Fine-Tuning of FACT by the Ubiquitin Proteasome System in Regulation of Transcriptional Elongation.

FACT (facilitates chromatin transcription), an evolutionarily conserved histone chaperone involved in transcription and other DNA transactions, is upregulated in cancers, and its downregulation is associated with cellular death. However, it is not clearly understood how FACT is fine-tuned for normal cellular functions. Here, we show that the FACT subunit Spt16 is ubiquitylated by San1 (an E3 ubiquitin ligase) and degraded by the 26S proteasome. Enhanced abundance of Spt16 in the absence of San1 impairs transcriptional elongation. Likewise, decreased abundance of Spt16 also reduces transcription. Thus, an optimal level of Spt16 is required for efficient transcriptional elongation, which is maintained by San1 via ubiquitylation and proteasomal degradation. Consistently, San1 associates with the coding sequences of active genes to regulate Spt16's abundance. Further, we found that enhanced abundance of Spt16 in the absence of San1 impairs chromatin reassembly at the coding sequence, similarly to the results seen following inactivation of Spt16. Efficient chromatin reassembly enhances the fidelity of transcriptional elongation. Taken together, our results demonstrate for the first time a fine-tuning of FACT by a ubiquitin proteasome system in promoting chromatin reassembly in the wake of elongating RNA polymerase II and transcriptional elongation, thus revealing novel regulatory mechanisms of gene expression.

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