MicroRNAs in Tetrahymena thermophila: An epigenetic regulatory mechanism in the response to cadmium stress.

Among the epigenetic mechanisms based on non-coding RNA are microRNAs (miRNAs) that are involved in the post-transcriptional regulation of mRNAs. In many organisms, the expression of genes involved in the cellular response to biotic or abiotic stress depends on the regulation, generally inhibitory, performed by miRNAs. For the first time in the eukaryotic microorganism (ciliate-model) Tetrahymena thermophila, miRNAs involved in the post-transcriptional regulation of transcripts linked to the response to cadmium have been isolated and analyzed. Forty de novo miRNAs (we named tte-miRNAs) have been isolated from control and Cd-treated populations (1 or 24 h exposures). An exhaustive comparative analysis of the features of these mature tte-miRNAs and their precursor sequences (pre-tte-miRNAs) confirms that they are true miRNAs. In addition to the three types of miRNA isoforms previously described in other organisms, two new types are also described among the tte-miRNAs studied. A certain percentage of the pre-tte-miRNA sequences are in introns from genes with many introns, and have been defined as 5', 3'-tailed mirtrons. A qRT-PCR analysis of selected tte-miRNAs together with some of their targets has validated them. Cd is one of the most toxic metals for the cell, which must defend itself against its toxicity by various mechanisms, such as expulsion by membrane pumps, chelation by metallothioneins, among others. Like other toxic metals, Cd also causes a well-known series of cellular effects such as intense proteotoxicity. Many of the targets that are regulated by the tte-miRNAs are transcripts encoding proteins that fit into these defense mechanisms and toxic metal effects.