Chromatin boundary elements serve as cis-acting regulatory DNA signals required 
to protect genes from the effects of the neighboring heterochromatin. In the 
yeast genome, boundary elements act by establishing barriers for heterochromatin 
spreading and are sufficient to protect a reporter gene from transcriptional 
silencing when inserted between the silencer and the reporter gene. Here we 
dissected functional topography of silencers and boundary elements within 
circular minichromosomes in Saccharomyces cerevisiae. We found that both HML-E 
and HML-I silencers can efficiently repress the URA3 reporter on a multi-copy 
yeast minichromosome and we further showed that two distinct heterochromatin 
boundary elements STAR and TEF2-UASrpg are able to limit the heterochromatin 
spreading in circular minichromosomes. In surprising contrast to what had been 
observed in the yeast genome, we found that in minichromosomes the 
heterochromatin boundary elements inhibit silencing of the reporter gene even 
when just one boundary element is positioned at the distal end of the URA3 
reporter or upstream of the silencer elements. Thus the STAR and TEF2-UASrpg 
boundary elements inhibit chromatin silencing through an antisilencing activity 
independently of their position or orientation in S. cerevisiae minichromosomes 
rather than by creating a position-specific barrier as seen in the genome. We 
propose that the circular DNA topology facilitates interactions between the 
boundary and silencing elements in the minichromosomes.