G. Kyrion et al., RAP1 AND TELOMERE STRUCTURE REGULATE TELOMERE POSITION EFFECTS IN SACCHAROMYCES-CEREVISIAE, Genes & development, 7(7A), 1993, pp. 1146-1159
To investigate the role of the yeast telomere-, silencing-, and UAS-bi
nding protein RAP1 in telomere position effects, we have characterized
two sets of mutant cells: (1) a set of rap1 alleles (termed the rap1t
alleles) that produce truncated RAP1 proteins missing the carboxy-ter
minal 144-165 amino acids; and (2) null mutants of the RIF1 gene, enco
ding a protein capable of interaction with the carboxyl terminus of RA
P1. The data presented here indicate that loss of the carboxyl terminu
s of RAP1 abolishes position effects at yeast telomeres and diminishes
silencing at the HML locus. Elimination of position effects in these
cells is associated with increased accessibility to the Escherichia co
li dam methylase in vivo. Thus, the carboxy-terminal domain of RAP1 is
required for telomere position effects. In contrast, rif1 deletion al
leles increase the frequency of repressed cells. Using the rap1t allel
es to generate wild-type cells differing only in telomere tract length
s, we also show that telomere position effects are highly sensitive to
changes in the size (or structure) of the telomeric tract. Longer pol
y(G1-3T) tracts can increase the frequency of transcriptional repressi
on at the telomere, suggesting that telomeric poly(G1-3T) tracts play
an active role in the formation or stability of subtelomeric transcrip
tional states.