ABSTRACT

Retroviruses contain two plus-strand genomic RNAs, which are stably but noncovalently joined in their 5′ regions by a dimer linkage structure (DLS). Two models have been put forward to explain the mechanisms by which the RNAs dimerize; each model emphasizes the role of specific molecular determinants. The kissing-loop model implicates interactions between palindromic sequences in the DLS region. The second model proposes that purine-rich stretches in the region form purine quartet structures. Here, we present an examination of the in vitro dimerization of Moloney murine sarcoma virus (MuSV) RNA in the context of these two models. Dimers were found to form spontaneously in a temperature-, time-, concentration-, and salt-dependent manner. In contrast to earlier reports, we found that deletion of neither the palindrome nor the consensus purine motifs (PuGGAPuA) affected the level of dimer formation at low concentrations of RNA. Rather, different purine-rich sequences, 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 quartet structures. We also found that the palindromic sequences could support dimerization at significantly higher RNA concentrations. In addition, the G-rich stretches were as important as the palindromic sequence for maintaining efficient viral replication. Overall, our data support a model that entails contributions from both of the previously proposed mechanisms of retroviral RNA dimerization.