Biology Department Seminar

Heterochromatin is composed of transposable elements (TE) and related repeats which silence genes located nearby, and play a major role in epigenetic regulation of the genome. Far from being inert, heterochromatin is transcribed and small interfering RNA corresponding to heterochromatic sequences can be detected in plants, animals and fission yeast. We have used tiling microarrays to examine these transcripts and their regulation in plants and fission yeast. In plants, expression of small interfering RNA (siRNA) corresponding to different classes of TE depends on DNA methyltransferase MET1, the SWI/SNF ATPase, DDM1, or both, but not on the histone deacetylase SIL1. All three genes are required for silencing transposons in the absence of RNAi. In fission yeast and in Arabidopsis, centromeric repeats are continually transcribed on one strand, but the transcripts are rapidly turned over by RNA interference, through the combined action of DNA dependent RNA polymerase, Argonaute and RNA dependent RNA polymerase, each of which is associated with heterochromatin. Histone H3 lysine-9 dimethylation (H3K9me2) depends on RNAi, mediated by the Rik1-Clr4 complex. Rik1 has similarity to both DNA and RNA binding proteins, and may play a role in RNA processing. In pombe, spreading of histone methylation into reporter genes that are silenced by position effect variegation (PEV) depends on co-transcription and slicing by Argonaute, leading to a model for PEV.