THE FULL-LENGTH TAT PROTEIN IS REQUIRED FOR TAR-INDEPENDENT, POSTTRANSCRIPTIONAL TRANSACTIVATION OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ENVGENE-EXPRESSION

Tat is a protein that dramatically increases the expression of all gen es expressed from the human immunodeficiency virus type 1 (HIV-1) long terminal repeat through interaction with a cis-acting target sequence referred to as TAR (for trans-acting responsive region). The tat gene is divided into two coding exons which, when translated, result in th e synthesis of an 86-amino-acid protein. However, the 72-amino-acid se gment encoded by the first coding exon of tat is sufficient to encode a fully active Tat protein in known assays. We examined expression of the env gene from an LTR that lacks TAR (designated dTAR-env). Surpris ingly, only the full-length Tat peptide trans activated expression of the env gene from dTAR-env. Comparison of RNA and protein expression o f the env gene in the presence of Tat indicated that the mechanism of trans activation is posttranscriptional rather than transcriptional. T o test whether the TAR-independent Tat function is specific to the HIV -1 env gene, we analyzed expression of heterologous genes from the lon g terminal repeat lacking TAR. These heterologous genes were not trans activated by Tat in the absence of a TAR element, which suggests that the second-exon peptide of Tat has a sequence-specific role in TAR-in dependent trans activation of the HIV-1 env gene. Analysis of a mutant in the 5' end of the env gene was used to identify a cis-acting seque nce required for Tat responsiveness.