| Yeast cells “remember”previous expression of certain genes, priming them for faster reactivation for generations after they have been repressed. GAL genes that have been transcribed and then repressed for up to 7 generations are activated much faster than if they have not previously been expressed. This epigenetic transcriptional memory results in changes in chromatin structure and gene positioning at the nuclear periphery. Two independent mechanisms promote GAL gene memory over different time scales: short-term memory (1 h after repression) and epigenetic memory (12 h after repression). Epigenetic memory results in localization of GAL genes at the nuclear periphery, requires the Gal1 protein, the histone variant H2A.Z and the glucose co-repressor Tup1, and leads to the association of poised RNA polymerase II with recently repressed GAL gene promoters. In cells in which the GAL genes are repressed in glucose, but not in raffinose, Gal1 protein is necessary and sufficient to confer GAL gene localization at the nuclear periphery. These results suggest that yeast cells use multiple independent inputs to establish multidimensional transcriptional memory, incorporating information about previous transcription, duration of repression and type of repression. |
