Translational strategies in potexviruses: Products encoded by clover yellow mosaic virus, foxtail mosaic virus, and viola mottle virus RNAs in vitro

Natural Infection of Sorghum by Foxtail Mosaic Virus in Kansas

Sorghum (Sorghum bicolor) was infected by a mechanically transmissible, flexuous, rod-shaped virus. Antiserum made against the purified virus reacted specifically in enzyme-linked immunosorbent assay to the virus and to the potexvirus foxtail mosaic virus (FoMV), indicating that the sorghum virus was an isolate of FoMV. Comparison of the sorghum isolate (H93) to FoMV PV 139 showed that H93 differed biologically by causing severe symptoms in sorghum, not readily infecting certain barley lines, and causing only faint symptoms in barley. At the molecular level, the capsid of H93 had a mass of 23.9 kDa and 217 amino acid residues compared with 23.7 kDa and 215 residues previously reported for the nucleic acid sequence of FoMV. The amino acid sequences of the two viruses were greater than 96% identical. They varied by having four substitutions, one deletion, and three insertions between residues 66 and 67. This is the first report of natural infection of sorghum by FoMV, thus extending its host range among cereal crops.

Nucleotide sequence and in vitro translation of the coat protein gene of cymbidium mosaic virus

The nucleotide sequence of the coat protein gene of cymbidium mosaic virus (CyMV) has been determined from a cDNA clone that encompassed the 3'-region of the virus genome. The 0.75 kb coat protein open reading frame (ORF) codes for a 24kD polypeptide. The coat protein coding region was separated from the 3'-poly(A) terminal of the viral genome by an 80 base noncoding region. The derived amino acid sequence of this ORF showed considerable homology with other potexvirus coat protein sequences, notably with narcissus mosaic virus (NMV), potato virus X (PVX), and white clover mosaic virus (WClMV). In vitro expression of the CyMV coat protein gene was carried out by T7 RNA polymerase on the linearized cDNA clone template, followed by in vitro translation of the purified transcript in wheat germ extract. One of the translated products was found to comigrate with the coat protein of the virus at about 24 kD, as determined by NaDodSO4-PAGE. This translated product was also found to crossreact with antibodies raised against CyMV.

Coat protein and protease activity as in vitro translation products of potato carlavirus M RNA

Genomic-size RNA isolated from purified potato carlavirus M (PVM) was translated in both the reticulocyte and the wheat germ cell-free, messenger-dependent systems. The PVM RNA translated the same set of major products in both in vitro systems. The Mr values of the most prominent polypeptides observed consistently were 185,000 (P185), 147,000 (P147), 94,000 (P94), 87,000 (P87), 72,000 (P72), 67,000 (P67), 52,000 (P52), 46,000 (P46), 35,000 (P35) and 25,000 (P25). Relatively low amounts of a translation product of Mr 200,000 (P200) were often detectable in both systems. The P35 polypeptide displayed the same molecular weight and one-dimensional peptide map as the virus coat protein (CP), and was precipitated by antibodies raised against PVM and PVM CP. The kinetics of appearance of the in vitro synthesized polypeptides suggested that primary translation products of high molecular weight undergo post-translational proteolytic cleavage.

Translational efficiency and competitive ability of mRNAs with 5'-untranslated alpha beta-leader of potato virus X RNA

The 5'-untranslated leader sequence of potato virus X (PVX) RNA (63 nucleotides apart from cap-structure) consists of two sub-sequences referred to as alpha-sequence (41 nucleotides with no G) and beta-sequence (42 nucleotides upstream from the first AUG). Computer-based folding predictions suggest that the 5'-proximal region of alpha beta-leader is unstructured. The second structural feature of alpha beta-leader is the presence of the sequences apparently complementary to the 3'-terminal region of 18S rRNA. The alpha beta-leader has been shown to strongly enhance the translation of the contiguous foreign gene (NPT1) transcripts in cell-free translation systems from rabbit reticulocytes (RRL), wheat germ (WG) and Krebs-2 ascite cell extract (KA). In competitive translation PVX, RNA strongly inhibited tobacco mosaic virus (TMV) RNA, in RRL and WG systems. No competition occurred between PVX and TMV RNAs in KA system. There was no correlation between the translational efficiency and competitive ability of PVX RNA in different cell-free translation systems. The competitive ability did not solely depend on the presence of alpha beta-leader in mRNA. We present evidence to suggest that alpha beta-leader together with about 150 bases of the coding sequence is responsible for the translation competitive ability of PVX RNA.

Control and expression of 3' open reading frames in clover yellow mosaic virus

The genomic RNA of clover yellow mosaic virus (CYMV) contains at least seven open reading frames (ORFs) which are organized in a more elaborate array than in other sequenced members of the potexvirus group. We have investigated the strategy by which ORFs located in the 3' region of CYMV's genomic RNA are differentially expressed by correlating the location of the 5' termini of the two abundant viral subgenomic RNAs with their coding potential. We have mapped the 5' termini of the subgenomic RNAs precisely to the nucleotide level and have shown that both are capped. The larger 2.1-kb subgenomic RNA encodes as its 5' ORF a 25-kDa polypeptide, whose function is unknown. The smaller 1.0-kb subgenomic RNA can encode only the 23-kDa coat protein. All four ORFs in the 3' 1095 residues of CYMV are efficiently expressed in vitro, but of these only coat protein, which can be expressed from a subgenomic RNA, is detectable in CYMV infected tissue. For this reason, we believe that expression of ORFs in the 3' one-third of CYMV RNA are controlled at the transcriptional level.