Figure 7.
UASrpg blocks the spread of SIR-dependent silent chromatin structure initiated at a silencer. (A) Diagram of the HML locus in strains in which separate segments of the locus can be simultaneously excised as circles for topological examination. (Thick arrows) FRT sites; (shaded box) the 149-bp fragment of PTEF2 containing UASrpg. (B) Topological states of separate parts of HML. DNA circles from the HML locus of each strain diagramed in A were analyzed as described in Materials and Methods. The topoisomers of the two circles in each strain are significantly different in size (∼1 vs. ∼3 kb) so that they can be fractionated without overlap in the same track of the gel. The nicked (from II) circles are indicated. The Gaussian center of each distribution of topoisomers is indicated (●).