K. Verhoef et al., OPTIMAL TAT-MEDIATED ACTIVATION OF THE HIV-1 LTR PROMOTER REQUIRES A FULL-LENGTH TAR RNA HAIRPIN, Nucleic acids research, 25(3), 1997, pp. 496-502
HIV-1 transcription from the LTR promoter is activated by the viral Ta
t protein through interaction with the nascent TAR RNA hairpin structu
re. The mechanism of Tat-mediated transcriptional activation has been
extensively investigated with LTR-CAT reporter genes in transient tran
sfections and, more recently, in infection experiments with mutant HIV
-1 variants, Several discrepancies between these two assay systems hav
e been reported. For instance, whereas opening of the lower part of th
e TAR RNA stem does not affect the promoter activity of an LTR-CAT pla
smid in transient assays, the corresponding virus mutant is fully repl
ication-impaired, With the aim to resolve this controversy, we have ex
amined the activity of a set of TAR RNA mutants in transient transfect
ion experiments with a variety of cell types, We now demonstrate that
truncated TAR motifs exhibit a severe, but cell-type dependent transcr
iption defect. Whereas full LTR activity is measured in COS cells that
have been used regularly in previous transfection assays, a severe de
fect is apparent in a variety of human cell lines, including T cell li
nes that are typically used in HIV-I replication studies, These result
s suggest the presence of a human protein that participates in Tat-med
iated transcriptional activation through binding to the lower part of
the TAR stem. Several candidate co-factors have been reported in liter
ature. This study resolves the discrepancy between transfection and in
fection studies on the requirements of the fewer TAR stem structure. T
he evidence also implies that LTR transcription studies should be perf
ormed preferentially in human cell types.