4S RNA. This is the RNA species which actually makes the connection between genetic code and amino acid. There are 61 different tRNA species -- one for each of the 64 possible codons except the termination signals. Each tRNA has an anticodon segment, with three exposed bases complementary to a particular codon. Each tRNA can be "charged" with a particular amino acid by a specific amino acyl-tRNA transferase. During translation, the tRNA which bears the appropriate anticodon base pairs with the mRNA which is being translated. The tRNA is then bound to the A-site on the small ribosomal subunit by elongation factor Tu. The ribosome then catalyzes the formation of a peptide bond between the tRNA-bound amino acid and the growing protein chain. Elongation factor G then moves the peptidyl-tRNA complex to the P-site of the ribosome. The tRNA-peptide link is cleaved on arrival of the next tRNA at the A site. The uncharged tRNA then moves to the E site and is released. Specific tRNAs are usually abbreviated with the 3-letter amino acid designation in subscript. Thus the tRNAs for alanine are abbreviated tRNAala. See also, aminoacyl-tRNA.