A Saccharomyces cerevisiae phleomycin-sensitive mutant, ph140, is defective in the RAD6 DNA repair gene

Bre1/RNF20 promotes Rad51-mediated strand exchange and antagonizes the Srs2/FBH1 helicases

Central to homologous recombination (HR) is the assembly of Rad51 recombinase on single-strand DNA (ssDNA), forming the Rad51-ssDNA filament. How the Rad51 filament is efficiently established and sustained remains partially understood. Here, we find that the yeast ubiquitin ligase Bre1 and its human homolog RNF20, a tumor suppressor, function as recombination mediators, promoting Rad51 filament formation and subsequent reactions via multiple mechanisms independent of their ligase activities. We show that Bre1/RNF20 interacts with Rad51, directs Rad51 to ssDNA, and facilitates Rad51-ssDNA filament assembly and strand exchange in vitro. In parallel, Bre1/RNF20 interacts with the Srs2 or FBH1 helicase to counteract their disrupting effect on the Rad51 filament. We demonstrate that the above functions of Bre1/RNF20 contribute to HR repair in cells in a manner additive to the mediator protein Rad52 in yeast or BRCA2 in human. Thus, Bre1/RNF20 provides an additional layer of mechanism to directly control Rad51 filament dynamics.

Upregulation of the Saccharomyces cerevisiae efflux pump Tpo1 rescues an Imp2 transcription factor-deficient mutant from bleomycin toxicity

Yeast mutants lacking the transcriptional co-activator Imp2 are hypersensitive to the anticancer drug bleomycin, although the gene targets involved in this process remain elusive. A search for multicopy suppressors that rescue the imp2Δ mutant from bleomycin toxicity revealed the transcriptional activator Yap1, which can turn on many target genes such as transporters involved in regulating drug resistance. We show that YAP1 overexpression stimulated the expression of the TPO1 gene encoding a polyamine efflux pump, and that Yap1 failed to rescue the imp2Δ mutant from bleomycin toxicity in the absence of the TPO1 gene. Moreover, TPO1 overexpression, and not the related transporter gene QDR3, conferred upon the tpo1Δ imp2Δ double mutant parental resistance to bleomycin. We conclude that YAP1 overexpression rescues the imp2Δ mutant from bleomycin toxicity by triggering Tpo1 expression to expel the drug. Our data provide the first evidence that bleomycin could be a substrate for the Tpo1 efflux pump.

A comparison of synthetic oligodeoxynucleotides, DNA fragments and AAV-1 for targeted episomal and chromosomal gene repair

Background

Current strategies for gene therapy of inherited diseases consist in adding functional copies of the gene that is defective. An attractive alternative to these approaches would be to correct the endogenous mutated gene in the affected individual. This study presents a quantitative comparison of the repair efficiency using different forms of donor nucleic acids, including synthetic DNA oligonucleotides, double stranded DNA fragments with sizes ranging from 200 to 2200 bp and sequences carried by a recombinant adeno-associated virus (rAAV-1). Evaluation of each gene repair strategy was carried out using two different reporter systems, a mutated eGFP gene or a dual construct with a functional eGFP and an inactive luciferase gene, in several different cell systems. Gene targeting events were scored either following transient co-transfection of reporter plasmids and donor DNAs, or in a system where a reporter construct was stably integrated into the chromosome.

Results

In both episomal and chromosomal assays, DNA fragments were more efficient at gene repair than oligonucleotides or rAAV-1. Furthermore, the gene targeting frequency could be significantly increased by using DNA repair stimulating drugs such as doxorubicin and phleomycin.

Conclusion

Our results show that it is possible to obtain repair frequencies of 1% of the transfected cell population under optimized transfection protocols when cells were pretreated with phleomycin using rAAV-1 and dsDNA fragments.

Protective mechanisms against the antitumor agent bleomycin: lessons from Saccharomyces cerevisiae

Bleomycin is a small glycopeptide antibiotic used in combination therapy for the treatment of a few types of human cancer. The antitumor effect of bleomycin is most likely caused by its ability to bind to DNA and induce the formation of toxic DNA lesions via a free radical reactive (Fe.bleomycin) complex. However, the chemotherapeutic potential of bleomycin is limited, as it causes pulmonary fibrosis and tumor resistance at high doses. The chemical structure and modes of action of bleomycin have been extensively studied and these provide a foundation towards improving the therapeutic value of the drug. This review provides a first account of the current state of knowledge of the cellular processes that can allow the yeast Saccharomyces cerevisiae to evade the lethal effects of bleomycin. This model organism is likely to provide rapid clues in our understanding of bleomycin resistance in tumor cells.

