Use of trinucleotide sequences as mRNA. (a) Twenty [14C]aminoacyl-tRNAs were formed in separate reactions, using the 20 different 14C-labeled amino acids, and each was added to a preparation of ribosomes, along with a three-nucleotide RNA of defined sequence (a codon). Individual reaction mixtures were then passed through a nitrocellulose filter that binds protein, trapping the [14C]aminoacyl-tRNA bound to the triplet RNA codon on the ribosome. The reaction shown here used [14C]Phe-tRNAPhe and the UUU codon. (b) Results obtained with several codons (UUU, AAA, CCC) and [14C]aminoacyl-tRNAs.