Characterization of DNA sequences associated with residual nuclei of Saccharomyces cerevisiae

Localization of a DNA replication origin and termination zone on chromosome III of Saccharomyces cerevisiae

Two-dimensional gel electrophoretic replicon mapping techniques were used to identify all functional DNA replication origins and termini in a 26.5-kbp stretch in the left arm of yeast chromosome III. Only one origin was detected; it coincided with an ARS element (ARS306), as have all previously mapped yeast origins. A replication termination region was identified in a 4.3-kbp stretch at the telomere-proximal end of the investigated region, between the origin identified in this paper and the neighboring, previously mapped, ARS305-associated origin (previously called the A6C origin). Termination does not occur at a specific site; instead, it appears to be the consequence of replication forks converging in a stretch of DNA of at least 4.3 kbp.

Localization of a DNA replication origin and termination zone on chromosome III of Saccharomyces cerevisiae

Two-dimensional gel electrophoretic replicon mapping techniques were used to identify all functional DNA replication origins and termini in a 26.5-kbp stretch in the left arm of yeast chromosome III. Only one origin was detected; it coincided with an ARS element (ARS306), as have all previously mapped yeast origins. A replication termination region was identified in a 4.3-kbp stretch at the telomere-proximal end of the investigated region, between the origin identified in this paper and the neighboring, previously mapped, ARS305-associated origin (previously called the A6C origin). Termination does not occur at a specific site; instead, it appears to be the consequence of replication forks converging in a stretch of DNA of at least 4.3 kbp.

Evidence suggesting that the ARS elements associated with silencers of the yeast mating-type locus HML do not function as chromosomal DNA replication origins

The silent mating-type loci of Saccharomyces cerevisiae, HML and HMR, are flanked by transcriptional silencers that have ARS activity (i.e., they function as replication origins when in plasmids). To test whether these ARS elements are chromosomal origins, we mapped origins near HML (close to the left telomere of chromosome III). Our results indicate that the HML-associated ARS elements either do not function as chromosomal replication origins or do so at a frequency below our detection level, suggesting that replication from a silencer-associated origin in each S phase is not essential for the maintenance of transcriptional repression at HML. Our results also imply that the ability of a DNA fragment to function as an ARS element in a plasmid does not ensure its ability to function as an efficient chromosomal replication origin. Telomere proximity is not responsible for inactivating these ARS elements, because they are not detectably functional as chromosomal origins even in genetically modified strains in which they are far from the telomere.

Evidence suggesting that the ARS elements associated with silencers of the yeast mating-type locus HML do not function as chromosomal DNA replication origins

The silent mating-type loci of Saccharomyces cerevisiae, HML and HMR, are flanked by transcriptional silencers that have ARS activity (i.e., they function as replication origins when in plasmids). To test whether these ARS elements are chromosomal origins, we mapped origins near HML (close to the left telomere of chromosome III). Our results indicate that the HML-associated ARS elements either do not function as chromosomal replication origins or do so at a frequency below our detection level, suggesting that replication from a silencer-associated origin in each S phase is not essential for the maintenance of transcriptional repression at HML. Our results also imply that the ability of a DNA fragment to function as an ARS element in a plasmid does not ensure its ability to function as an efficient chromosomal replication origin. Telomere proximity is not responsible for inactivating these ARS elements, because they are not detectably functional as chromosomal origins even in genetically modified strains in which they are far from the telomere.

Mutation in a gene for type I procollagen (COL1A2) in a woman with postmenopausal osteoporosis: evidence for phenotypic and genotypic overlap with mild osteogenesis imperfecta

