- Volume 69, Issue 7, 1988

Following intracerebral infection of hamsters with scrapie agent replication started with or without a very short lag phase. Infectivity titres increased exponentially within 35 to 40 days post-infection to a maximum level of 3 × 109 LD50 per brain and then remained constant until death. Minimal detectable amounts of scrapie-associated fibrils (SAF) appeared at 42 days and reached high levels 56 days after inoculation. The first clinical symptoms were diagnosed at about 65 days and animals died after 85 to 95 days. These data confirm earlier results in which peripheral infection first revealed agent replication, then SAF formation and finally clinical disease. Unconventional virus diseases, therefore, can best be described as virus-induced, organ-specific amyloidoses.

Head-to-tail dimers of cloned DNA A and DNA B of tomato golden mosaic virus (TGMV) were integrated between the T-DNA border sequences of a broad host range binary vector and transferred into cells of Nicotiana benthamiana seedlings using an Agrobacterium tumefaciens-mediated delivery system. Most of the inoculated plants developed golden-yellow mosaic and leaf curling symptoms typical of TGMV infection. Extracts of infected leaves were shown to contain both double-stranded and single-stranded TGMV DNA forms of genome length and the virus capsid polypeptide. Infection was also achieved by inoculating plants with mixtures of Agrobacterium strains containing dimers of DNA A or DNA B, with a strain containing a partial dimer of DNA A and a dimer of DNA B and with a strain containing a dimer of DNA B and a partial dimer of DNA A with a 603 bp deletion in the coat protein gene. In the latter case, mosaic symptoms were mild and leaves did not curl. Transgenic N. tabacum cv. Samsun plants containing head-to-tail dimers of DNA A (A2 plants) or DNA B (B2 plants) were produced by transformation with Ti plasmid vectors. A2 plants and B2 plants were agroinfected with dimeric DNA B and dimeric DNA A, respectively. In both cases, symptoms typical of TGMV infection were induced and viral single-stranded DNA of both components was detected in the systemically infected tissue.

Mild proteolysis by trypsin of particles of six potyviruses (bean yellow mosaic virus, clover yellow vein virus, Johnson grass mosaic virus, passion-fruit woodiness virus, potato virus Y and watermelon mosaic virus II) revealed that the N- and C-terminal regions of their coat protein are exposed on the particles’ surfaces. The enzyme treatment removed the N-terminal region (30 to 67 amino acids long, depending on the virus) and 18 to 20 amino acids from the C terminus of the coat proteins, leaving a fully assembled virus particle composed of coat protein cores consisting of 216 or 218 amino acid residues. These core particles were indistinguishable from untreated native particles in an electron microscope and were still infectious. The core particles lacked the virus-specific surface epitopes that are recognized by the bulk of the polyclonal antibodies raised against the whole virus particles. Epitopes thought to be group-specific were located in the trypsin-resistant core protein region. The implications of these findings are discussed in relation to the similar surface location of the N- and C-terminal regions of the coat protein of other rod-shaped plant viruses and the observed common structural features displayed by isometric plant and animal viruses.

The sequence of 1232 nucleotides from the 3′ region of the plum pox virus (PPV) RNA has been determined. This sequence contains one long open reading frame (ORF). The sequence of the 17 amino-terminal amino acids of the PPV capsid protein was determined chemically. An identical amino acid sequence was found in the translation product of the large ORF, starting at amino acid -330, which is alanine. Our data suggest that the PPV capsid protein, like that of other potyviruses, is a product of the maturation of a large polyprotein. The putative cleavage site is at a glutamine-alanine dipeptide. The capsid protein gene consists of 990 nucleotides and corresponds to a region coding for 330 amino acids which have a combined calculated M r of 36593. The adjacent 3′ untranslated region is of 215 nucleotides and ends in a polyadenylate tract. PPV capsid protein molecules are thus larger than those of the other potyviruses that have been characterized. The amino acid sequence of PPV coat protein is 47 to 60% homologous to other potyvirus coat proteins and most of the variations are in the amino-terminal region.

The sequence of the 7356 nucleotide [excluding the 3′ poly(A)] RNA-1 of tomato black ring virus (TBRV) was determined from overlapping cDNA clones. A putative initiation codon at nucleotide 261 was considered to be the start of an open reading frame which terminates at a UAG codon at position 7053. The predicted translation product is a protein of 2264 amino acids with an M r of 253680 (254K). Comparison of the amino acid sequence of this 254K protein with other viral proteins revealed three regions, each having about 60% homology with a region of the three cowpea mosaic virus (CPMV) proteins (58K, 24K, 87K) which are thought to be involved in replication of the CPMV RNAs. The same regions show similarities, although less extended, with proteins 2C, 3C and 3D of picornaviruses. From these results we suggest that the genome organization of TBRV is similar to that of comoviruses and picornaviruses.