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Sequence and organization of the nonstructural proteins and noncoding regions of melon necrotic spot virus Rivière, Carol Jeanne


Melon necrotic spot virus (MNSV), is an isometric plant virus with a monopartite, positive-sense, single-stranded RNA genome. This thesis reports the nucleotide sequence of regions of the MNSV genome which are noncoding or which encode nonstructural proteins, as well as the number and approximate sizes of the subgenomic RNAs produced by MNSV during infection. These data show that the MNSV genome comprises at least 4,262 nucleotides encoding, in two different reading frames, three or four nonstructural proteins for which there is some evidence of in vivo expression. These proteins, listed in order of their location on the genome from the 5' terminus, have molecular weights of ca. 29,000 (p29), 89,000 (p89), 7,000 (p7) and 14,000 (pl4). The p29 and P89 proteins are probably translated from genomic length RNA, while p7 and pl4 are probably translated from a ca. 1.9 kb subgenomic RNA. A second subgenomic RNA of ca. 1.6 kb is the likely template for translation of the 3' proximal, 42 kDa coat protein. Expression of p89 and p14 requires read-through of the amber termination codons of p29 and p7, respectively. MNSV p89 is the putative viral replicase; its read-through domain contains the GDD motif characteristic of the RNA-dependent RNA polymerases of several plant and animal viruses. The MNSV replicase shows very little amino acid sequence similarity with the replicases of viruses from either of the two established virus supergroups. The MNSV polymerase, however, shows a high degree of similarity with the polymerases of maize chlorotic mottle virus as well as carmo-, tombus- and luteoviruses which have been suggested to form a third virus supergroup. The role of p29 is unknown, but it presumably functions in replication. The functions of p7 and p14 are also unknown, but may be related to virus transport. Although MNSV shows amino acid sequence similarity in some of its proteins with viruses in several plant virus groups, it shows the most extensive similarities with members of the carmovirus group. MNSV also closely resembles these viruses in the number, sizes and genomic organization of its proteins as well as in its probable translation strategy. For these reasons, as well as previously reported physico-chemical similarities with the carmoviruses, MNSV should be classified as a member of the carmovirus group.

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