BEET SOIL-BORNE VIRUS-RNA-3 - A FURTHER EXAMPLE OF THE HETEROGENEITY OF THE GENE CONTENT OF FUROVIRUS GENOMES AND OF TRIPLE GENE BLOCK-CARRYING RNAS

The complete nucleotide sequence of RNA 3 of the Ahlum isolate of beet soil-borne virus (BSBV) was determined from cDNAs obtained with immun ocaptured virus particles and denatured preparations of dsRNA. BSBV RN A 3 is unique among the plant virus RNAs studied so far in containing apparently only the coding sequences of a triple gene block (TGB). The derived amino acid sequences of the three putative TGB-encoded protei ns showed the highest level of sequence similarities with those of the corresponding proteins of potato mop top furovirus (PMTV) followed by those of peanut clump furovirus and barley stripe mosaic hordeivirus. Progressively fewer similarities were found with the TGB-encoded prot eins of beet necrotic yellow vein virus (uncertain classification), po tato X potexvirus, and potato M carlavirus. The 3'-terminal 78 nucleot ides of BSBV RNA 3 can be folded into a tRNA-like structure and a high degree of sequence similarity exists between the 122 3'-terminal nucl eotides of BSBV RNA 3 and PMTV RNA 2. In other regions, however, no pr onounced sequence similarities were found between the two RNAs, and PM TV RNA 2 contains an additional putative gene for a cysteine-rich prot ein downstream of the TGB. The two viruses are unrelated serologically . BSBV RNA 3 adds a further variant to the heterogeneity of the gene c ontent of furovirus genomes and of triple gene block-carrying RNAs. (C ) 1996 Academic Press, Inc.