Silencing at native yeast telomeres, in which the subtelomeric elements are 
intact, is different from silencing at terminal truncations. The repression of 
URA3 inserted in different subtelomeric positions at several chromosome ends was 
investigated. Many ends exhibit very little silencing close to the telomere, 
while others exhibit substantial repression in limited domains. Silencing at 
native ends is discontinuous, with maximal repression found adjacent to the ARS 
consensus sequence in the subtelomeric core X element. The level of repression 
declines precipitously towards the centromere. Mutation of the ARS sequence or 
an adjacent Abf1p-binding site significantly reduces silencing. The subtelomeric 
Y' elements are resistant to silencing along their whole length, yet silencing 
can be re-established at the proximal X element. Deletion of PPR1, the 
transactivator of URA3, and SIR3 overexpression do not increase repression or 
extend spreading of silencing to the same extent as with terminally truncated 
ends. sir1Delta causes partial derepression at X-ACS, in contrast to the lack of 
effect seen at terminal truncations. orc2-1 and orc5-1 have no effect on natural 
silencing yet cause derepression at truncated ends. X-ACS silencing requires the 
proximity of the telomere and is dependent on SIR2, SIR3, SIR4 and HDF1. The 
structures found at native yeast telomeres appear to limit the potential of 
repressive chromatin.