Mutations in the two genes for type I collagen (COL1A1 or COL1A2) cause osteogenesis imperfecta (OI), a heritable disease characterized by moderate to extreme brittleness of bone early in life. Here we show that a 52-year-old postmenopausal woman with severe osteopenia and a compression fracture of a thoracic vertebra had a mutation in the gene for the alpha 2(I) chain of type I collagen (COL1A2) similar to mutations that cause OI. cDNA was prepared from the woman's skin fibroblast RNA and assayed for the presence of a mutation by treating DNA heteroduplexes with carbodiimide. The results indicated a sequence variation in the region encoding amino acid residues 660-667 of the alpha 2(I) chain. Further analysis demonstrated a single-base mutation that caused a serine-for-glycine substitution at position 661 of the alpha 2(I) triple-helical domain. The substitution produced posttranslational overmodification of the collagen triple helix, as is seen with most glycine substitutions that cause OI. The patient had a history of five previous fractures, slightly blue sclerae, and slight hearing loss. Therefore, the results suggest that there may be phenotypic and genotypic overlap between mild osteogenesis imperfecta and postmenopausal osteoporosis, and that a subset of women with postmenopausal osteoporosis may have mutations in the genes for type I procollagen.

Transcript levels of the Saccharomyces cerevisiae DNA repair gene RAD18 increase in UV irradiated cells and during meiosis but not during the mitotic cell cycle

We have examined the transcript levels of the Saccharomyces cerevisiae DNA repair gene RAD18 in UV irradiated cells, in the mitotic cell cycle, and during meiosis. Levels of RAD18 mRNA increased upon UV irradiation, but remained constant during the mitotic cell cycle. During meiosis, RAD18 mRNA levels rose about 4 fold at a stage coincident with the period when high levels of genetic recombination occur. RAD18 resembles the other DNA repair genes, RAD2, RAD6, RAD7, RAD23, and RAD54, all of which also exhibit increased transcription in response to DNA damage and during meiosis. Comparisons of sequences in 5' flanking regions of RAD genes suggest that different upstream sequences are involved in regulating the expression of DNA repair genes belonging to different epistasis groups.

Transcript levels of the Saccharomyes cerevisiae DNA repair gene RAD23 increase in response to UV light and in meiosis but remain constant in the mitotic cell cycle

The RAD23 gene of Saccharomyces cerevisiae is required for excision-repair of UV damaged DNA. In this paper, we determine the location of the RAD23 gene in a cloned DNA fragment, identify the 1.6 kb RAD23 transcript, and examine RAD23 transcript levels in UV damaged cells, during the mitotic cell cycle, and in meiosis. The RAD23 mRNA levels are elevated 5-fold between 30 to 60 min after 37 J/m2 of UV light. RAD23 mRNA levels rise over 6-fold during meiosis at a stage coincident with high levels of genetic recombination. This response is specific to sporulation competent MATa/MAT alpha diploid cells, and is not observed in asporogenous MATa/MATa diploids. RAD23 mRNA levels, however, remain constant during the mitotic cell cycle.

The Saccharomyces cerevisiae DNA repair gene RAD2 is regulated in meiosis but not during the mitotic cell cycle

The expression of the RAD2 gene of Saccharomyces cerevisiae is elevated upon DNA damage. Here, we show that RAD2 transcript levels also rise approximately eightfold during meiosis but remain constant during the mitotic cell cycle. The period of maximal RAD2 mRNA accumulation during meiosis is consistent with a possible role of RAD2 in a late stage of recombination, in mismatch repair of heteroduplexes, or both.

The Saccharomyces cerevisiae DNA repair gene RAD2 is regulated in meiosis but not during the mitotic cell cycle

The expression of the RAD2 gene of Saccharomyces cerevisiae is elevated upon DNA damage. Here, we show that RAD2 transcript levels also rise approximately eightfold during meiosis but remain constant during the mitotic cell cycle. The period of maximal RAD2 mRNA accumulation during meiosis is consistent with a possible role of RAD2 in a late stage of recombination, in mismatch repair of heteroduplexes, or both.

