A new retroelement constituted by a natural alternatively spliced RNA of murine replication-competent retroviruses

Replication of simple retroviruses depends on the recruitment of a single large primary transcript toward splicing, transport/packaging and translation regulations. In this respect, we studied the novel SD' 4.4 kb RNA of murine leukemia retroviruses (MLV) which results from alternative splicing of the primary transcript. We showed that SD' RNA was required for optimal replication since expression of a pre-spliced SD' RNA trans-complemented the impaired infectivity of a SD'-defective mutant. We monitored the fate of this novel transcript throughout early and late events of the viral life cycle. SD' RNA was specifically incorporated into virions demonstrating that the unspliced RNA was not the unique viral RNA present in virions. Furthermore, SD' RNA was reverse transcribed and its DNA copy integrated into the host genome, thus constituting a new splice donor-associated retroelement (SDARE) in infected cells. Finally, we showed that SD' mRNA encoded a 50 kDa polyprotein, and to a lower extent an additional 60 kDa polyprotein, which harbored Gag and integrase domains.