Dynamic transcription of ubiquitin genes under basal and stressful conditions and new insights into the multiple UBC transcript variants

Ubiquitin (Ub) is a small 76-amino acid protein that is engaged in many different pathways within the cell, including protein turnover. During proteotoxic stress, when the demand of clearing damaged/misfolded proteins strongly increases, cells activate Ub gene transcription to face the need of extra ubiquitin. This paper shows the contribution of the four ubiquitin coding genes (UBB, UBC, UBA52, RPS27A) to the ubiquitin RNA pool under basal and stressful conditions. Our results reveal that UBC and RPS27A represent the major fraction of the Ub transcriptome in different cell lines, but when converted to the coding potential, polyubiquitin genes UBC and UBB mainly contribute to determine the intracellular ubiquitin content under basal conditions. Both the polyubiquitin genes UBB and UBC are upregulated upon proteasome inhibition and oxidative stress, with markedly higher responses from the UBC promoter. A similar output, with lower fold-inductions, is detected in heat-stressed cells, with UBC acting as the main contributor to thermotolerance. By contrast, upon these stressors, the levels of UBA52 and RPS27A mRNAs remain unchanged. Remarkably, UV irradiation fails to induce Ub gene transcription, but rather seems to act at the post-transcriptional level, by stabilizing ubiquitin mRNAs at UV doses which induce rapid degradation of other RNA molecules. Moreover, the evidence that the UBC core promoter contains multiple transcription start sites and their responsiveness to stress, is here reported for the first time.