Fig. 5. Model of one possible TEL repression complex. (A) The domain structure of TEL. (B) A possible complex of TEL with chromatin. The TEL–SAM polymer structure from this work is shown as a space-filling model in blue. The C-termini are colored red and orient away from the axis of the polymer. This is the only known structure in the model shown. Spheres representing the co-repressor and DNA binding domains of TEL are placed on the outside of the polymer with the same helical pitch as the TEL–SAM polymer. The sphere volumes were determined assuming a partial specific volume of 0.7 ml/g. A coil representing chromatin is wrapped around the polymer, interacting with the DNA binding domains. The thickness of the coil is the same as the width of a nucleosome core particle (54 Å) (Luger et al., 1997). We note that this is a highly speculative model and serves only to illustrate one way that polymerization could lead to spreading of transcriptional repression. Many other models are possible.