Characterization of the RNA-binding domains in the replicase proteins of tomato bushy stunt virus.

Tomato bushy stunt virus (TBSV), a tombusvirus with a nonsegmented, plus-stranded RNA genome, codes for two essential replicase proteins. The sequence of one of the replicase proteins, namely p33, overlaps with the N-terminal domain of p92, which contains the signature motifs of RNA-dependent RNA polymerases (RdRps) in its nonoverlapping C-terminal portion. In this work, we demonstrate that both replicase proteins bind to RNA in vitro based on gel mobility shift and surface plasmon resonance measurements. We also show evidence that the binding of p33 to single-stranded RNA (ssRNA) is stronger than binding to double-stranded RNA (dsRNA), ssDNA, or dsDNA in vitro. Competition experiments with ssRNA revealed that p33 binds to a TBSV-derived sequence with higher affinity than to other nonviral ssRNA sequences. Additional studies revealed that p33 could bind to RNA in a cooperative manner. Using deletion derivatives of the Escherichia coli-expressed recombinant proteins in gel mobility shift and Northwestern assays, we demonstrate that p33 and the overlapping domain of p92, based on its sequence identity with p33, contain an arginine- and proline-rich RNA-binding motif (termed RPR, which has the sequence RPRRRP). This motif is highly conserved among tombusviruses and related carmoviruses, and it is similar to the arginine-rich motif present in the Tat trans-activator protein of human immunodeficiency virus type 1. We also find that the nonoverlapping C-terminal domain of p92 contains additional RNA-binding regions. Interestingly, the location of one of the RNA-binding domains in p92 is similar to the RNA-binding domain of the NS5B RdRp protein of hepatitis C virus.