Studies were conducted to directly test whether the introduction of telomerase protects cancer-prone human mammary epithelial cells from chromosomal instability and spontaneous immortalization. Using a model for Li Fraumeni Syndrome (LFS), infection of human telomerase resulted in maintenance of telomere lengths, extension of in vitro lifespan, and prevention of spontaneous immortalization.
Proceedings of the National Academy of Sciences of the United States of America
Chromosome ends in the lower eukaryotes terminate in variable numbers of tandem, simple DNA repeats. We tested predictions of a model in which these telomeric repeats provide a substrate for the addition of more repeats by a terminal transferase-like mechanism that, in concert with DNA polymerase and primase, effectively counterbalances the loss of DNA due to degradation or incomplete replication.
We have analyzed the de novo telomere synthesis catalyzed by the enzyme telomere terminal transferase (telomerase) from Tetrahymena. Oligonucleotides representing the G-rich strand of telomeric sequences from five different organisms specifically primed the addition of TTGGGG repeats in vitro, suggesting that primer recognition may involve a DNA structure unique to these oligonucleotides. The sequence at the 3' end of the oligonucleotide primer specified the first nucleotide added in the reaction.
The germ-line micronuclear genome of the ciliate Tetrahymena thermophila contains approximately 10(2) chromosome-internal blocks of tandemly repeated C4A2 sequences (mic C4A2). This repeated sequence is the telomeric sequence in the somatic macronucleus. Each of six cloned micC4A2 was found to be adjacent to a conserved 30 bp sequence, which we propose is the terminal inverted repeat of a family of DNA elements (the Tel-1 family).
Macronuclear DNA from the protozoan G. chattoni, a holotrichous ciliate, was analyzed. Most, if not all, of the macronuclear DNA is subchromosomal, ranging in size from above 100 kb down to 2.1 kb, with molecules in the lower molecular weight range being resolvable by gel electrophoresis into reproducible, specific, discrete size classes. A prominent class of linear 9.3 kb molecules consists of single free rRNA genes. Upon denaturation of total macronuclear DNA was found as single-stranded circles.
We have constructed a linear yeast plasmid by joining fragments from the termini of Tetrahymena ribosomal DNA to a yeast vector. Structural features of the terminus region of the Tetrahymena rDNA plasmid maintained in the yeast linear plasmid include a set of specifically placed single-strand interruptions within the cluster of hexanucleotide (C4A2) repeat units. An artificially constructed hairpin terminus was unable to stabilize a linear plasmid in yeast.
The DNA of telomeres--the terminal DNA-protein complexes of chromosomes--differs notably from other DNA sequences in both structure and function. Recent work has highlighted its remarkable mode of synthesis by the ribonucleoprotein reverse transcriptase, telomerase, as well as its ability to form unusual structures in vitro. Moreover, telomere synthesis by telomerase has been shown to be essential for telomere maintenance and long-term viability.
To investigate the developmentally programmed telomere addition that accompanies chromosome fragmentation during macronuclear differentiation in Tetrahymena thermophila, five representative telomeric regions from the macronucleus were cloned and characterized in detail. The sequences adjacent to the telomeric (C4A2:T2G4) repeats on these five macronuclear ends had no significant sequence homology or shared secondary structure.
We explored the ability of S. cerevisiae to utilize heterologous DNA sequences as telomeres by cloning germline (micronuclear) DNA from Tetrahymena thermophila on a linear yeast plasmid that selects for telomere function. The only Tetrahymena sequences that functioned in this assay were (C4A2)n repeats. Moreover, these repeats did not have to be derived from Tetrahymena telomeres, although we show that micronuclear telomeres (like macronuclear telomeres) of Tetrahymena terminate in (C4A2)n repeats.