A Kaposi's sarcoma virus RNA element that increases the nuclear abundance of intronless transcripts

The Kaposi's sarcoma‐associated herpesvirus produces a 1077 nucleotide noncoding, polyadenylated, exclusively nuclear RNA called PAN that is highly expressed in lytically infected cells. We report that PAN contains a novel post‐transcriptional element essential for its abundant accumulation. The element, PAN–ENE (PAN RNA e xpression and n uclear retention e lement), increases the efficiency of 3′‐end formation in vivo and is sufficient to enhance RNA abundance from an otherwise inefficiently expressed intronless β‐globin construct. The PAN–ENE does not concomitantly increase the production of encoded protein. Rather, it retains the unspliced β‐globin mRNA in the nucleus. Tethering of export factors can override the nuclear retention of the PAN–ENE, supporting a mechanism whereby the PAN–ENE blocks assembly of an export‐competent mRNP. The activities of the PAN–ENE are specific to intronless constructs, since inserting the PAN–ENE into a spliced β‐globin construct has no effect on mRNA abundance and does not affect localization. This is the first characterization of a cis ‐acting element that increases RNA abundance of intronless transcripts but inhibits assembly of an export‐competent mRNP.