Localization and footprinting of an enhancer within the avian sarcoma virus gag gene

A cis-acting regulatory element within the gag gene of avian retroviruses has been localized by deletion analysis, and sites of protein interaction have been studied by DNase I footprinting. Unidirectional deletions were made from both the 5' and 3' ends of a 656-base-pair fragment of the gag gene of Fujinami sarcoma virus. These deletion mutants were tested for enhancer activity in a chloramphenicol acetyltransferase transient expression assay. A sharp 5' boundary for enhancer activity was observed between 776 and 786 nucleotides downstream from the transcription initiation site. In contrast, deletion from the 3' side resulted in a gradual loss of enhancer activity, reaching a near basal level of activity by nucleotide 868. Internal deletion of 76 nucleotides just downstream of the 5' boundary abolished enhancement. Mutagenesis of a consensus enhancer core sequence (GTGGTTTG) showed that this sequence was not necessary for enhancer activity in our transient assays. DNase I footprinting with both a highly purified enhancer-binding protein from rat liver (EBP20) and a partially purified chicken liver nuclear extract showed specific protection of nucleotides 813 to 872 within the localized enhancer region. Footprinting of unidirectional deletion mutants that had lost activity indicated that this binding was not sufficient to confer enhancement.