rqh1+, a fission yeast gene related to the Bloom's and Werner's syndrome genes, is required for reversible S phase arrest

Abstract

of nucleotides in the cell are known to block DNA Department of Genetics, Harvard Medical School, 200 Longwood elongation (Vassilev and Russev, 1984; Friedberg et al Hartwell, 1995;Longhese et al., 1996). Here we rqh1 mutants. rqh1 ⍣ , previously known as hus2 ⍣ , demonstrate that reversible S phase arrest also requires encodes a putative DNA helicase related to the Escherprotective functions that are distinct from cell cycle ichia coli RecQ helicase, with particular homology to checkpoint controls. the gene products of the human BLM and WRN genesWe have previously studied S phase arrest in the fission and the Saccharomyces cerevisiae SGS1 gene. BLM and yeast Schizosaccharomyces pombe, by isolating mutants WRN are mutated in patients with Bloom's syndrome that are sensitive to hydroxyurea (HU), a drug that blocks and Werner's syndrome respectively. Both syndromes DNA replication by depleting deoxynucleotides. Our are associated with genomic instability and cancer screening strategy was based on the observation that susceptibility. We show that, like BLM and SGS1, checkpoint-defective cells undergo an aberrant mitosis rqh1 ⍣ is required to prevent recombination and that ('cut') when treated with HU (Enoch and Nurse, 1990). in fission yeast suppression of inappropriate recombinUnder the same conditions normal cells cease DNA ation is essential for reversible S phase arrest.synthesis and arrest cell division, displaying an elongated Keywords: cell cycle/hus2/recombination/RecQ DNA cell morphology. By screening for mutants that 'cut' in helicase/Schizosaccharomyces pombe HU, a number of checkpoint-defective HU-sensitive (hus) mutants were identified (Enoch et al., 1992). Several of the mutants were found to be allelic to previously known

Cell cycle regulation of the endogenous wild type Bloom's syndrome DNA helicase

“…Even though the BLM gene and several mutations responsible for Bloom's syndrome have been identi®ed in the last 4 years, the physiological function of the gene product remains uncharacterized. In contrast, functional data for the BLM homologs Rqh1 in ®ssion yeast and Sgs1p in budding yeast, have become available that suggest a crucial role for these proteins during the cell cycle (Stewart et al, 1997;Murray et al, 1997;Frei and Gasser, 2000). These data have prompted us to investigate a regulation of BLM during the cell cycle.…”

“…The BLM protein has been identi®ed as a member in the DExH boxcontaining RecQ helicase subfamily (Ellis et al, 1995). Its primary sequence presents similarities with all known members in the RecQ subfamily, including Escherichia coli RecQ (Umezu et al, 1990), Saccharomyces cerevisiae Sgs1p (Ganglo et al, 1994;Watt et al, 1995), Schizosaccharomyces pombe Rqh1 (Stewart et al, 1997), Drosophila melanogaster DmBLM (Kusano et al, 1999), human RecQL (Puranam and Blackshear, 1994;Seki et al, 1994), human Wrn, the product of the Werner's syndrome gene (Yu et al, 1996) and the recently identi®ed human RecQ4 and RecQ5 proteins (Kitao et al, 1998). Chester et al (1998) developed a mouse model for the human Bloom's syndrome, and showed that mouse embryos homozygous for a targeted mutation in the murine Bloom's syndrome gene die by embryonic day 13.5, but little is known about function(s) of BLM.…”

“…In the RecQ subfamily, phylogenetic analyses have revealed that the BLM protein is most closely related to Schizosaccharomyces pombe Rqh1 and to Saccharomyces cerevisiae Sgs1p (Murray et al, 1997;Kitao et al, 1998). Rqh1, which prevents excessive recombination (Stewart et al, 1997) and takes part in a S phase mechanism allowing the bypass of unrepaired DNA damage during DNA synthesis (Murray et al, 1997), is crucial for recovery from S phase arrest in ®ssion yeast. In budding yeast, Sgs1p is directly involved in the S phase checkpoint response (Frei and Gasser, 2000).…”

Werner's Syndrome Protein Is Required for Correct Recovery after Replication Arrest and DNA Damage Induced in S-Phase of Cell Cycle

“…The effect of HU in WS cells is interesting because the sensitivity to HU is one of the phenotypical hallmarks of the yeast mutants in the orthologs of WRN, sgs1 and Rqh1. In the RecQ yeast mutants the sensitivity to replication arrest, as induced by HU, results in hyperrecombination and in the formation of the "cut" mitotic phenotype (Stewart et al, 1997;Yamagata et al, 1998). Such a phenotype, resulting from unequal distribution of the genetic material between the two daughter cells, is also associated with mutations in the Rad genes, which are responsible of the checkpoint response (Enoch and Nurse, 1990).…”

“…Furthermore, in yeast, the RecQlike proteins seem involved in suppression of hyperrecombination, S-phase checkpoint control, and correct DNA segregation (Gangloff et al, 1994, Watt et al, 1995Stewart et al, 1997;Davey et al, 1998;Yamagata et al, 1998;Frei and Gasser, 2000). Noteworthy, yeast mutants in such RecQ-like helicases are extremely sensitive to agents that cause replication arrest, such as hydroxyurea because of uncontrolled illegitimate recombinational events (Stewart et al, 1997;Yamagata et al, 1998). The observed sensitivity of WS cells to 4-nitroquinoline-1-oxide (Gebhart et al, 1988;Ogburn et al (Lebel and Leder, 1998;Poot et al, 1999;Pichierri et al, 2000).…”