Current Biology
Volume 17, Issue 23, 4 December 2007, Pages 2041-2046
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A Yeast Catabolic Enzyme Controls Transcriptional Memory

https://doi.org/10.1016/j.cub.2007.10.044Get rights and content
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Summary

It has been postulated that chromatin modifications can persist through mitosis and meiosis, thereby securing memory of transcriptional states 1, 2, 3, 4. Whether these chromatin marks can self-propagate in progeny independently of relevant trans-acting factors is an important question in phenomena related to epigenesis. “Adaptive cellular memory” displayed by yeast cells offers a convenient system to address this question. The yeast GAL genes are slowly activated by Gal4 when cells are first exposed to galactose, but their progeny, grown in glucose media, exhibit a fast activation mode upon re-exposure to this sugar [5]. This “galactose memory” persists for several generations and was recently proposed to involve chromatin modifications and perinuclear topology of the GAL genes cluster 5, 6. Here, we perform a heterokaryon assay demonstrating that this memory does not have a chromatin basis but is maintained by cytoplasmic factor(s) produced upon previous galactose induction. We show that Gal3, the cytoplasmic rate-limiting factor that releases the Gal4 activator, is dispensable for preserving galactose memory. Instead, the important memory determinant is a close Gal3 homolog, the highly expressed Gal1 galactokinase, the residual activity of which preserves memory in progeny cells by rapidly turning on the Gal4 activator upon cells' re-exposure to galactose.

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