The human immunodeficiency virus type 1 (HIV-1) TAR element is critical for the activation of gene expression by the transactivator protein, Tat. Mutagenesis has demonstrated that a stable stem-loop RNA structure containing both loop and bulge structures transcribed from TAR is the major target for tat activation. Though transient assays have defined elements critical for TAR function, no studies have yet determined the role of TAR in viral replication because of the inability to generate viral stocks containing mutations in TAR. In the current study, we developed a strategy which enabled us to generate stable 293 cell lines which were capable of producing high titers of different viruses containing TAR mutations. Viruses generated from these cell lines were used to infect both T-lymphocyte cell lines and peripheral blood mononuclear cells. Viruses containing TAR mutations in either the upper stem, the bulge, or the loop exhibited dramatically decreased HIV-1 gene expression and replication in all cell lines tested. However, we were able to isolate lymphoid cell lines which stably expressed gene products from each of these TAR mutant viruses. Though the amounts of virus in these cell lines were roughly equivalent, cells containing TAR mutant viruses were extremely defective for gene expression compared with cell lines containing wild-type virus. The magnitude of this decrease in viral gene expression was much greater than previously seen in transient expression assays using HIV-1 long terminal repeat chloramphenicol acetyltransferase gene constructs. In contrast to the defects in viral growth found in T-lymphocyte cell lines, several of the viruses containing TAR mutations were much less defective for gene expression and replication in activated peripheral blood mononuclear cells. These results indicate that maintenance of the TAR element is critical for viral gene expression and replication in all cell lines tested, though the cell type which is infected is also a major determinant of the replication properties of TAR mutant viruses.
In vitro transcription by vesicular stomatitis virus nucleocapsids is inhibited by enzymatic dephosphorylation of the NS protein. We provide evidence that specific, partial dephosphorylation of NS molecules is the only detectable change in nucleocapsids treated with bacterial alkaline phosphatase under conditions that prevent the action of adventitious protease. Dephosphorylation appeared to affect only the rate of transcription; there were no changes in sedimentation rates of transcripts. To identify the sites of phosphorylation required for NS activity in transcription, we examined phosphopeptides produced by chymotrypsin digestion of the two electrophoretic classes of NS molecules found in virions and infected cells. The electrophoretically slower class, NS1, abundant in the intracellular soluble pool, has a lower activity in transcription; it contained six chymotryptic phosphopeptides. Five of these peptides contained both phosphoserine and phosphothreonine, indicating that this peptide cluster represents at least 11 separate sites of phosphorylation. In the electrophoretically faster nucleocapsid-associated NS2 class of molecules, which support a higher rate of transcription, another group of eight phosphopeptides was superimposed on this pattern. Two of these peptides contained both phosphoserine and phosphothreonine, so this cluster of peptides represents at least 10 additional phosphorylation sites. These sites were especially sensitive to dephosphorylation by bacterial alkaline phosphatase. One or more of them appears to be responsible for the higher transcription rates medicated by NS2 molecules.
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