We developed a sensitive vector system for the analysis of weak promot
er activities. This promoter assay is based on the transcriptional act
ivator protein, Tat, of human immunodeficiency virus type 1 (HIV-1). H
igh-level expression of HIV requires activation in trans by Tat of the
promoter in the long terminal repeat (LTR). Here we describe the cons
truction of a promoterless pTat vector. Foreign promoter elements can
be inserted upstream from the tat gene, and expression of Tat protein
is measured in trans on a co-transfected LTR-CAT reporter plasmid. We
show that this binary system is more sensitive than standard pCAT repo
rter assays.