1. Rabbit reticulocytes or reticulocyte extracts incorporate cysteine into cysteinyl transfer ribonucleic acid (Cys-tRNA) having a blocked α-amino group (X-Cys-rRNA). The lack of a free α-amino group is revealed in the electrophoretic characteristics of X-cysteinyladenosine (X-Cys-A) obtained from X-Cys-tRNA by treatment with pancreatic ribonuclease, and in the failure of the X-Cys-A to react with acetic anhydride. The electrophoretic characteristics of X-Cys-A and its reactivity with diazomethane indicate that the blocking group X might have a free carboxyl group. X-Cys-A does not react with iodoacetate, possibly indicating that its sulfhydryl group is blocked. 2. Both high and low molecular weight fractions of the reticulocyte extract (obtained by fractionation by gel filtration through a Sephadex G-25 column) are needed for converting Cys-tRNA into X-Cys-tRNA. X-Cys-tRNA was not found in Escherichia coli extracts, but E. coli Cys-tRNA is converted to X-Cys-tRNA when added to a reticulocyte extract. 3. NaF is thought to inhibit a step in peptide chain initiation and to lead to the accumulation of initiation complexes (W. Hoerz and K. S. McCarthy, Proc. Natl. Acad. Sci. U.S., 63 (1969) 1206). A tRNA species involved in chain initiation in reticulocytes is tRNAMetf. Intact reticulocytes treated with NaF contain about 7 times more X-Cys-tRNA and 4 times more tRNAMetf (at least partially acylated with methionine) bound to ribosomes than control cells. The amount bound to ribosomes of several other aminoacyl-tRNA's tested (including Cys-tRNA), however, decreases upon treatment with NaF. 4. The structure, biosynthesis, and function of X-Cys-tRNA remain to be established.
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