Use of nucleotide analogs by class I and class II CCA-adding enzymes (tRNA nucleotidyltransferase): Deciphering the basis for nucleotide selection

Stereochemistry of CCA addition. Addition of the R_P and S_P isomers of CTP or ATP to tRNA lacking CA or A was assayed under standard conditions using 100 μM CTP (R_P and S_P) or 200 μM ATP (R_P and S_P). Although modest addition was observed when the concentration of the R_P isomer of ATP was increased to 600 μM (data not shown), this presumably reflects incorporation of S_P isomer slightly contaminating the R_P preparation; mixing experiments suggested that a 10% contamination of R_P with S_P could account for the data.