A second-site mutation that restores replication of a Tat-defective human immunodeficiency virus.

We previously constructed a large set of mutants of the human immunodeficiency virus type 1 (HIV-1) regulatory protein Tat with conservative amino acid substitutions in the activation domain. These Tat variants were analyzed in the context of the infectious virus, and several mutants were found to be defective for replication. In an attempt to obtain second-site suppressor mutations that could provide information on the Tat protein structure, some of the replication-impaired viruses were used as a parent for the isolation of revertant viruses with improved replication capacity. Sequence analysis of revertant viruses frequently revealed changes within the tat gene, most often first-site reversions either to the wild-type amino acid or to related amino acids that restore, at least partially, the Tat function and virus replication. Of 30 revertant cultures, we identified only one second-site suppressor mutation. The inactive Y26A mutant yielded the second-site suppressor mutation Y47N that partially restored trans-activation activity and virus replication. Surprisingly, when the suppressor mutation was introduced in the wild-type Tat background, it also improved the trans-activation function of this protein about twofold. We conclude that the gain of function measured for the Y47N change is not specific for the Y26A mutant, arguing against a direct interaction of Tat amino acids 26 and 47 in the three-dimensional fold of this protein. Other revertant viruses did not contain any additional Tat changes, and some viruses revealed putative second-site Tat mutations that did not significantly improve Tat function and virus replication. We reason that these mutations were introduced by chance through founder effects or by linkage to suppressor mutations elsewhere in the virus genome. In conclusion, the forced evolution of mutant HIV-1 genomes, which is an efficient approach for the analysis of RNA regulatory motifs, seems less suited for the analysis of the structure of this small transcription factor, although protein variants with interesting properties can be generated.