The role of PcG and TrxG proteins in executing epigenetic memory
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Cells with identical genomes can have very different identities and functions. This variability is in part controlled by epigenetic mechanisms that regulate differential gene expression, stabilize genetic programs throughout the lifetime of a cell, and pass these programs on to daughter cells. Here, I postulate three basic properties that any candidate regulator needs to fulfill in order to be considered epigenetic. These criteria are: a predictive value in genome function; a stable retention throughout the cell cycle; and a reliable reproduction into the next generation. This review examines whether histone modifications, which are often considered epigenetic marks, satisfy all three criteria. Additionally, an alternative candidate for epigenetic memory is proposed, in the form of chromatin binding PcG and TrxG proteins. I conclude that although histone modifications are good candidates for epigenetic memory, an independent, non-mutually exclusive epigenetic function might be fulfilled directly by PcG and TrxG proteins.