Epigenetics of Ciliates

Research using ciliates revealed early examples of epigenetic phenomena and continues to provide novel findings. These protozoans maintain separate germline and somatic nuclei that carry transcriptionally silent and active genomes, respectively. Examining the differences in chromatin within distinct nuclei of Tetrahymena identified histone variants and established that transcriptional regulators act by modifying histones. Formation of somatic nuclei requires both transcriptional activation of silent chromatin and large-scale DNA elimination. This somatic genome remodeling is directed by homologous RNAs, acting with an RNA interference (RNAi)-related machinery. Furthermore, the content of the parental somatic genome provides a homologous template to guide this genome restructuring. The mechanisms regulating ciliate DNA rearrangements reveal the surprising power of homologous RNAs to remodel the genome and transmit information transgenerationally. Outline 1 Ciliates: Single cells with two different genomes 2 Conjugation: Differentiation of the germline and somatic genomes 3 Macronuclei and micronuclei: A model for active versus silent chromatin 4 Homology-dependent gene silencing in ciliates 5 Genome-wide rearrangements occur during macronuclear development 6 DNA elimination is guided by homologydependent mechanisms 7 DNA elimination is guided by small RNA-directed trans-nuclear comparison of whole genomes 8 scnRNA-induced heterochromatin formation precedes DNA excision 9 DNA unscrambling is templated by maternal RNAs 10 Biological roles of programmed genome rearrangements 11 Epigenetic regulation of mating-type determination 12 Conclusion