THE POSTTRANSCRIPTIONAL CONTROL ELEMENT OF THE SIMIAN RETROVIRUS TYPE-1 FORMS AN EXTENSIVE RNA SECONDARY STRUCTURE NECESSARY FOR ITS FUNCTION

It was previously shown that a 240-nucleotide (nt) RNA element (cis-ac ting transactivation element [CTE]) located between the env gene and t he 3' long terminal repeat of simian retrovirus type 1 (SRV-1) can fun ctionally replace posttranscriptional activation directed by Rev and t he Rev-responsive element (RRE) when inserted into a Rev- and RRE-defi cient molecular clone of human immunodeficiency virus type 1, resultin g in efficient virus replication. Here, we analyze the molecular and s tructural requirements for function of this RNA element, Deletion muta genesis demonstrated that the core element spans 173 nt, SRV-2 and Mas on-Pfizer monkey virus have highly homologous elements, which function similarly when inserted into the Rev/RRE-deficient human immunodefici ency virus type 1, Computer prediction indicated that the core CTEs of all three viruses have similar extensive secondary structures, Mutage nesis of the SRV-1 CTE revealed that both sequence and secondary struc ture are essential for function. Nuclease probing of the SRV-1 CTE fur ther supported the genetic analysis and confirmed the predicted struct ural features of the RNA element, Sequence analysis of the 240-nt SRV- 1 CTE, after continuous long-term propagation of the Rev-independent v iruses, revealed that the genetically defined core element remained un changed, while regions outside the core CTE underwent deletions or dup lications, These data further support our in vitro mutagenesis data an d demonstrate the importance of the sequence and structure of the SRV- 1 CTE for appropriate function.