Identification of a post-transcriptional regulatory element in the human endogenous retroviral syncytin-1

Retroviral transcripts have cis-acting elements that interact with host and viral proteins to enable efficient nuclear export and/or translation; however, it is poorly understood whether the transcripts of human endogenous retroviral genes retain such elements. Here, we show that human syncytin-1, which is derived from human endogenous retrovirus W, requires a 3' untranslated region (3'UTR) for efficient gene expression and retains a post-transcriptional regulatory element (named SPRE). The insertion of SPRE markedly increased a reporter gene (human immunodeficiency virus type 1 Gag) expression without affecting the amounts of nuclear or cytoplasmic transcript. Deletion analysis identified a required sequence for SPRE activity, and the prediction of the RNA secondary structure demonstrated a common secondary structure found among active SPRE sequences. Another human syncytin, syncytin-2, also requires a 3'UTR for efficient gene expression. These data provide insights into post-transcriptional regulation in endogenous retroviral gene expression.