RNA-BINDING ACTIVITIES OF BARLEY STRIPE MOSAIC-VIRUS GAMMA-B FUSION PROTEINS

The barley stripe mosaic virus (BSMV) gamma b gene encodes a 17 kDa cy steine-rich protein known to affect virulence and to have a role in re gulating viral gene expression. We have constructed recombinant gamma b-glutathione S-transferase fusion proteins in Escherichia coli and ha ve determined the ability of the purified fusion proteins and various mutant derivatives to bind nucleic acids in vitro. Gel-shift analyses revealed that the wild-type gamma b-fusion protein is able to bind RNA cooperatively. The binding affinity is highly selective for single-st randed RNA because double-stranded RNA, single-stranded and double-str anded DNA, and transfer RNA were unable to compete for binding with th e labelled RNA probes. However, BSMV-specific sequence binding was not observed since a chloroplast RNA competed for binding with P-32-label led transcripts derived from the BSMV genome. The first 44 amino acids of the 152 amino acid gamma b fusion protein encompassing one of two cysteine-rich 'zinc finger-like' moths, and a basic region separating the finger-like moths are required for RNA binding. Site-specific amin o acid substitutions within two groups of lysine and arginine residues located in the basic moth reduced the binding affinity of the fusion protein greatly, but cysteine and histidine substitutions designed to disrupt the finger-like moths failed to have appreciable effects on bi nding. These findings indicate that the regulatory properties of gamma b may be mediated in part by RNA binding activities.