Nonsense-mediated mRNA decay in Tetrahymena is EJC independent and requires a protozoa-specific nuclease

Nonsense-mediated mRNA decay (NMD) is essential for removing premature termination codon (PTC)-containing transcripts from cells. Studying the NMD pathway in model organisms can help to elucidate the NMD mechanism of humans and improves our understanding of how this biologically important process has evolved. Protozoa are among the earliest branching eukaryotes. Their NMD mechanism is poorly understood and may be primordial. We demonstrate that highly conserved Upf proteins (Upf1a, Upf2, and Upf3) are involved in the NMD pathway of the ciliate, Tetrahymena thermophila. We further show that a novel protozoa-specific nuclease, Smg6L, is responsible for destroying many NMD-targeted transcripts. The transcriptome-wide identification and characterization of NMD-targeted transcripts in vegetative Tetrahymena cells showed that many have exon–exon junctions downstream of the termination codon. However, Tetrahymena homologs of exon junction complex (EJC) core components do not form a complex and are dispensable for NMD, suggesting that NMD is EJC independent in this early branching eukaryote. Overall design: Examination of transcriptional variation associated with deletion of UPF1a, SMG6L, MAG1 genes and truncation of the NYN nucelase domain of Smg6L protein in Tetrahymena thermophila