The U3 regions of the long terminal repeats (LTRs) of simian immunodeficiency viruses SIVmac251 and SIVmac239 were analyzed for basal transcriptional activity and for interaction with cellular factors in the T-cell line HUT-78 and the monocyte/macrophage cell line U937. A number of 5' deletions and mutations were made in the U3 regions of the two LTRs, and these constructs were placed upstream of a plasmid containing the bacterial chloramphenicol acetyltransferase reporter gene. The nucleotide sequences between -225 and +18 were sufficient to maintain full transcriptional activity of both LTRs in HUT-78 and U937 cells. Nucleotide sequence analysis revealed several differences between SIVmac251 and SIVmac239 within this region. Analysis of deletion mutants revealed that an additional removal of bases, from -124 to -225, had little effect on the transcriptional activity of the clone 239 LTR, whereas this deletion resulted in a significant reduction of activity in the clone 251 LTR. DNase protection assays using nuclear extracts from HUT-78 and U937 cells showed that bases within this region bound cellular factors. In addition, the NF-kappa B site was protected in DNase assays with HUT-78 cells and 12-O-tetradecanoylphorbol-13-acetate-treated U937 cells. An additional DNase footprint was detected in SIVmac239, at -52 to -38, just upstream of the TATA box. This site overlaps the 3' half of the 3'-most Sp-1 site and is downstream of 11 bases that are found in SIVmac239 but not SIVmac251. Thus, differences in the sequences in the U3 region of the LTRs of SIVmac251 and SIVmac239 have been identified which appear to alter the transcriptional activity of these promoters as well as changing the interaction of cellular proteins with sequences in the LTRs
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