Evolution of the human immunodeficiency virus type 1 long terminal repeat promoter by conversion of an NF-kappa B enhancer element into a GABP binding site

Human immunodeficiency virus type 1 (HIV-1) transcription is regulated by t he viral Tat protein and cellular factors, of which the concentration and a ctivity may depend on the cell type. Viral long terminal repeat (LTR) promo ter sequences are therefore optimized to suit the specific nuclear environm ent of the target host cell. In long-term cultures of a Tat-defective, poor ly replicating HIV-1 mutant, we selected for a faster-replicating virus wit h a l-nucleotide deletion in the upstream copy of two highly conserved NF-k appa B binding sites. The variant enhancer sequence demonstrated a severe l oss of NF-kappa B binding in protein binding assays. Interestingly, we obse rved a new binding activity that is specific for the variant NF-kappa B seq uence and is present in the nuclear extract of unstimulated cells that lack NF-kappa B. These results suggest that inactivation of the NF-kappa B site coincides with binding of another transcription factor. Pine mapping of th e sequence requirements for binding of this factor revealed a core sequence similar to that of Ets binding sites, and supershift assays with antibodie s demonstrated the involvement of the GABP transcription factor. Transient transfection experiments with LTR-chloramphenicol acetyltransferase constru cts indicated that the variant LTR promoter is specifically inhibited by GA BP in the absence of Tat, but this promoter was dramatically more responsiv e to Tat than the wild-type LTR. Introduction of this GABP site into the LA I virus yielded a specific gain of fitness in SupT1 cells, which contain li ttle NF-kappa B protein. These results suggest that GABP potentiates Tat-me diated activation of LTR transcription and viral replication in some cell t ypes. Conversion of an NF-kappa B into a GABP binding site is likely to hav e occurred also during the worldwide spread of HIV-1, as we noticed the sam e LTR modification in subtype E isolates from Thailand. This typical LTR pr omoter configuration may provide these viruses with unique biological prope rties.