Retroviral constitutive transport element evolved from cellular TAP(NXF1)-binding sequences

The constitutive transport element (CTE) of type D retroviruses serves as a signal of nuclear export of unspliced viral RNAs, The human TAP(NXF1) prot ein, a cellular mRNA export factor, directly binds to CTE and mediates nucl ear export of CTE-containing RNAs, Here, we use genomic SELEX (systematic e volution of ligands by exponential enrichment) to show that the human genom e encodes a family of high-affinity TAP ligands. These TAP-binding elements (TEE) are 15-bp minisatellite repeats that are homologous to the core TAP- binding sites in CTE, The repeats are positioned similarly in the RNA secon dary structures of CTE and TEE. Like CTE, TEE is an active nuclear export s ignal. CTE elements of different species share sequence similarities to TEE in the regions that are neutral for CTE function. This conservation points to a possible common ancestry of the two elements, and in fact, TEE has pr operties expected from a primordial CTE, Additionally, a molecular fossil o f a TEE-like minisatellite is found in the genome of a modern retroelement, These findings constitute direct evidence of an evolutionary link between TEE-related minisatellites and CTE.