Heterogeneity in the 3 '-terminal untranslated region of tobacco mild green mosaic tobamoviruses from Nicotiana glauca resulting in variants with three or six pesudoknots

Isolates of tobacco mild green mosaic tobamovirus (TMGMV) were obtained fro m 58 plants of Nicotiana glauca in southern California and placed in one of two groups (Small type and Large type) based on the size of the subgenomic RNA for the coat protein (CP). The CP sequence differed by no more than on e amino acid for the two types, and the Small type was identical to that pu blished for TMGMV. Thirty-six of the isolates had a double-stranded (ds)RNA profile that matched that of type TMGMV, and the nucleotide sequence of th e 3' untranslated region (3'UTR) of six of these isolates was similar to th e published sequence of TMGMV. Twenty-two isolates had a larger dsRNA for t he CP subgenomic RNA. Six of these were sequenced and all had a repeat sequ ence of between 147 and 165 bases in the part of the 3'UTR that is involved in the formation of pseudoknots. These novel but common isolates are predi cted to have six rather than three pseudoknots, Small types (three pseudokn ots = type TMGMV) yielded twice as much virus after purification as Large t ypes (six pseudoknots). The two groups of isolates could be distinguished i n N. rustica (Large type, but not Small type gave a systemic infection), an d N. clevelandii (Small type but not Large type induced systemic lethal nec rosis), Almost all isolates of TMGMV used in this study were initially asso ciated with satellite tobacco mosaic virus (STMV), and both types supported STMV experimentally.