HO Endonuclease-induced Double-strand Break Research Articles

RAD1 and RAD10, but Not Other Excision Repair Genes, Are Required for Double-Strand Break-Induced Recombination in Saccharomyces cerevisiae

HO endonuclease-induced double-strand breaks (DSBs) in the yeast Saccharomyces cerevisiae can be repaired by the process of gap repair or, alternatively, by single-strand annealing if the site of the break is flanked by directly repeated homologous sequences. We have shown previously (J. Fishman-Lobell and J. E. Haber, Science 258:480-484, 1992) that during the repair of an HO-induced DSB, the excision repair gene RAD1 is needed to remove regions of nonhomology from the DSB ends. In this report, we present evidence that among nine genes involved in nucleotide excision repair, only RAD1 and RAD10 are required for removal of nonhomologous sequences from the DSB ends. rad1 delta and rad10 delta mutants displayed a 20-fold reduction in the ability to execute both gap repair and single-strand annealing pathways of HO-induced recombination. Mutations in RAD2, RAD3, and RAD14 reduced HO-induced recombination by about twofold. We also show that RAD7 and RAD16, which are required to remove UV photodamage from the silent HML, locus, are not required for MAT switching with HML or HMR as a donor. Our results provide a molecular basis for understanding the role of yeast nucleotide excision repair gene and their human homologs in DSB-induced recombination and repair.

Lethality induced by a single site-specific double-strand break in a dispensable yeast plasmid.

Cells of the yeast Saccharomyces cerevisiae are delayed in the G2 phase of the cell cycle following chromosomal DNA damage. This arrest is RAD9-dependent and suggests a signaling mechanism(s) between chromosomal lesions and cell cycling. We examined the global nature of growth inhibition caused by an HO endonuclease-induced double-strand break (DSB) at a 45-bp YZ sequence (from MAT YZ) in a non-yeast region of a dispensable single-copy plasmid. The presence of an unrepaired DSB results in cellular death even though the plasmid is dispensable. Loss of cell viability is partially dependent on the RAD9 gene product. Following induction of the DSB, 40% of RAD+ and 49% of rad9 delta cells [including both unbudded (G1) and budded (S plus G2) cells] did not progress further in the cell cycle. The remaining RAD+ cells progressed to form microcolonies (< 30 cells) containing aberrantly shaped inviable cells. For the rad9 delta mutant, the majority of the remaining cells produced viable colonies accounting for the greater survival of the rad9 delta strain. Based on the profound effects of a single nonchromosomal DNA lesion, this system provides a convenient means for studying the signaling effects of a DNA lesion, as well as for designing strategies for modulating cell proliferation.

Two alternative pathways of double-strand break repair that are kinetically separable and independently modulated.

HO endonuclease-induced double-strand breaks in Saccharomyces cerevisiae can undergo recombination by two distinct and competing pathways. In a plasmid containing a direct repeat, in which one repeat is interrupted by an HO endonuclease cut site, gap repair yields gene conversions while single-strand annealing produces deletions. Consistent with predictions of the single-strand annealing mechanism, deletion formation is not accompanied by the formation of a reciprocal recombination product. Deletions are delayed 60 min when the distance separating the repeats is increased by 4.4 kb. Moreover, the rate of deletion formation corresponds to the time at which complementary regions become single stranded. Gap repair processes are independent of distance but are reduced in rad52 mutants and in G1-arrested cells.

Two alternative pathways of double-strand break repair that are kinetically separable and independently modulated.

HO endonuclease-induced double-strand breaks in Saccharomyces cerevisiae can undergo recombination by two distinct and competing pathways. In a plasmid containing a direct repeat, in which one repeat is interrupted by an HO endonuclease cut site, gap repair yields gene conversions while single-strand annealing produces deletions. Consistent with predictions of the single-strand annealing mechanism, deletion formation is not accompanied by the formation of a reciprocal recombination product. Deletions are delayed 60 min when the distance separating the repeats is increased by 4.4 kb. Moreover, the rate of deletion formation corresponds to the time at which complementary regions become single stranded. Gap repair processes are independent of distance but are reduced in rad52 mutants and in G1-arrested cells.

Genetic and physical analysis of double-strand break repair and recombination in Saccharomyces cerevisiae.

We have investigated HO endonuclease-induced double-strand break (DSB) recombination and repair in a LACZ duplication plasmid in yeast. A 117-bp MATa fragment, embedded in one copy of LACZ, served as a site for initiation of a DSB when HO endonuclease was expressed. The DSB could be repaired using wild-type sequences located on a second, promoterless, copy of LACZ on the same plasmid. In contrast to normal mating-type switching, crossing-over associated with gene conversion occurred at least 50% of the time. The proportion of conversion events accompanied by exchange was greater when the two copies of LACZ were in direct orientation (80%), than when inverted (50%). In addition, the fraction of plasmids lost was significantly greater in the inverted orientation. The kinetics of appearance of intermediates and final products were also monitored. The repair of the DSB is slow, requiring at least an hour from the detection of the HO-cut fragments to completion of repair. Surprisingly, the appearance of the two reciprocal products of crossing over did not occur with the same kinetics. For example, when the two LACZ sequences were in the direct orientation, the HO-induced formation of a large circular deletion product was not accompanied by the appearance of a small circular reciprocal product. We suggest that these differences may reflect two kinetically separable processes, one involving only one cut end and the other resulting from the concerted participation of both ends of the DSB.