Model for activation of HTA1-HTB1. At early times, there are at least two possible configurations for the HTA1-HTB1 promoter, because the binding of SBF and Spt10 at UAS3/UAS4 is mutually exclusive (indicated by “OR”). Both Spt10 and SBF bind before HTA1 and HTB1 transcription is activated and are therefore inactive in arrested cells. SBF is activated first, driving small peaks of expression of both genes. Soon afterwards, Spt10 is activated, driving the major peaks of expression of both genes. After 60 min, little Swi4 is bound, and Spt10 is reduced to ∼20% of its initial level. Numbered boxes, UASs; black box, NEG region; pair of ovals, Spt10 dimer; single rectangle, SBF. Inactive Spt10 and SBF are shaded dark; activated forms are shaded light. Arrows indicate activated promoters.