Bromodeoxyuridine 5'-monophosphate incorporation into yeast nuclear and mitochondrial deoxyribonucleic acid

New vectors for simplified construction of BrdU-Incorporating strains of Saccharomyces cerevisiae

The thymidine analogue BrdU is a powerful tool for analysing nucleotide incorporation in studies of DNA replication or repair. S. cerevisiae lacks the thymidine salvage pathway that enables efficient cellular uptake and incorporation of thymidine analogues into DNA. Recent in vivo reconstitution of this pathway in yeast by high-level expression of Herpes simplex virus thymidine kinase (HSV-TK) or combined expression of HSV-TK and human equilibrative nucleoside transporter (hENT1) has enabled analysis of BrdU incorporation in yeast. While the BrdU incorporation systems are highly valuable, the construction and use of strains utilizing these systems can be complicated by specific requirements of the available systems. We have created a set of vectors that simplify the construction and use of BrdU-Incorporating (BrdU-Inc) strains of budding yeast. Each vector in the set contains HSV-TK and hENT1 under the control of promoters that express constitutively, and one of four different selectable markers (HIS3, TRP1, LEU2 or URA3) for genomic integration. With these BrdU-Inc vectors, one-step integration of a single copy produces yeast that efficiently incorporate BrdU upon its addition to the medium. These vectors ease strain construction and maintenance, thereby facilitating routine use of BrdU for analysis in yeast.

Efficient labeling of fission yeast Schizosaccharomyces pombe with thymidine and BUdR

In this paper we report the construction of a Schizosaccharomyces pombe strain that facilitates analysis of replicating DNA. The strain co-expresses the Herpes simplex virus thymidine kinase gene (hsv-tk) and a human equilibrative nucleoside transporter (hENT1). The double integrant efficiently incorporates 3H-thymidine into nuclear DNA as monitored by scintillation counting. These strains also incorporate the thymidine analog Bromodeoxy uridine (BUdR) into newly replicated DNA, which can be detected by immunofluorescence and flow cytometry. This strain provides a valuable tool for direct study of DNA replication in S.pombe.

Saccharomyces cerevisiae cdc2 mutants fail to replicate approximately one-third of their nuclear genome

Chromosomal DNA replication was examined in temperature-sensitive mutants of Saccharomyces cerevisiae defective in a gene required for the completion of S phase at the nonpermissive temperature, 37 degrees C. Based on incorporation of radioactive precursors and density transfer experiments, strains carrying three different alleles of cdc2 failed to replicate approximately one-third of their nuclear genome at 37 degrees C. Whole-cell autoradiography experiments demonstrated that 93 to 96% of the cells synthesized DNA at 37 degrees C. Therefore, all cells failed to replicate part of their genome. DNA isolated from terminally arrested cells was of normal size as measured on neutral and alkaline sucrose gradients, suggesting that partially replicated DNA molecules do not accumulate and that DNA strands are ligated properly in cdc2 mutants. In addition, electron microscopic examination of the equivalent of more than one genome's DNA from arrested cells failed to reveal any partially replicated molecules. The sequences which failed to replicate at 37 degrees C were not highly specific; eight different cloned sequences replicated to the same extent as total DNA. The 2-microns plasmid DNA and rDNA replicated significantly less well than total DNA, but approximately one-half of these sequences replicated at 37 degrees C. These observations suggest that cdc2 mutants are defective in an aspect of initiation of DNA replication common to all chromosomes such that a random fraction of the chromosomes fail to initiate replication at 37 degrees C, but that once initiated, replication proceeds normally.

Saccharomyces cerevisiae cdc2 mutants fail to replicate approximately one-third of their nuclear genome

Chromosomal DNA replication was examined in temperature-sensitive mutants of Saccharomyces cerevisiae defective in a gene required for the completion of S phase at the nonpermissive temperature, 37 degrees C. Based on incorporation of radioactive precursors and density transfer experiments, strains carrying three different alleles of cdc2 failed to replicate approximately one-third of their nuclear genome at 37 degrees C. Whole-cell autoradiography experiments demonstrated that 93 to 96% of the cells synthesized DNA at 37 degrees C. Therefore, all cells failed to replicate part of their genome. DNA isolated from terminally arrested cells was of normal size as measured on neutral and alkaline sucrose gradients, suggesting that partially replicated DNA molecules do not accumulate and that DNA strands are ligated properly in cdc2 mutants. In addition, electron microscopic examination of the equivalent of more than one genome's DNA from arrested cells failed to reveal any partially replicated molecules. The sequences which failed to replicate at 37 degrees C were not highly specific; eight different cloned sequences replicated to the same extent as total DNA. The 2-microns plasmid DNA and rDNA replicated significantly less well than total DNA, but approximately one-half of these sequences replicated at 37 degrees C. These observations suggest that cdc2 mutants are defective in an aspect of initiation of DNA replication common to all chromosomes such that a random fraction of the chromosomes fail to initiate replication at 37 degrees C, but that once initiated, replication proceeds normally.

Replication of bromodeoxyuridylate-substituted mitochondrial DNA in yeast

The DNA of several strains of Saccharomyces cerevisiae was labeled by growing the culture in medium supplemented with thymidylate and bromodeoxyuridylate. It was thus possible to follow the course of mitochondrial DNA replication in density shift experiments by determining the buoyant density distribution of unreplicated and replicated DNAs in analytical CsCl gradients. DNA replication was followed for three generations after transfer of cultures from light medium to heavy medium and heavy medium to light medium. Under both conditions, the density shifts observed for mitochondrial DNA were those expected for semiconservative, nondispersive replication. This was further confirmed by analysis of the buoyant density of alkali-denatured hybrid mitochondrial DNA. With this method, no significant recombination between replicated and unreplicated DNA was detected after three generations of growth.