Previously we showed that encapsidated potato virus X (PVX) RNA is nontranslatable in vitro, but can be converted into a translatable form after binding to PVX particles of PVX-coded movement protein, the product of the first gene of triple gene block (TGBp1). Here we report that a similar effect occurs via in situ phosphorylation of the PVX coat protein (CP) by Ser/Thr protein kinase (PK) C, the mixture of casein kinases I and II or by cytoplasmic PK(s) from Nicotiana glutinosa leaves. Immunochemical analyses indicated that phosphorylation induced conformational changes in PVX CP. The N-terminal region of the PVX CP, rich in Ser and Thr residues, is exposed at the virion surface and can be removed by treatment with trypsin. We showed that (i) trypsin treatment removed the bulk of (32)P-radioactivity from in situ phosphorylated PVX CP, (ii) PVX containing N-terminally truncated CP (PVX-Ptd) failed to be translationally activated by phosphorylation, and (iii) the specific infectivity of PVX-Ptd was reduced. However, the PVX-Ptd RNA remained intact and PVX-Ptd could be translationally activated by the PVX MP TGBp1. We hypothesize that phosphorylation of the parental PVX by cytoplasmic PK(s) in vivo renders PVX RNA translatable in primary inoculated cells, whereas translational activation of the progeny virions destined for plasmodesmata trafficking is triggered by TGBp1.

We found that a 2-h incubation of potato virus X (PVX) virions in 10 mM Tris-HCl buffer pH 7.5 at -20 degrees C results in a strong but reversible drop in virion stability. Under these conditions, the PVX virions are completely disrupted by low (starting from 50 mM) concentrations of LiCl and CaCl(2) but not of NaCl. Incubation of PVX samples with 0.05-2 M LiCl at +4 degrees C did not result in virion disassembly and the virions were not disrupted upon incubation at -20 degrees C in 10 mM Tris-HCl buffer pH 7.5 without LiCl. We suggest that a 2-h incubation of the PVX virions at -20 degrees C in 10 mM Tris-HCl pH 7.5 results in a structural transition in the virus particles. A revised model of the three-dimensional organization of coat protein subunits in the PVX virions is proposed. This two-domain model explains better the high plasticity of the PVX CP structure.

Genomic RNA sequence of a tobamovirus infecting cruciferae plants (cr-TMV) was determined. The RNA is composed of 6312 nucleotides and contains four ORFs encoding the proteins of 122K (ORFI), 178K (ORF2), 29K (ORF3) and 18K (capsid protein, 0RF4). ORF4 overlaps 0RF3 by 74 nucleotides and the overlapping region can be folded into a stable hairpin structure. The 3'-terminal region of the cr-TMV RNA preceding the tRNA-like structure was shown to form six potentially stable pseudoknots.

The complete nucleotide sequence of Plantago asiatiea mosaic virus (P1AMV) genomic RNA has been determined. The 6128 nucleotide sequence contains five open reading frames (ORFs) coding for proteins of Mr 156K (ORF1), 25K (ORF2), 12K (ORF3), 13K (ORF4) and 22K (ORF5). The sequences of these PIAMV proteins exhibit strong homology to the proteins of the other potexviruses. Phylogenetic trees based on the multiple sequence alignments of three conserved domains in ORF1 product and capsid protein reveal a close relationship of P1AMV to papaya mosaic virus and clover yellow mosaic virus. The PIAMV genomic RNA and a major subgenomic RNA (sgRNA) of 0.9 kb have been detected in infected leaves by Northern blot hybridization. The latter sgRNA is the messenger for virus capsid protein and its 5' terminus has been located 23 nucleotides upstream of the initiator codon of the coat protein gene. The P1AMV virion RNA and RNA transcript resembling the 0-9 kb sgRNA have been translated in vitro giving rise to a single major 170K product and a major 22K product, respectively.