Moloney murine sarcoma virus genomic RNAs dimerize via a two-step process:a concentration-dependent kissing-loop interaction is driven by initial contact between consecutive guanines

Retroviruses contain two plus-strand genomic RNAs, which are stably but non covalently joined in their 5' regions by a dimer linkage structure (DLS). T wo models have been put forward to explain the mechanisms by which the RNAs dimerize; each model emphasizes the role of specific molecular determinant s. The kissing-loop model implicates interactions between palindromic seque nces in the DLS region. The second model proposes that purine-rich stretche s in the region form purine quartet structures. Here, we present an examina tion of the in vitro dimerization of Moloney murine sarcoma virus (MuSV) RN A in the context of these two models. Dimers were found to form spontaneous ly in a temperature-, time-, concentration-, and salt-dependent manner. In contrast to earlier reports, we found that deletion of neither the palindro me nor the consensus purine motifs (PuGGAPuA) affected the bevel of dimer f ormation at low concentrations of RNA. Rather, different purine-rich sequen ces, i.e., consecutive stretches of guanines, were found to enhance both in vitro RNA dimerization and in vivo viral replication. Biochemical evidence further suggests that these guanine-rich (G-rich) stretches form guanine q uartet structures. We also found that the palindromic sequences could suppo rt dimerization at significantly higher RNA concentrations. In addition, th e G-rich stretches were as important as the palindromic sequence for mainta ining efficient vir al replication. Overall, our data support a model that entails contributions from both of the previously proposed mechanisms of re troviral RNA dimerization.