UNUSUAL ANTICODON LOOP STRUCTURE FOUND IN ESCHERICHIA-COLI LYSINE TRANSFER-RNA

Although both tRNA(Lys) and tRNA(Glu) of E.coli possess similar antico don loop sequences, with the same hypermodified nucleoside 5-methylami nomethyl-2-thiouridine (mnm(5)s(2)U) at the first position of their an ticodons, the anticodon loop structures of these two tRNAs containing the modified nucleoside appear to be quite different as judged from th e following observations. (I) The CD band derived from the mnm(5)s(2)U residue is negative for tRNA(Glu), but positive for tRNA(LyS). (2) Th e mnm(5)s(2)U monomer itself and the mnm(5)s(2)U-containing anticodon loop fragment of tRNA(Lys) show the same negative CD bands as that of tRNA(Glu) . (3) The positive CD band of tRNA(Lys) changes to negative when the temperature is raised. (4) The reactivity of the mnm(5)s(2)U residue toward H2O2 is much lower for tRNA(Lys) than for tRNA(Glu). Th ese features suggest that tRNA(Lys) has an unusual anticodon loop stru cture, in which the mnm(5)s(2)u residue takes a different conformation from that of tRNA(Glu); whereas the mnm(5)s(2)U base of tRNA(Glu) has no direct bonding with other bases and is accessible to a solvent, th at of tRNA(Lys) exists as if in some way buried in its anticodon loop. The limited hydrolysis of both tRNAs by various RNases suggests that some differences exist in the higher order structures of tRNA(Lys) and tRNA(Glu). The influence of the unusual anticodon loop structure obse rved for tRNA(Lys) On it, function in the translational process is als o discussed.