FIG. 3.

Two potential mechanisms to define a zone of silencing. The left model predicts that PTB-PTB interactions between binding sites flanking an exon sequesters an exon, thus precluding the definition of this exon. The model on the right suggests that PTB can oligomerize across an exon, resulting in the coating of the exon, which will also antagonize its definition. The model on the right could also explain the silencing of exons that have a PTB site only on one flank; PTB could multimerize, covering a region of the RNA determined by interactions with other factors.