Spn1 Regulates the Recruitment of Spt6 and the Swi/Snf Complex during Transcriptional Activation by RNA Polymerase II

Department of Biochemistry and Molecular Biology, Colorado State University, Fort Collins, CO 80523, USA.
Molecular and Cellular Biology (Impact Factor: 4.78). 03/2008; 28(4):1393-403. DOI: 10.1128/MCB.01733-07
Source: PubMed


We investigated the timing of the recruitment of Spn1 and its partner, Spt6, to the CYC1 gene. Like TATA binding protein and RNA polymerase II (RNAPII), Spn1 is constitutively recruited to the CYC1 promoter, although levels of transcription from this gene, which is regulated postrecruitment of RNAPII, are low. In contrast,
Spt6 appears only after growth in conditions in which the gene is highly transcribed. Spn1 recruitment is via interaction
with RNAPII, since an spn1 mutant defective for interaction with RNAPII is not targeted to the promoter, and Spn1 is necessary for Spt6 recruitment.
Through a targeted genetic screen, strong and specific antagonizing interactions between SPN1 and genes encoding Swi/Snf subunits were identified. Like Spt6, Swi/Snf appears at CYC1 only after activation of the gene. However, Spt6 significantly precedes Swi/Snf occupancy at the promoter. In the absence
of Spn1 recruitment, Swi/Snf is constitutively found at the promoter. These observations support a model whereby Spn1 negatively
regulates RNAPII transcriptional activity by inhibiting recruitment of Swi/Snf to the CYC1 promoter, and this inhibition is abrogated by the Spn1-Spt6 interaction. These findings link Spn1 functions to the transition
from an inactive to an actively transcribing RNAPII complex at a postrecruitment-regulated promoter.

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    • "Studies in yeast and mammalian systems suggested that IWS1 is involved in chromatin remodeling and histone modifications. In yeast, IWS1 recruits transcription elongation factor Spt6 and chromatin-remodeling complex SWI/ SNF to induce CYC1 gene expression (Zhang et al., 2008). "
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    • "In vitro, binding of Spn1 reduces the affinity of Spt6 toward nucleosomes, suggesting a regulatory role for Spn1 in Spt6-mediated chromatin remodeling (36). Recent studies also indicate that Spn1 plays a key role in RNAPII recruitment at the CYC1 gene (37). Aside from Ser2/Ser5 phosphorylation, in vitro peptide pull-down studies indicate that the tSH2 domain of Spt6 also binds to Tyr1-phosphorylated peptides (38), and recent evidence indicates that Tyr1 phosphorylation is required for the recruitment of Spt6 during transcription elongation and prevents premature termination by inhibiting the binding of termination factors, thus acting as a molecular CTD ‘switch’ (19). "
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    • "The association of these elongation factors, which include Paf1, Spt16, Spt4, Spt5, Spt6, Spn1, and Elf1, occurs concurrently on all Pol II genes and is independent of gene length, type, or expression [146]. The recruitment of these factors is essential for transcription processivity (Spt4/5) [147– 149], histone regulation (Spt6/16, Spn1, Elf1) [150] [151] [152] [153] [154] [155] [156], and gene activation/3 processing (Paf1) [157]. Similarly, mammalian P-TEFb complex is recruited to Pol II at this stage of transcription [158] [159] [160] [161]. "
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