Functional telomere formation in yeast using synthetic C4A2 sequences

Yeast linear plasmids with T2AG3 telomeres: TEL+CEN antagonism and genetic and molecular stability

A linear plasmid containing ARS1, CEN4, and 48 bp of vertebrate (T2AG3) telomeric sequences at each end was used to transform Saccharomyces cerevisiae. Only circular plasmids that had lost the centromere and had retained the T2AG3 sequences were obtained, indicating that the vertebrate T2AG3 sequences and the yeast CEN4 could not be simultaneously present in this vector. This hypothesis was verified by removing the CEN4 sequence from the construct. In fact, the resulting transformants contained two classes of efficiently replicating linear plasmids: one of the expected size and one about twice as large. During subsequent growth, plasmids of the former, but not latter, class were subjected to concatemer formation. This can best be explained by recombination events involving the T2AG3 sequences at the ends of the molecule, since very similar centric and acentric linear plasmids bearing Tetrahymena telomeric ends replicated faithfully.

Replication of circular and linear SV40-based plasmids in monkey cells

Three plasmids were derived from a common SV40-based parent. A circular plasmid (pYACneoC) contained the SV40 ori and two sets of 50 bp of human telomeric sequences. By differential enzyme digestion, two linear plasmids were generated from the circular form, one (pYACneoL) terminating with, and the other (pYACneoN) free of telomeric sequences. The replicative features of the circular and of both linear plasmids were assayed by transfecting COS-7 cells. At various times after transfection, the low-molecular-weight DNA was extracted, and the fraction of molecules that had replicated was determined by Dpnl digestion. We demonstrate that about half of the circular plasmid molecules replicate, but only during a short time interval immediately following transfection. No replication was observed in the case of the two linear plasmids. However, the function of the SV40 origin is potentially present in the molecules, since circular forms that do replicate can be recovered from both linear plasmids. The extent of replication of circularized pYACneoL is similar to that of pYACneoC, whereas a lower fraction of circularized pYACneoN molecules replicate. These results are discussed in terms of the possible influence of the DNA structure on the viral ori, and of the influence of the host cell functions on viral replication.

On the level of plasmid-bearing cells in transformed cultures of Saccharomyces cerevisiae

LC1, a YIP5-derived plasmid containing a human DNA fragment with ARS activity in yeast, has been used to study the replication of ARS plasmids in Saccharomyces cerevisiae. ARS plasmids carried in yeast hosts are normally mitotically unstable. In transformed cultures the fraction of cells that contain plasmid, measured by plating on selective media, is lower than would be expected from measured rates of plasmid loss. In the case of S. cerevisiae carrying either the plasmid LC1 or YRP17, the assay yields values of the order of 10-20% or 30-50% respectively. We have found that by doing a double nutritional upshift that involves conditioned medium and casamino acids, a population of cells can be defined that carry plasmid but are unable to grow on media that select for the plasmid marker. Thus the total fraction of cells that can be shown to contain plasmid increases to greater than 70%. To distinguish between the inability of plasmid to replicate in these cells and lack of expression of the selectable gene, cultures grown from single cells were analysed for the presence of plasmid DNA. In a substantial fraction of the population, plasmid DNA could be detected only by polymerase chain reaction and not by standard blotting and hybridization. These results suggest that plasmid is unable to replicate in these cells. Growth kinetics experiments with transformed cultures are consistent with the notion that only a small fraction of the cells contains plasmid capable of replication upon dilution into selective medium. Possible explanations for the phenomena observed are discussed.

A human DNA telomeric fragment contains yeast ARS and mitotic stabilizing sequences

A minilibrary of human DNA fragments was prepared in the vector YIP5 from a DNA preparation enriched for telomeric sequences. Screening of the library produced one clone that hybridized to the TTAGGG sequence. The cloned DNA fragment was shown to be telomeric by a number of criteria. In situ hybridization to metaphase human chromosomes showed that the fragment hybridized to the tips of all human chromosomes. The fragment contained at least two yeast autonomously replicating sequences (ARS) and stabilizing sequences, since it transformed Saccharomyces cerevisiae with high efficiency, giving rise to clones which were mitotically stable under non-selective growth.