The tobacco necrosis virus p7a protein is a nucleic acid-binding protein

The two centrally located open reading frames (ORFs) of necroviruses may, by analogy with the similarly located and related ORFs of carmoviruses, be expected to have a function in virus movement in plants. In the case of tobacco necrosis virus (TNV) strain D these proteins both have a molecular mass of approximately 7 kDa and are thus known as p7a and p7b. We overexpressed p7a in Escherichia coli, separated it from bacterial proteins and renatured it on gels, and showed that p7a was able to bind single-stranded RNA and single-stranded DNA, but was unable to bind double-stranded DNA. These protein-nucleic acid complexes were stable at moderately high salt concentrations. Protein p7b could not be expressed in a number of bacterial systems. We speculate that in TNV, unlike some other viruses which encode a single movement protein with separate functional domains for RNA binding and plasmodesmatal targeting, p7a and p7b may respectively provide these functions on separate proteins.