The anticodon of an ochre-suppressing derivative of E. coli tRNATyrL, previously identified as UUA, can contain a modified uridlne (U+) in the first position. The novel modified nudeotide has been identified by twodimensional thin layer chromatography following RNase T2 digestion of anticodon contalnlng fragments. U+R is found in less than stolchiometric molar yields in preparations of tRNATyrL Su+Oc. The electrophoretlc mobility of U+P is the same as Up at pH 3.5 and PH 7.5. U+ probably does not contain sulfur since it cannot be labeled with 35S in vivo incorporation experiments.
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