title: The snail repressor inhibits release, not elongation, of paused Pol II in the Drosophila embryo.
abstract: The development of the precellular Drosophila embryo is characterized by exceptionally rapid transitions in gene activity, with broadly distributed maternal regulatory gradients giving way to precise on/off patterns of gene expression within a one-hour window, between two and three hours after fertilization [1]. Transcriptional repression plays a pivotal role in this process, delineating sharp expression patterns (e.g., pair-rule stripes) within broad domains of gene activation. As many as 20 different sequence-specific repressors have been implicated in this process, yet the mechanisms by which they silence gene expression have remained elusive [2]. Here we report the development of a method for the quantitative visualization of transcriptional repression. We focus on the Snail repressor, which establishes the boundary between the presumptive mesoderm and neurogenic ectoderm [3]. We find that elongating Pol II complexes complete transcription after the onset of Snail repression. As a result, moderately sized genes (e.g., the 22 kb sog locus) are fully silenced only after tens of minutes of repression. We propose that this "repression lag" imposes a severe constraint on the regulatory dynamics of embryonic patterning and further suggest that posttranscriptional regulators, like microRNAs, are required to inhibit unwanted transcripts produced during protracted periods of gene silencing.
authors.affiliation: Biophysics Graduate Group, Center for Integrative Genomics, Division of Genetics, Genomics and Development, University of California, Berkeley, Berkeley, CA 94720, USA. bothma@berkeley.edu
authors.forename: Jacques P
authors.initials: JP
authors.lastname: Bothma
authors.forename: Joe
authors.initials: J
authors.lastname: Magliocco
authors.forename: Michael
authors.initials: M
authors.lastname: Levine
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000235:genetics; D004257:DNA Polymerase II / Q000378:metabolism* / Q000502:physiology; D004330:Drosophila / Q000196:embryology / Q000235:genetics* / Q000378:metabolism; D047108:Embryonic Development / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D012097:Repressor Proteins / Q000502:physiology*; D000071250:Snail Family Transcription Factors; D013997:Time Factors; D014157:Transcription Factors / Q000502:physiology*; D014158:Transcription, Genetic / Q000502:physiology
chemical_list: D012097:Repressor Proteins; D000071250:Snail Family Transcription Factors; D014157:Transcription Factors; C000604451:sna protein, Drosophila; D004257:DNA Polymerase II