Protein Synthesis with Ribosomes Selected for the Incorporation of ß-Amino Acids.

In an earlier study, ß³-puromycin was used for the selection of modified ribosomes, which were utilized for the incorporation of five different ß-amino acids into Escherichia coli dihydrofolate reductase (DHFR). The selected ribosomes were able to incorporate structurally disparate ß-amino acids into DHFR, in spite of the use of a single puromycin for the selection of the individual clones. In this study, we examine the extent to which the structure of the ß³-puromycin employed for ribosome selection influences the regio- and stereochemical preferences of the modified ribosomes during protein synthesis; the mechanistic probe was a single suppressor tRNA(CUA) activated with each of four methyl-ß-alanine isomers (1-4). The modified ribosomes were found to incorporate each of the four isomeric methyl-ß-alanines into DHFR but exhibited a preference for incorporation of 3(S)-methyl-ß-alanine (ß-mAla; 4), i.e., the isomer having the same regio- and stereochemistry as the O-methylated ß-tyrosine moiety of ß³-puromycin. Also conducted were a selection of clones that are responsive to ß²-puromycin and a demonstration of reversal of the regio- and stereochemical preferences of these clones during protein synthesis. These results were incorporated into a structural model of the modified regions of 23S rRNA, which included in silico prediction of a H-bonding network. Finally, it was demonstrated that incorporation of 3(S)-methyl-ß-alanine (ß-mAla; 4) into a short α-helical region of the nucleic acid binding domain of hnRNP LL significantly stabilized the helix without affecting its DNA binding properties.