A negative feedback mechanism links UBC gene expression to ubiquitin levels by affecting RNA splicing rather than transcription.

ABSTRACT

UBC gene plays a critical role in maintaining ubiquitin (Ub) homeostasis. It is upregulated under stress conditions, and herein we report that it is downregulated upon Ub overexpression. Downregulation occurs in a dose-dependent manner, suggesting the existence of a fine-tuned Ub sensing mechanism. This "sensor" requires a conjugation competent ubiquitin to detect Ub levels. Searching the sensor among the transcription factors involved in basal and stress-induced UBC gene expression was unsuccessful. Neither HSF1 and HSF2, nor Sp1 and YY1 are affected by the increased Ub levels. Moreover, mutagenesis of their binding sites in the UBC promoter-driven reporter constructs does not impair the downmodulation effect. Epigenetic studies show that H2A and H2B ubiquitination within the UBC promoter region is unchanged upon ubiquitin overexpression. Noteworthy, quantification of nascent RNA molecules excludes that the downmodulation arises in the transcription initiation step, rather pointing towards a post-transcriptional mechanism. Indeed, a significantly higher fraction of unspliced UBC mRNA is detected in ubiquitin overexpressing cells, compared to empty vector transfected cells. Our findings suggest how increasing cellular ubiquitin levels may control the expression of UBC gene by negatively affecting the splicing of its pre-mRNA, providing a straightforward feedback strategy for the homeostatic control of ubiquitin pools.