Uncapped mRNAs during heat stress

Tudor staphylococcal nuclease (TSN or Tudor-SN; also known as SND1) is an evolutionarily conserved protein involved in transcriptional and post-transcriptional regulation of gene expression in animals. Although TSN was found to be indispensable for normal plant development and stress tolerance, the molecular mechanisms underlying these functions remain elusive. Here we show that Arabidopsis thaliana TSN is essential for integrity and function of cytoplasmic messenger ribonucleoprotein (mRNP) complexes called stress granules (SG) and processing bodies (PB), sites of post-transcriptional gene regulation during stress. TSN associates with SG following their microtubule-dependent assembly and plays a scaffolding role in both SG and PB. The enzymatically active tandem repeat of four SN domains is crucial for targeting TSN to the cytoplasmic mRNA complexes, and is sufficient for the cytoprotective function of TSN during stress. Furthermore, our work connects the cytoprotective function of TSN with its positive role in stress-induced mRNA decapping. While stress led to a pronounced increase in the accumulation of uncapped mRNAs in wild type plants, this increase was abrogated in TSN knockout plants. Taken together, our results establish TSN as a key enzymatic component of catabolic machinery responsible for the processing of mRNAs in the cytoplasmic mRNP complexes during stress. Five-day Arabidopsis seedlings were grown vertically on MS agar plates under 16/8-h light/dark cycle and light intensity 150 µE m−2 s−1 for 5 days. For heat stress, plates with seedlings were incubated for 40 min on a thermoblock at 39 ºC. Root tissue was collected and used to purify uncapped and total mRNA. Two independent sets of biological samples were used for the experiments.