The AU-Rich RNA Recombination Hot Spot Sequence of Brome Mosaic Virus Is Functional in Tombusviruses: Implications for the Mechanism of RNA Recombination

RNA recombination can be facilitated by recombination signals present in viral RNAs. Among such signals are short sequences with high AU contents that constitute recombination hot spots in Brome mosaic virus (BMV) and retroviruses. In this paper, we demonstrate that a defective interfering (DI) RNA, a model template associated with Tomato bushy stunt virus (TBSV), a tombusvirus, undergoes frequent recombination in plants and protoplast cells when it carries the AU-rich hot spot sequence from BMV. Similar to the situation with BMV, most of the recombination junction sites in the DI RNA recombinants were found within the AU-rich region. However, unlike BMV or retroviruses, where recombination usually occurred with precision between duplicated AU-rich sequences, the majority of TBSV DI RNA recombinants were imprecise. In addition, only one copy of the AU-rich sequence was essential to promote recombination in the DI RNA. The selection of junction sites was also influenced by a putative cis-acting element present in the DI RNA. We found that this RNA sequence bound to the TBSV replicase proteins more efficiently than did control nonviral sequences, suggesting that it might be involved in replicase "landing" during the template switching events. In summary, evidence is presented that a tombusvirus can use the recombination signal of BMV. This supports the idea that common AU-rich recombination signals might promote interviral recombination between unrelated viruses.