Expression of the Saccharomyces cerevisiae DNA repair gene RAD6 that encodes a ubiquitin conjugating enzyme, increases in response to DNA damage and in meiosis but remains constant during the mitotic cell cycle

The RAD6 gene of Saccharomyces cerevisiae encodes a ubiquitin-conjugating (E2) enzyme and is required for the repair of damaged DNA, mutagenesis, and sporulation. Here, we report our studies on the regulation of RAD6 gene expression after UV damage, during the mitotic cell cycle, in meiosis, and following heat shock and starvation. RAD6 mRNA levels became elevated in cells exposed to UV light, and at all UV doses the increase in mRNA levels was rapid and occurred within 30 min after exposure to UV. RAD6 mRNA levels also increased in sporulating MATa/MAT alpha cells, and the period of maximal accumulation of RAD6 mRNA during meiosis is coincident with the time during which recombination occurs. However, RAD6 mRNA levels showed no periodic fluctuation in the mitotic cell cycle, were not elevated upon heat shock, and fell in cells in the stationary phase of growth. These observations suggest that RAD6 activity is required throughout the cell cycle rather than being restricted to a specific stage, and that during meiosis, high levels of RAD6 activity may be needed at a stage coincident with genetic recombination. The observation that RAD6 transcription is not induced by heat and starvation, treatments that activate stress responses, suggests that the primary role of RAD6 is in the repair of damaged DNA rather than in adapting cells to stress situations.

Chromosomal ARS and CEN elements bind specifically to the yeast nuclear scaffold

We describe here for the first time the isolation of a yeast nuclear scaffold that maintains specific interactions with yeast genomic DNA sequences. The scaffold-DNA interaction is reversible and saturable, and some binding sites are conserved between yeast and Drosophila KC cells. Second, we find that the specific sequences bound to the yeast nuclear scaffold are the putative origins of replication (ARS elements) and a chromosomal centromere, CENIII. The scaffold association has been closely mapped at the ARS1 locus, and appears to include the 11 bp ARS consensus, but not the ABF-1 binding site. Competition studies show that ARS1 does not compete for CENIII binding, allowing us to distinguish two classes of scaffold attachment sites by functional and structural criteria.

Close association of a DNA replication origin and an ARS element on chromosome III of the yeast, Saccharomyces cerevisiae

Two dimensional gel electrophoretic techniques were used to locate all functional DNA replication origins in a 22.5 kb stretch of yeast chromosome III. Only one origin was detected, and that origin is located within several hundred bp of an ARS element.

Plasmid associations with residual nuclear structures in Saccharomyces cerevisiae

Acentric yeast plasmids are mitotically unstable, apparently because they cannot freely diffuse after replicating and therefore are not included in the daughter nucleus. This behavior could result if plasmids remain attached to structural elements of the nucleus after replicating. Since DNA replication is believed to take place on the nuclear matrix, we tested whether there was a correlation between the mitotic stability of a given plasmid and the extent to which it was found associated with residual nuclear structures. Residual nuclei were prepared from yeast nuclei by extraction with either high salt, 2 M NaCl, or low salt, 10 mM lithium diiodosalicylate (LIS). Hybridization analysis was used to estimate the fraction of plasmid molecules remaining after nuclei were extracted. We examined the extent of matrix association of three ARS1 plasmids, Trp1-RI circle (1.45 kb), YRp7 (5.7 kb) and p lambda BAT (45.1 kb) with mitotic loss rates ranging from 3-25%. In addition we examined the matrix binding of the endogenous 2 micron plasmid and the 2 micron-derived YEp13 which is relatively stable in the presence of 2 micron and less stable in cir degree strains. Among the ARS1 plasmids we observed a negative correlation between stability and matrix association, consistent with models in which binding to the nuclear matrix prevents passive segregation of ARS1 plasmid molecules. No such correlation was observed among the 2 micron plasmids. Among all plasmids examined there is a positive correlation between size and matrix association.

The in vivo replication origin of the yeast 2 microns plasmid

We have used two-dimensional neutral/alkaline agarose gel electrophoresis to separate the nascent strands of replicating yeast 2 micron plasmid DNA molecules according to extent of replication, away from nonreplicating molecules and parental strands. Analysis of the lengths of nascent strands by sequential hybridization with short probes shows that replication proceeds bidirectionally from a single origin at map position 3700 +/- 100, coincident with the genetically mapped ARS element. The two recombinational isomers of 2 microns plasmid (forms A and B) replicate with equal efficiency. These results suggest that ARS elements may prove to be replication origins for chromosomal DNA.