G1/S transcription factors assemble in discrete clusters that increase in number as cells grow

Abstract

The spatio-temporal organization of transcription factor (TF)-promoter interactions is critical for the coordination of transcriptional programs. In budding yeast, the main G1/S transcription factors, SBF and MBF, are limiting with respect to target promoters in small G1 phase cells and accumulate as cells grow, raising the question of how SBF/MBF are dynamically distributed across the G1/S regulon. Super-resolution Photo-Activatable Localization Microscopy (PALM) mapping of the static positions of SBF/MBF subunits revealed that 85% were organized into discrete clusters containing ~8 copies regardless of cell size, while the number of clusters increased with growth. Stochastic simulations with a mathematical model based on colocalization of promoters in clusters recapitulated observed cluster behavior. A prediction of the model that SBF/MBF should exhibit both fast and slow dynamics was confirmed in PALM experiments on live cells. This spatio-temporal organization of the TFs that activate the G1/S regulon may help coordinate commitment to division.

Swi4-dependent SWI4 transcription couples cell size to cell cycle commitment

“…In this case growth is slowed more than the accumulation of Swi4 such that cells reach the Swi4 threshold at a smaller size. This SBF promoter titration hypothesis is reminiscent of a previous titration model in which G1/S promoters are titrated by Cln3 (37).…”