In vitro assembly of the Tomato bushy stunt virus replicase requires the host Heat shock protein 70.

To gain insights into the functions of a viral RNA replicase, we have assembled in vitro and entirely from nonplant sources, a fully functional replicase complex of Tomato bushy stunt virus (TBSV). The formation of the TBSV replicase required two purified recombinant TBSV replication proteins, which were obtained from E. coli, the viral RNA replicon, rATP, rGTP, and a yeast cell-free extract. The in vitro assembly of the replicase took place in the membraneous fraction of the yeast extract, in which the viral replicase-RNA complex became RNase- and proteinase-resistant. The assembly of the replicase complex required the heat shock protein 70 (Hsp70 = yeast Ssa1/2p) present in the soluble fraction of the yeast cell-free extract. The assembled TBSV replicase performed a complete replication cycle, synthesizing RNA complementary to the provided RNA replicon and using the complementary RNA as template to synthesize new TBSV replicon RNA.