A chromodomain protein, Swi6, performs imprinting functions in fission yeast during mitosis and meiosis

Inheritance of stable states of gene expression is essential for cellular d ifferentiation. In fission yeast, an epigenetic imprint marking the mating- type (mat2/3) region contributes to inheritance of the silenced state, but the nature of the imprint is not known. We show that a chromodomain-contain ing Swi6 protein is a dosage-critical component involved in imprinting the mat locus. Transient overexpression of Swi6 alters the epigenetic imprint a t the mat2/3 region and heritably converts the expressed state to the silen ced state. The establishment and maintenance of the imprint are tightly cou pled to the recruitment and the persistence of Swi6 at the mat2/3 region du ring mitosis as well as meiosis. Remarkably, Swi6 remains bound to the mat2 /3 interval throughout the cell cycle and itself seems to be a component of the imprint. Our analyses suggest that the unit of inheritance at the mat2 /3 locus comprises the DNA plus the associated Swi6 protein complex.