IDENTIFICATION OF LIMITING STEPS FOR EFFICIENT TRANSACTIVATION OF HTV-I PROMOTER BY TAT IN SACCHAROMYCES-CEREVISIAE

Cellular context is an important determinant for the activity of Tat, the trans-activator of human immunodeficiency virus (HIV). We have inv estigated HIV-1 promoter expression and trans-activation in Saccharomy ces cerevisiae to provide clues about the limiting steps for Tat activ ity in this organism. A minimal 43-nucleotide HIV promoter (HIV43) has the activity of a weak yeast promoter in the presence or absence of v arious enhancer binding sites (bs), whereas the entire long terminal r epeat is not expressed. None of these constructs could be trans-activa ted by Tat. Fusion proteins Gal4 binding domain (BD)-Tat48 and Ga14BD- Tat72 are active with different efficiencies on various yeast promoter s that have Gal4 bs. They have 70 and 50% of Gal4 wild type activity o n hybrid HIV promoters fused to Gal4 bs only in the presence of API bs . This study shows that trans-activation of the HIV-1 promoter by Tat occurs in yeast when Tat is targeted to the promoter and a functional enhancer activity is present. Spl function and Tat transfer from the R NA to the promoter are two major elements for in vivo trans-activation of HIV-1 that are defective in S. cerevisiae but can be replaced by f unctional equivalents.