Transient expression analysis of the reticuloendotheliosis virus long terminal repeat element

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.