RNA RECOGNITION AND REGULATION OF HIV-1 GENE-EXPRESSION BY VIRAL FACTOR TAT

Viral transcription factor Tat is a small nuclear protein containing a large number of basic amino acids. The tat gene consists of two exons but only the first encoding 72-amino acid polypeptide is necessary fo r protein activity. Since the second exon is poorly conservative the t otal number of amino acids among Tat proteins from different strains o f HIV-1 varies from 86 to 130. Tat protein acts as trans-activator of HIV genome transcription. It is absolutely required for viral function ing. Tat increases processivity of RNA-polymerase II by abolition of t ranscription blockade, which appears after polycondensation of the fir st 60-70 nucleotides of either HIV mRNA, i.e., it acts as antiterminat or. For manifestation of its activity Tat specifically binds to the do uble stranded RNA fragment called TAR which is located at the 5'-termi nus of all HIV mRNAs. The TAR structure contains a hairpin and a side loop. The Tat-binding region includes only a site of the loop; manifes tation of Tat activity in vivo requires the full TAR and additional ce llular co-factors.