  Efficient replication and assembly of virus particles are integral to the
establishment of infection. In addition to the primary role of the capsid
protein (CP) in encapsidating the RNA progeny, experimental evidence on
positive sense single-stranded RNA viruses suggests that the CP also regulates
RNA synthesis. Here, we demonstrate that replication of Satellite tobacco
mosaic virus (STMV) is controlled by the cooperative interaction between STMV
CP and the helper virus (HV) Tobacco mosaic virus (TMV) replicase. We
identified that the STMV CP-HV replicase interaction requires a positively
charged residue at the third position (3R) in the N-terminal 13 amino acid (aa)
motif. Far-Northwestern blotting showed that STMV CP promotes binding between
HV-replicase and STMV RNA. An STMV CP variant having an arginine to alanine
substitution at position 3 in the N-terminal 13aa motif abolished replicase-CP
binding. The N-terminal 13aa motif of the CP bearing alanine substitutions for
positively charged residues located at positions 5, 7, 10 and 11 are defective
in packaging full-length STMV, but can package a truncated STMV RNA lacking the
3' terminal 150 nt region. These findings provide insights into the mechanism
underlying the regulation of STMV replication and packaging.
