Subsenescent telomere lengths in fibroblasts immortalized by limiting amounts of telomerase.

Human fibroblasts expressing the catalytic component of human telomerase (hTERT) have been followed for 250-400 population doublings. As expected, telomerase activity declined in long term culture of stable transfectants. Surprisingly, however, clones with average telomere lengths several kilobases shorter than those of senescent parental cells continued to proliferate. Although the longest telomeres shortened, the size of the shortest telomeres was maintained. Cells with subsenescent telomere lengths proliferated for an additional 20 doublings after inhibiting telomerase activity with a dominant-negative hTERT mutant. These results indicate that, under conditions of limiting telomerase activity, cis-acting signals may recruit telomerase to act on the shortest telomeres, argue against the hypothesis that the mortality stage 1 mechanism of cellular senescence is regulated by telomere positional effects (in which subtelomeric loci silenced by long telomeres are expressed when telomeres become short), and suggest that catalytically active telomerase is not required to provide a protein-capping role at the end of very short telomeres.

Nonsequence-specific inhibition of bacterial luminescence by phosphorothioate oligodeoxyribonucleotides.

To evaluate the effect of synthetic DNA oligomers on regulation of bacterial genes in vivo, we tested 63 oligomers of variable length and chemistry for their ability to selectively suppress light production in the bioluminescent marine organism, Vibrio fischeri. Phosphodiester, phosphorothioate, and mixed backbone oligomers were designed to be lux gene targeted or nontargeted (negative) controls. Although significant suppression of luminescence was observed, most notably with the phosphorothioate oligomers, there was no correlation between inhibitory activity and oligomer sequence. The phosphorothioate oligomer that was most potent for inhibition of luminescence in bacterial culture had no effect on the activity of purified luciferase. Mechanisms other than sequence-specific inhibition of gene expression or direct interaction with luciferase are discussed.