Retroviridae are unique among virus families in that one virion packages two full-length copies of its genome. However, each infectious event generates only one provirus; thus, retroviruses are pseudodiploid. We do not fully understand the benefit of pseudodiploidy, and we propose to examine the benefit of packaging two RNAs in one virion. Additionally, the exact mechanisms that retroviruses use to package two RNAs are not currently understood. We have performed experiments examining the mechanisms of copackaged RNA partner selection and have demonstrated that the DIS sequence plays an important role in this process. In our experimental system, we can increase or decrease heterozygous virus formation by manipulating the DIS sequences of the two viruses. These results suggest that Gag recognizes and packages an RNA dimer rather than two monomers. We will continue to dissect elements important to packaging the RNA dimer, including cis-acting elements such as DIS and trans-acting elements such as Gag proteins. Using a cell-fusion assay, we will elucidate when and where selection of copackaged RNA partners occur. These studies will shed light on one of the most fundamental features of retroviral replication and will illustrate both the advantages and the limitations of pseudodiploidy. [Corresponds to Hu Project 2 in the April 2007 site visit report of the HIV Drug Resistance Program]
