Protein interactions with DNA elements in variant equine infectious anemia virus enhancers and their impact on transcriptional activity

Abstract

The long terminal repeats (LTRs) from various cloned equine infectious anemia virus (EIAV) proviruses differ significantly, but all contain cis-acting DNA elements identical to MDBP-, PEA2-, AP-1-, and PU.1 (ets)-binding sites. A prototype EIAV LTR would contain one of each of these conserved elements. The LTR variations originate from the insertion of novel sequences between the PEA2 and AP-1 elements in the transcriptional enhancer unit. Viewed in this way, the LTR from provirus clone lambda 12 has an 11-bp insertion containing a PEA2 site and the LTR of the lambda 6 provirus has a 31-bp insertion/duplication containing PEA2, AP-1, and PU.1 sites. Two other LTRs were cloned by amplification of cDNAs from the persistently infected cell line, EIAV-FEA. A third LTR was generated by site-directed mutagenesis of one of the LTRs from EIAV-FEA cells. The latter three had a single base change in the element next to the TATA box that abolished PU.1 binding; however, the variable regions of these LTRs were shown by gel mobility shift assays to contain one or two PU.1 sites. One variable region was shown to have an octamer site overlapping its tandem PU.1 elements. Basal, PMA-activated, and Tat trans-activated transcriptional activities of the LTRs were compared in several different cell lines by transient transfection. The various promoters displayed different relative levels of activity depending on the cell line used and the condition of activation. This natural set of variant promoters may help define how changes in the components of the transcription complex influence transactivation by Tat. The diverse LTRs could endow their respective proviruses with a unique pattern of expression and activation in vivo.