Figure 3.

Fob1 is required for rDNA silencing at NTS1 but not at NTS2/18S. Silencing was assessed by monitoring the growth of 10-fold serial dilutions of cells on –URA medium. Complete medium was used as a plating control. (A) Both FOB1 and SIR2 are required for Ty1–mURA3 silencing at NTS1. Silencing was assayed using strains containing a Ty1–mURA3 insertion either outside rDNA or at NTS1 (Smith and Boeke 1997). The approximate location of this reporter (R1) within rDNA is shown in Figure 1A. (B,C) In another reporter gene system, FOB1 and SIR2 are both required for silencing at NTS1 (B, R3 reporter), but only SIR2 is required for silencing at NTS2, near the 35S coding region (C, R4 reporter). See Figure 1A for the locations of R3 and R4 reporter genes. (D) The levels of Net1–TAP and Sir2–TAP proteins do not change in the absence of Fob1 as shown by Western blotting of whole-cell extracts. Actin is shown as a loading control.