Dynamic regulation of murine RNA polymerase III transcription during heat shock stress

Cells respond to many different types of stresses by overhauling gene expression patterns, both at the transcriptional and translational levels. Under heat stress, global transcription and translation are inhibited, while the expression of chaperone proteins is preferentially favored. As the direct link between mRNA transcription and protein translation, transfer RNA (tRNA) expression is intricately regulated during the stress response. Despite extensive research into the heat shock response (HSR), the regulation of tRNA expression by RNA polymerase III (Pol III) transcription has yet to be fully elucidated in mammalian cells. Here, we examine the regulation of Pol III transcription during different stages of heat shock stress in mouse embryonic stem cells. We observe that Pol III transcription is downregulated after 30 min of heat shock, followed by an overall increase in transcription after 60 min of heat shock. This effect is more evident in tRNAs, although other Pol III gene targets are also similarly affected. Notably, we show that the downregulation at 30 min of heat shock is independent of HSF1, the master transcription factor of the HSR, but that the subsequent increase in expression at 60 min requires HSF1. Taken together, these results demonstrate an adaptive RNA Pol III response to heat stress and an intricate relationship between the canonical HSR and tRNA expression.