The small cysteine-rich protein P14 of beet necrotic yellow vein virus regulates accumulation of RNA 2 in cis and coat protein in trans.

The effect of null mutations of the small cysteine-rich protein P14 encoded by RNA 2 of beet necrotic yellow vein virus has been investigated using in vitro transcripts of viral RNA to infect Chenopodium quinoa protoplasts. The P14 mutations down-regulated RNA 2 accumulation by approximately 10- to 50-fold. Accumulation of minus-strand RNA 2 was also diminished but RNA 1 accumulation was much less affected. The inhibition of RNA 2 accumulation could not be complemented in trans by providing P14 from another source (either a second molecule of RNA 2 or an RNA 3-based replicon) containing and expressing the P14 gene. The P14 null mutations dramatically inhibited accumulation of viral coat protein, which is encoded by the 5'-proximal gene on RNA 2, but this effect could be complemented in trans, indicating that it occurs by a mechanism distinct from that affecting RNA 2 accumulation. Transient expression experiments were also carried out in which a plasmid expressing P14 and plasmids expressing a reporter gene placed downstream of potential translational control sequences (the 5'-noncoding sequences of RNAs 2, 3, or 4) were introduced into C. quinoa or Nicotiana tabacum leaves by microprojectile bombardment. Coexpression of P14 produced a 3- to 4-fold stimulation of reporter gene expression levels for all the constructs. The lack of sequence specificity suggests that this phenomenon is not directly related to the RNA 2-specific stimulation of coat protein accumulation observed in a viral infection.