FIG. 1.

Model of the roles of endo- and exoribonucleases in tombusvirus RNA degradation and recombination. During or after replication of TBSV DI-72 repRNA, a host endoribonuclease cleaves some of the viral RNAs (at places marked by arrows), producing 5′Fr and 3′Fr degRNAs. These cleaved repRNA products are then rapidly removed by the host Xrn1p 5′-3′ exoribonuclease. The rapid removal of the recombination substrates inhibits viral RNA recombination. The not-yet-degraded 3′Fr RNAs can participate in replicase-driven recombination events. Note that 5′Fr lacks important cis-acting replication sequences and does not seem to participate in RNA recombination.