- Volume 79, Issue 2, 1998

The consensus secondary structure of the virion proteins (VPs) of tymoviruses was predicted from their amino acid sequences using a combination of computer methods: profile alignment, hydropho- bicity moment and ‘PredictProtein’. All methods predicted that they were eight-stranded anti-parallel β-barrels with two α-helical regions. The predicted structure was used to design recombinants of turnip yellow mosaic virus (TYMV) in which selected parts of its VP were replaced with homologous regions of belladonna mottle virus (BeMV) in a cDNA clone encoding the genome of TYMV. Six of ten such recombinants were fully viable and most gave symptoms in Chinese cabbage indistinguishable from those of TYMV, although they did not always infect plants systemically and none infected hosts of BeMV or of other tymoviruses. A TYMV recombinant with the N-terminal part of its VP replaced with the E71 epitope of Plasmodium falciparum was also viable, but others with the same region replaced with the V3 region of the lentivirus human immunodeficiency virus type 1 were not. Epitope analysis of antisera prepared against the virions of parental TYMV and some of the recombinants showed that, although the N terminus of the VP is immuno- genically dominant, it is not exposed at the surface of the virion, a finding confirmed by comparing the electrophoretic mobilities of the virions. The recently published structure of the TYMV VP determined by X-ray crystallography confirms the accuracy of the predicted secondary structure of the VP, and hence the utility of the methods used.

We have determined the nucleotide sequence and located the major in vivo transcript termini of the Lacanobia oleracea granulovirus (LoGV) egt gene. The open reading frame encodes a 460-amino acid polypeptide having extensive sequence similarity to ten nucleopolyhedrovirus (NPV) ecdysteroid UDP- glucosyltransferase (EGT) proteins; the degree of similarity is particularly high within several previously identified EGT‘domains’, and eight invariant amino acid residues are conserved. A phylogenetic tree, constructed by the neighbour joining method, showed LoGV EGT to be the most highly diverged of the eleven baculovirus sequences compared. Database searching revealed that part of a published DNA sequence from Cryptophlebia leucotreta granulovirus appears to encode the N-terminal region of EGT, and the relative genomic locations of the egt and granulin genes in that virus were compared with their positions in LoGV. In infected L. oleracea larvae, egt is transcribed predominantly as a1 6 kb mRNA. Primer extension analysis suggested that the major egt 5 transcription terminus is located within a baculovirus late gene promoter motif (GTAAG), in contrast to the early gene promoter contexts determined by others for three NPV egt mRNA 5′ ends. An early transcriptional start site is also used in LoGV egt expression, but at a much lower level. The 3′ terminus of egt mRNA was identified by sequencing DNA fragments generated by rapid amplification of cDNA ends, and is located 58–62 nucleotides beyond the translation stop codon.