Reverse Recruitment: A New Model for Eukaryotic Gene Regulation
Regulation of gene transcription is a key feature of developmental, homeostatic, and oncogenic processes. The recruitment model for gene activation presumes that DNA is a platform on which the requisite components of the transcriptional machinery are assembled. In contrast to this idea, the reverse recruitment model stipulates that the nuclear periphery is an enriched zone of pre-assembled transcriptional regulators and that target genes become active by moving to contact these peripherally anchored transcription factories. Surprisingly, many nucleoporins activate transcription themselves when fused to a heterologous DNA-binding domain. Nucleoporin activation may be a widespread eukaryotic phenomenon, because it was first detected as a consequence of oncogenic rearrangements in Acute Myeloid Leukemia (AML) and related syndromes in humans. Transcriptional activation of the Rap1/Gcr1/Gcr2 target genes occurs in association with the Nup84 subcomplex. The Rap1 coactivators Gcr1 and Gcr2 form an important bridge between the yeast Nuclear Pore Complex (NPC) and the transcriptional machinery. The nuclear periphery is also the site of key events in the regulation of glucose-repressed genes, which together compose one-sixth of the Saccharomyces cerevisiae genome. Transcriptional activation by a subset of nucleoporins is glucose repressed and requires the Snf1/Gal83/Snf4 kinase complex for complete derepression in the absence of glucose. In the absence of glucose the positively acting Snf1 kinase complex is activated and all three of its subunits (α, β, and γ) localize to the nuclear periphery. The glucose repressed gene SUC2 , which is the best characterized target of the Snf1 kinase complex, associates tightly with the nuclear periphery when transcriptionally active but is highly mobile in the nucleoplasm when repressed. The repressor Mig1 is associated with components of the NPC and this association with the nuclear periphery is important for Mig1 to act as a repressor. Strikingly, the regulation of SUC2 is impaired when components of the NPC and the perinuclear transcription factor Gcr1 are removed. Another example of gene regulation at the nuclear periphery that I discuss is the GAL system. The activator of GAL genes, Ga14, is physically and functionally associated with the nuclear periphery. These findings allowed me to propose that the "reverse recruitment" model is likely a central feature of eukaryotic gene regulation.