A region of the Reticuloendotheliosis virus (REV) long terminal repeat (LTR) harbouring single or duplicated copies of 46-bp and 26-bp sequence elements is implicated in enhancer activity. Sequences residing upstream from the proviral 3′ LTR did not contribute to activity of the intact LTR. Gene expression regulated by a combination of REV enhancer and SV40 early region promoter was 50-fold less than from the analogous construct containing the chicken syncytial virus promoter. Deletion of LTR sequences immediately downstream of the CAP site, which include a region capable of forming a stable hairpin in the mRNA, decreased expression by 70%. Expression assays and S1 nuclease mapping showed that a second transcriptional start site, identified by transcription in vitro using HeLa cell lysates and purified DNA templates, was not used in vivo in the cell lines examined.
ASJC Scopus subject areas