A modified uridine in the anticodon of E. coli $${\hbox{ tRNA }}_{\hbox{ 1 }}^{\hbox{ Tyr }}\hbox{ \hspace }{\hbox{ su }}_{\hbox{ oc }}^ $$

The anticodon of an ochre-suppressing derivative of E. coli tRNA I Tyr, previously identified as UUA, can contain a modified uridine (U+) in the first position. The novel modified nucleotide has been identified by two-dimensional thin layer chromatography following RNase T2 digestion of anticodon-containing fragments. Up+ is found in less than stoichiometric molar yields in preparations of tRNA I Tyr su + oc. The electrophoretic mobility of Up+ 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.