Normal human telomeres are not late replicating

Telomeres in yeast are late replicating. Genes placed next to telomeres in yeast can be repressed (telomere positional effects), leading to the hypoth esis that telomeres may be heterochromatic and may control the expression o f subtelomeric genes. In addition, yeast telomeres are processed to have a transient long overhang at the end of S phase. The applicability of the yea st data to human biology was examined by determining the timing of telomere replication and processing in normal human diploid fibroblasts. Telomeres were purified from synchronized cells that had been labeled with 5-bromodeo xyuridine (BrdU) at hourly intervals, and the fraction of labeled telomeres was analyzed by retrieval with anti-BrdU antibodies. We determined that no rmal human telomeres replicate throughout S phase rather than being very la te replicating. Furthermore, the overall timing of replication was unaffect ed by telomere length in young versus old cells or cells whose telomeres ha d been elongated following transfection with the catalytic subunit of telom erase. Finally, the asymmetry in the length of the G-rich overhang in daugh ter telomeres produced by leading versus lagging strand synthesis was shown to be established within 1 h of telomere replication, indicating there is no significant delay between synthesis and the processing events that contr ibute to the establishment of asymmetric overhangs. Therefore, the timings of replication and processing of human telomeres are very different from th ose of yeast. (C) 1999 Academic Press.