Imbalance in dosage of the genes for the heterochromatin components Sir3p and histone H4 results in changes in the length and sequence organization of yeast telomeres

Telomeric heterochromatin plays an essential role in telomere function, including the regulation of telomere length. We observe that in Saccharomyces cerevisiae an imbalance in the dosage of genes for two protein components of heterochromatin (namely Sir3p and histone H4) causes modifications in telomere length and telomere sequence organization. The effects of Sir3p/H4 imbalance were analyzed in yeast strains in which the wild-type SIR3 gene (normally a single-copy gene) was either absent or present in 20-30 copies, and both histone H4 genes (HHF1 and HHF2) were present or HHF1 was deleted, thus covering a wide range of viable gene-dosage combinations. Modifications of telomeres and of subtelomeric regions were identified by analyzing both the overall telomere population and by focusing on two single telomeric regions: the left telomere of chromosome III (LIII) and the right telomere of chromosome XI (RXI). The modifications induced by alteration of the Sir3p/H4 ratio consist of a reduction in the length and an increase in the instability of the terminal block of (C(1-3)A)n repeats and in susceptibility to insertion of Y' elements into this repeat element. Restoration of the wild-type gene ratio (by removal of the extra copies of SIR3 or by complementation with the missing second copy of HHF) restored the original telomere organization, both with respect to the length of the (C(1-3)A)n repeat stretch and the absence of Y' elements. This behavior shows that the stability of the wild-type sequence organization requires maintenance of the normal structure of telomeric heterochromatin.