Molecular biology of potexviruses: recent advances

Generation of infectious transcripts from Korean strain and mild mottle strain of potato virus X

Full-length cDNAs of two different strains of Potato virus X (PVX-Kr and PVX-Mo) have been directly amplified by long template reverse transcription polymerase chain reaction (RT-PCR) using the 5'-end primer containing a SP6 or T7 RNA promoter sequence and the virus-specific 3'-end primer, and then constructed in plasmid vectors. Capped in vitro transcripts from cloned full-length cDNAs as well as those RT-PCR amplicons proved to be infectious systemically on tobacco plants. Symptom expression on tobacco plants from PVX-Mo transcripts was faster and severer than that from PVX-Kr. In replication stability test of transcripts derived from PVX clones, progeny viruses showed stable replication according to sequencing through passages. This highly infectious transcript system from the full-length cDNA clones for PVX can be useful for recombinant molecules for functional analysis of viral proteins in plant-virus interaction study as well as for expression of foreign protein in planta.

Molecular characterization of Lolium latent virus, proposed type member of a new genus in the family Flexiviridae

Lolium latent virus (LoLV) was recently detected in the USA for the first time in ryegrass hybrids (Lolium perenne x Lolium multiflorum). The genome of one USA isolate, LoLV-US1, has now been fully sequenced. The positive strand genomic RNA is 7674 nucleotides (nt) long and is organized in five open reading frames (ORFs) encoding the replication-associated protein, the movement-associated triple gene block proteins and the coat protein (CP). The genome organization is similar to that of viruses in the genera Potexvirus and Foveavirus; however, analysis of the complete LoLV genomic sequence, phylogenetic analyses of the deduced amino acid (aa) sequences of the polymerase and the CP, presence of a putative ORF 6, and the in vivo detection of two CPs in equimolar amounts, highlight features peculiar to LoLV. These characteristics indicate that LoLV forms a monotypic group separate from existing genera and unassigned species within the family Flexiviridae, for which we propose the genus name Lolavirus. One-step RT-PCR was developed for quick and reliable LoLV detection.

Characterization of a flowering cherry strain of Cherry necrotic rusty mottle virus

The host range and complete nucleotide sequences of two Cherry necrotic rusty mottle virus (CNRMV) isolates (FC4 and FC5) infecting flowering cherry accessions imported from Japan are described. Of the plants tested, cherry, peach, apricot and almond became infected, but only sweet cherry cv. 'Canindex', Nanking cherry and apricot cv. 'Tilton' showed a mild foliar mottle. The genomic sequences of CNRMV-FC4 and CNRMV-FC5 are 8,430 and 8,429 nt in length, excluding the 3' poly (A) tail. They contain seven open reading frames encoding for a putative virus replicase, "triple gene block" proteins, a coat protein and two proteins with unknown functions. The two CNRMV-FC isolates share 96% identity in the genomic sequences, and their genome organizations are virtually identical to that of a German CNRMV isolate (CNRMV-GER). However, they differ from CNRMV-GER by 14% in the overall nucleotide sequence and 2% (ORF2) to 30% (ORF5a) in the derived amino acid sequences of individual gene products.

Triple gene block protein interactions involved in movement of Barley stripe mosaic virus

Barley stripe mosaic virus (BSMV) encodes three movement proteins in an overlapping triple gene block (TGB), but little is known about the physical interactions of these proteins. We have characterized a ribonucleoprotein (RNP) complex consisting of the TGB1 protein and plus-sense BSMV RNAs from infected barley plants and have identified TGB1 complexes in planta and in vitro. Homologous TGB1 binding was disrupted by site-specific mutations in each of the first two N-terminal helicase motifs but not by mutations in two C-terminal helicase motifs. The TGB2 and TGB3 proteins were not detected in the RNP, but affinity chromatography and yeast two-hybrid experiments demonstrated that TGB1 binds to TGB3 and that TGB2 and TGB3 form heterologous interactions. These interactions required the TGB2 glycine 40 and the TGB3 isoleucine 108 residues, and BSMV mutants containing these amino acid substitution were unable to move from cell to cell. Infectivity experiments indicated that TGB1 separated on a different genomic RNA from TGB2 and TGB3 could function in limited cell-to-cell movement but that the rates of movement depended on the levels of expression of the proteins and the contexts in which they are expressed. Moreover, elevated expression of the wild-type TGB3 protein interfered with cell-to-cell movement but movement was not affected by the similar expression of a TGB3 mutant that fails to interact with TGB2. These experiments suggest that BSMV movement requires physical interactions of TGB2 and TGB3 and that substantial deviation from the TGB protein ratios expressed by the wild-type virus compromises movement.