Disruption of the Saccharomyces cerevisiae cell-wall pathway gene SLG1 causes hypersensitivity to the antitumor drug bleomycin

Bleomycin is an antitumor drug that damages DNA via a free radical-dependent mechanism, and yeast mutants defective in DNA repair are hypersensitive to the drug. To identify possible pathways that may contribute to bleomycin resistance in yeast, we characterized a panel of bleomycin-sensitive mutants that were previously isolated by insertion mutagenesis using the transposon miniTn3::Leu2::LacZ::AMP( R). One of these mutants harbored a single insertion in the SLG1 gene, which encodes a cell membrane protein that senses cell wall stress, and functions to maintain cell wall function by activating the protein kinase C signaling pathway. Deletion of the SLG1 gene in parental strains caused hypersensitivity to bleomycin, and this correlated with an accumulation of damaged DNA. A plasmid that expresses the native SLG1 gene or that increases PKC1 gene dosage restored bleomycin resistance to the slg1Delta mutant. Two-dimensional gel electrophoresis revealed that exposure to bleomycin triggered the expression of certain proteins, presumably to maintain cell wall function, in a Slg1-dependent manner. In addition, mutants lacking cell wall function were found to be hypersensitive to bleomycin. We conclude that mutants deficient in proteins that maintain cell wall function are severely compromised in their ability to limit bleomycin entry into the cell. Therefore, these mutants are burdened with increased genotoxicity upon exposure to bleomycin in the medium. Our results show that major mechanisms other than DNA repair are operating in yeast to mediate bleomycin resistance.

Bre1, an E3 ubiquitin ligase required for recruitment and substrate selection of Rad6 at a promoter

Ubiquitination of histone H2B catalyzed by Rad6 is required for methylation of histone H3 by COMPASS. We identified Bre1 as the probable E3 for Rad6's role in transcription. Bre1 contains a C3HC4 (RING) finger and is present with Rad6 in a complex. The RING finger of Bre1 is required for ubiquitination of histone H2B, methylation of lysine 4 and 79 of H3 and for telomeric silencing. Chromatin immunoprecipitation experiments indicated that both Rad6 and Bre1 are recruited to a promoter. Bre1 is essential for this recruitment of Rad6 and is dedicated to the transcriptional pathway of Rad6. These results suggest that Bre1 is the likely E3 enzyme that directs Rad6 to modify chromatin and ultimately to affect gene expression.

DNA damage-inducible and RAD52-independent repair of DNA double-strand breaks in Saccharomyces cerevisiae

Chromosomal repair was studied in stationary-phase Saccharomyces cerevisiae, including rad52/rad52 mutant strains deficient in repairing double-strand breaks (DSBs) by homologous recombination. Mutant strains suffered more chromosomal fragmentation than RAD52/RAD52 strains after treatments with cobalt-60 gamma irradiation or radiomimetic bleomycin, except after high bleomycin doses when chromosomes from rad52/rad52 strains contained fewer DSBs than chromosomes from RAD52/RAD52 strains. DNAs from both genotypes exhibited quick rejoining following gamma irradiation and sedimentation in isokinetic alkaline sucrose gradients, but only chromosomes from RAD52/RAD52 strains exhibited slower rejoining (10 min to 4 hr in growth medium). Chromosomal DSBs introduced by gamma irradiation and bleomycin were analyzed after pulsed-field gel electrophoresis. After equitoxic damage by both DNA-damaging agents, chromosomes in rad52/rad52 cells were reconstructed under nongrowth conditions [liquid holding (LH)]. Up to 100% of DSBs were eliminated and survival increased in RAD52/RAD52 and rad52/rad52 strains. After low doses, chromosomes were sometimes degraded and reconstructed during LH. Chromosomal reconstruction in rad52/rad52 strains was dose dependent after gamma irradiation, but greater after high, rather than low, bleomycin doses with or without LH. These results suggest that a threshold of DSBs is the requisite signal for DNA-damage-inducible repair, and that nonhomologous end-joining repair or another repair function is a dominant mechanism in S. cerevisiae when homologous recombination is impaired.

An allele of the yeast RPB7 gene, encoding an essential subunit of RNA polymerase II, reduces cellular resistance to the antitumor drug bleomycin

Bleomycin is an antitumor drug that kills cells by introducing lesions in DNA. Thus, normal cells exposed to bleomycin must rely on efficient DNA repair mechanisms to survive. In the yeast Saccharomyces cerevisiae, the transcriptional activator Imp2 is required to fend off the toxic effects of bleomycin. However, it remains unclear whether Imp2 controls the expression of a protein that either repairs bleomycin-induced DNA lesions, or detoxifies the drug, and or both. To gain further insight into the mechanisms by which yeast cells mount a response towards bleomycin, we began to sequentially characterize the genetic defect in a collection of bleomycin-sensitive mutants that were previously isolated by mini-Tn3 transposon mutagenesis. A rescue plasmid designed to integrate at the site of the mini-Tn3 insertion was used to identify the defective gene in one of the mutant strains, HCY53, which was not allelic to IMP2. We showed that in strain HCY53, the mini-Tn3 was inserted at the distal end of an essential gene RPB7, which encodes one of the two subunits, Rpb4-Rbp7, that forms a subcomplex with RNA polymerase II. Since rpb7 null mutants are nonviable, it would appear that the rpb7::mini-Tn3 allele produces a protein that retains partial biological function thus permitting cell viability, but which is unable to provide bleomycin resistance to strain HCY53. The defective phenotype of strain HCY53 could be corrected by a plasmid bearing the entire RPB7 gene. Two dimensional gel analysis revealed that the expression of several proteins were diminished or absent in the rpb7::mini-Tn3 mutant when challenged with bleomycin. These results are in accord with our previous report that bleomycin resistance in yeast is controlled at the transcriptional level.Key words: yeast, oxidants, bleomycin, transcription, DNA damage.