstrated that the 5'-flanking sequence of the S. cerevisiae CYC-1 gene works in S. pombe as a regulatory element.

Unscheduled DNA synthesis in xeroderma pigmentosum cells after microinjection of yeast photoreactivating enzyme

Photoreactivating enzyme (PRE) from yeast causes a light-dependent reduction of UV-induced unscheduled DNA synthesis (UDS) when injected into the cytoplasm of repair-proficient human fibroblasts (Zwetsloot et al., 1985). This result indicates that the exogenous PRE monomerizes UV-induced dimers in these cells competing with the endogenous excision repair. In this paper we present the results of the injection of yeast PRE on (residual) UDS in fibroblasts from different excision-deficient XP-strains representing complementation groups A, C, D, E, F, H and I (all displaying more than 10% of the UDS of wild-type cells) and in fibroblasts from two excision-proficient XP-variant strains. In fibroblasts belonging to complementation groups C, F and I and in fibroblasts from the XP-variant strains UDS was significantly reduced, indicating that pyrimidine dimers in these cells are accessible to and can be monomerized by the injected yeast PRE. The UDS reduction in the XP-variant strains is comparable with the effect in wild-type cells. In cells from complementation groups C, F and I the reduction is less than in wild-type and XP-variant cells. Fibroblasts belonging to groups A, D, E and H did not show any reduction in UDS level after PRE injection and illumination with photoreactivating light. These results give evidence that the genetic repair defect in some XP-strains is probably due to an altered accessibility of the UV-damaged sites.

Evidence for the presence of an essential arginine residue in photoreactivating enzyme from Streptomyces griseus

Butane-2,3-dione inhibits the enzymic activity of Streptomyces griseus photoreactivating enzyme (PRE). Some characteristics of the inhibition, notably the enhancement by borate buffer and the reversibility, indicate that arginine residues are modified. From the kinetics of inhibition it can be concluded that a single essential arginine residue is involved. U.v.-irradiated DNA, the substrate for PRE, protects the enzyme against inactivation by butane-2,3-dione. This suggests that the essential arginine residue is situated in or near the u.v.-irradiated-DNA-binding site. Non-irradiated DNA at higher concentrations also protects against inactivation, indicating that PRE can form non-specific complexes. From the ratio of complex constants obtained from protection experiments with non-irradiated and u.v.-irradiated DNA it appears that PRE preferably binds to dimer sites.

Microinjected photoreactivating enzymes from Anacystis and Saccharomyces monomerize dimers in chromatin of human cells

Photoreactivating enzymes (PRE) from the yeast Saccharomyces cerevisiae and the cyanobacterium Anacystis nidulans have been injected into the cytoplasm of repair-proficient human fibroblasts in culture. After administration of photoreactivation light, PRE-injected cells displayed a significantly lower level of UV-induced unscheduled DNA synthesis (UDS) than non-injected cells. This indicates that monomerization of the UV-induced pyrimidine dimers in the mammalian chromatin had occurred as a result of photoreactivation by the injected PRE at the expense of repair by the endogenous excision pathway. Purified PRE from yeast is able to reduce UDS to 20-25% of the UDS found in non-injected cells, whereas the in vitro more active PRE from A. nidulans gives a reduction to only 70%. This suggests that the eukaryotic enzyme is more efficient in the removal of pyrimidine dimers from mammalian chromatin than its equivalent purified from the prokaryote A. nidulans.

Expression of an Escherichia coli phr gene in the yeast Saccharomyces cerevisiae

A 2 kb DNA fragment, containing the photoreactivation gene phr1 from Escherichia coli, was inserted at the BamH1 site in the tet gene of the yeast--E. coli shuttle vector pJDB207. Photoreactivation--deficient Saccharomyces cerevisiae cells transformed with this plasmid showed photoreactivation of killing after UV irradiation of the cells, while extracts of transformed cells exhibited photoreactivating activity in vitro. Far more photoreactivating enzyme molecules were found when the gene was inserted in the plasmid in the opposite orientation to the tet gene as compared with a plasmid carrying the inserted gene in the same orientation. Photoreactivating enzyme encoded by the E. coli phr1 gene and produced in transformed yeast cells has characteristics of the E. coli photoreactivating enzyme (flavoprotein) as judged from the influence of ionic strength on photoreactivating activity.