An epigenetic activation role of Piwi and a Piwi-associated piRNA in Drosophila melanogaster.
Heterochromatin, representing the silenced state of transcription, consists largely of transposon-enriched and highly repetitive sequences. Implicated in heterochromatin formation and transcriptional silencing in Drosophila are Piwi (P-element induced wimpy testis) and repeat-associated small interfering RNAs (rasiRNAs). Despite this, the role of Piwi in rasiRNA expression and heterochromatic silencing remains unknown. Here we report the identification and characterization of 12,903 Piwi-interacting RNAs (piRNAs) in Drosophila, showing that rasiRNAs represent a subset of piRNAs. We also show that Piwi promotes euchromatic histone modifications and piRNA transcription in subtelomeric heterochromatin (also known as telomere-associated sequence, or TAS), on the right arm of chromosome 3 (3R-TAS). Piwi binds to 3R-TAS and a piRNA uniquely mapped to 3R-TAS (3R-TAS1 piRNA). In piwi mutants, 3R-TAS loses euchromatic histone modifications yet accumulates heterochromatic histone modifications and Heterochromatin Protein 1a (HP1a). Furthermore, the expression of both the 3R-TAS1 piRNA and a white reporter gene in 3R-TAS becomes suppressed. A P element inserted 128 base pairs downstream of the 3R-TAS1 piRNA coding sequence restores the euchromatic histone modifications of 3R-TAS and the expression of 3R-TAS1 piRNA in piwi mutants, as well as partly rescuing their defects in germline stem-cell maintenance. These observations suggest that Piwi promotes the euchromatic character of 3R-TAS heterochromatin and its transcriptional activity, opposite to the known roles of Piwi and the RNA-mediated interference pathway in epigenetic silencing. This activating function is probably achieved through interaction with at least 3R-TAS1 piRNA and is essential for germline stem-cell maintenance.
Yin
Hang
H
Department of Cell Biology, Duke University Medical School, Durham, North Carolina 27710, USA.
Lin
Haifan
H
D000818:Animals; D060565:Argonaute Proteins; D029721:Drosophila Proteins; D004331:Drosophila melanogaster / Q000166:cytology / Q000235:genetics* / Q000502:physiology; D044127:Epigenesis, Genetic / Q000235:genetics*; D022041:Euchromatin / Q000235:genetics / Q000378:metabolism; D005127:Eye Color / Q000235:genetics / Q000502:physiology; D005260:Female; D020868:Gene Silencing; D005854:Germ Cells / Q000166:cytology / Q000378:metabolism; D006570:Heterochromatin / Q000378:metabolism; D011506:Proteins / Q000235:genetics* / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000235:genetics* / Q000378:metabolism; D034743:RNA-Induced Silencing Complex; D013234:Stem Cells / Q000166:cytology / Q000378:metabolism; D016615:Telomere / Q000235:genetics / Q000378:metabolism
D060565:Argonaute Proteins; D029721:Drosophila Proteins; D022041:Euchromatin; D006570:Heterochromatin; D011506:Proteins; D034741:RNA, Small Interfering; D034743:RNA-Induced Silencing Complex; C116498:piwi protein, Drosophila