A model of secondary structure is proposed for the 3′-terminal sequence of the satellite tobacco mosaic virus (STMV) RNA on the basis of phylogenetic comparisons with tobacco mosaic virus (TMV) genomic RNA. Sequence homologies and compensatory base changes found between the two related viral RNAs imply that the 3′-end of STMV RNA folds into a tRNAlike domain similar to that found in the TMV RNA. Accordingly, functional assays showed that STMV RNA can be aminoacylated in vitro with histidine by yeast histidyl-tRNA synthetase to plateaus reaching 30%. Histidylation properties of STMV RNA were compared to those of TMV RNA and of a canonical yeast tRNAHis transcript which both are chargeable to nearly 100% plateau levels. Kinetic data indicate an excellent catalytic efficiency of STMV RNA charging expressed as VmaxIKm ratio, quasi-equivalent to that of TMV RNA, and only 17-fold reduced as compared to that of the yeast tRNAHis transcript. Biological implications of the structural mimicry between the tRNA-like regions of TMV and STMV RNAs are discussed in the light of the relationships of a satellite virus with its helper virus. This is the first report on a chargeable tRNA-like structure at the 3′-end of a satellite virus RNA.

Author notes

+;Present address: Department of Biochemistry, University of California at Riverside, Riverside, CA 92521, USA