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 over-expressed 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.