Multiple copies of the Mason-Pfizer monkey virus constitutive RNA transport element lead to enhanced HIV-1 Gag expression in a context-dependent manner

Retroviral gene expression requires nuclear export and translation of incom pletely spliced RNA. In the case of human immunodeficiency virus (HIV), thi s is facilitated by the viral Rev protein binding to its cognate RNA respon se element (RRE), while other retroviruses contain constitutive transport e lements (CTE) binding to cellular factors. These CTE can substitute for the HIV-1 Rev/RRE system, albeit with reduced efficiency. Here, we show that m ultimeric copies of the CTE restore HIV-1 protein expression to levels comp arable to or higher than Rev/RRE in various cell lines from different speci es. We suggest that multimerization of export factors is important for CTE function, as reported for Rev. CTE function was not affected when the eleme nt was displaced from its natural position close to the poly(A) signal, whi le insertion of an intron into the 3'-untranslated region (3'-UTR) severely reduced CTE activity. In this case, cytoplasmic RNA degradation was observ ed, which may be mediated by nonsense-mediated RNA decay. In contrast, Rev- dependent gene expression was insensitive to an intron in the 3'-UTR. Final ly, we show that the putative CTE-binding protein RNA helicase A is not spe cifically translocated into the cytoplasm upon overexpression of CTE-contai ning RNA.