Biomedical subjects

L L Sandell

Publications and source records attributed to L L Sandell.

5 recordsLinked to original sources

PubMed2001

Repair of chromosome ends after telomere loss in Saccharomyces.

Removal of a telomere from yeast chromosome VII in a strain having two copies of this chromosome often results in its loss. Here we show that there are three pathways that can stabilize this broken chromosome: homologous recombination, nonhomologous end joining, and de novo telomere addition. Both in a wild-type and a recombination deficient rad52 strain, most stabilization events were due to homologous recombination, whereas nonhomologous end joining was exceptionally rare. De novo telomere addition was relatively rare, stabilizing <0.1% of broken chromosomes. Telomere addition took place at a very limited number of sites on chromosome VII, most occurring close to a 35-base pair stretch of telomere-like DNA that is normally approximately 50 kb from the left telomere of chromosome VII. In the absence of the Pif1p DNA helicase, telomere addition events were much more frequent and were not concentrated near the 35-base pair tract of telomere-like DNA. We propose that internal tracts of telomere-like sequence recruit telomerase by binding its anchor site and that Pif1p inhibits telomerase by dissociating DNA primer-telomerase RNA interactions. These data also show that telomeric DNA is essential for the stable maintenance of linear chromosomes in yeast.

Base Sequence↗

PubMed1994

Transcription of a yeast telomere alleviates telomere position effect without affecting chromosome stability.

Telomeres are required for the stable maintenance of chromosomes in the yeast Saccharomyces cerevisiae. Telomeres also repress the expression of genes in their vicinity, a phenomenon known as telomere position effect. In an attempt to construct a conditional telomere, an inducible promoter was introduced adjacent to a single telomere of a chromosome such that transcription could be induced toward the end of the chromosome. Transcription toward two other essential chromosomal elements, centromeres and origins of replication, eliminates their function. In contrast, transcription toward a telomere did not affect the stability function of the telomere as measured by the loss rate of the transcribed chromosome. Transcription proceeded through the entire length of the telomeric tract and caused a modest reduction in the average length of the transcribed telomere. Transcription of the telomere substantially reduced the frequency of cells in which an adjacent URA3 gene was subject to telomere position effect. These results indicate that telomere position effect can be alleviated without compromising chromosome stability.

Centromere↗

PubMed1993

Loss of a yeast telomere: arrest, recovery, and chromosome loss.

Yeast strains were constructed in which a single telomere could be eliminated from the end of a dispensable chromosome. In wild-type cells, elimination of a telomere caused a RAD9-mediated cell cycle arrest, indicating that telomeres help cells to distinguish intact chromosomes from damaged DNA. However, many cells recovered from the arrest without repairing the damaged chromosome, replicating and segregating it for as many as ten cell divisions prior to its eventual loss. Telomere elimination caused a dramatic increase in loss of the chromosome in all strains examined, demonstrating that yeast telomeres are also essential for maintaining chromosome stability. Thus, in spite of checkpoint and DNA damage repair systems, many chromosomes that lose a telomere are themselves destined for loss.

Cell Cycle↗

PubMed1992

Telomeric position effect in yeast.

Telomeres are the physical ends of chromosomes. In yeast, when a gene is placed near a telomere, its transcription is repressed. Genes under the influence of this telomeric position effects switch between a repressed state and a transcriptionally active state, each of which is stable for many cell generations. Telomeric position effect may provide a model system for the study of heritable gene regulation in other, more complex organisms.

Journal Article↗

PubMed1989

Extension of mismatched 3' termini of DNA is a major determinant of the infidelity of human immunodeficiency virus type 1 reverse transcriptase.

The unusually high error rate of human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT) suggests that polymerization errors by this enzyme contribute to the genetic variability of the AIDS virus. We have analyzed the mechanism for HIV-1 RT infidelity by studying two distinct steps that might lead to base substitution mutations: nucleotide misinsertions and elongation from 3'-terminal DNA mispairs. Our results indicate that the capacity of HIV-1 RT to polymerize nucleotides onto mispaired termini is a major factor in the production of mutations by this enzyme. When a noncomplementary dAMP was inserted opposite a template adenine by HIV-1 RT, the nascent 3'-terminal A.A mispair was readily extended by subsequent incorporation of the next complementary nucleotide. The frequencies of nucleotide addition onto 3'-terminal A-A, A-C, and A-G mispairs were determined by quantitating the amount of extended primers with a gel electrophoresis assay and by measuring mutagenesis after hybridization of mismatched primers opposite an amber mutation in bacteriophage phi X174 DNA. The mispair extension frequencies are approximately 50-fold higher by HIV-1 RT than by the mammalian replicative enzyme DNA polymerase alpha.

Base Composition↗