An important 2 '-OH group for an RNA-protein interaction

We have-investigated the role of 2'-OH groups in the specific interaction b etween the acceptor stem of Escherichia coli tRNA(Cys) and cysteine-tRNA sy nthetase,This interaction provides for the high aminoacylation specificity observed for cysteine-tRNA synthetase, A synthetic RNA microhelix that reca pitulates the sequence of the acceptor stem was used as a substrate and var iants containing systematic replacement of the 2'-OH by 2'-deoxy or 2'-O-me thyl groups were tested. Except for position U73, all substitutions had lit tle effect on aminoacylation, Interestingly; the deoxy substitution at posi tion U73 had no:effect on aminoacylation, but the 2'-O-methyl substitution decreased aminoacylation by 10-fold and addition of the even bulkier 2'-O-p ropyl group decreased aminoacylation by another 2-fold. The lack of an effe ct by the deoxy substitution suggests that the hydrogen bonding potential o f the 2'-OH at position U73 is unimportant for aminoacylation. The decrease in activity upon alkyl substitution suggests that the 2'-OH group instead provides a monitor of the steric environment during the RNA-synthetase inte raction. The steric role was confirmed in the context of a reconstituted tR NA and is consistent with the observation that the U73 base is the single m ost important determinant for aminoacylation and therefore is a:site that i s likely to be in close contact with cysteine-tRNA synthetase. A steric rol e is supported by an NMR-based structural model of the acceptor stem, toget her with biochemical studies of a closely related microhelix, This role sug gests that the U73 binding site for cysteine-tRNA synthetase is sterically optimized to accommodate a 2'-OH group in the: backbone, but that the hydro xyl group itself is not:involved,in specific hydrogen bonding interactions.