Close proximity of the MPMV CTE to the polyadenylation sequences is important for efficient function in the subgenomic context

The constitutive transport element (CTE) of Mason-Pfizer monkey virus (MPMV) is a short cis-acting sequence element critical for virus gene expression. Analogous to the Rev/Rev Responsive Element (RRE) of primate lentiviruses, CTE allows the nucleocytoplasmic transport of unspliced viral mRNAs. In fact, CTE can functionally replace Rev/RRE in the genomic context and has been used successfully in the expression of viral and cellular genes from expression vectors as well. However, unlike RRE, CTE accomplishes this by interacting with cellular factors, making CTE function independent of co-expressed trans factors. Thus, CTE has proven to be a valuable tool in the expression of heterologous genes. Our previous studies have shown that close proximity of CTE to the polyadenylation sequences is important for CTE function in the genomic context. However, it is controversial whether CTE needs to be located spatially close to the polyadenylation sequences in the subgenomic context. Since CTE is being frequently used in expression vectors, we investigated the position dependency of CTE in the heterologous, subgenomic background using both genetic and structural analyses. Our results reveal that similar to the genomic situation, close proximity of CTE to the polyadenylation sequences is important for its function in the heterologous subgenomic context.