FIG. 9.

A model for Spt10 and Spt21 in activation of HTA2-HTB2. In G₁ (top), Spt10 is incapable of interacting with the HTA2-HTB2 promoter because Spt21 is not present and, perhaps, because an unidentified site-specific DNA-binding factor is not present. During S phase, Spt21 is present (left) and interacts with Spt10, thereby helping it bind to the HTA2-HTB2 promoter. Spt21 may be necessary for Spt10 binding either by a modification of Spt10, such as by a conformational change, or by contributing directly to interactions with Spt10 and with other factors, such as a DNA-binding factor, at the HTA2-HTB2 promoter. If Spt21 is not present (right), then Spt10 is not recruited to the HTA2-HTB2 promoter.