Role of the constant uridine in binding of yeast tRNA^Phe anticodon arm to 30S ribosomes

Twenty-two anticodon arm analogues were prepared by joining different tetra penta and hexaribonucleotides to a nine nucleotide fragment of yeast tRNA^Phe with T4 RNA ligase. The oligomer with the same sequence as the anticodon arm of tRNA^Phe binds poly U programmed 30S ribosomes with affinity similar to intact tRNA^Phe. Analogues with an additional nucleotide in the loop bind ribosomes with a weaker affinity whereas analogues with one less nucleotide in the loop do not bind ribosomes at all. Reasonably tight binding of anticodon arms with different nucleotides on the 5' side of the anti-codon suggest that positions 32 and 33 in the tRNA^phe sequence are not essential for ribosome binding. However, differences in the binding constants for anticodon arms containing modified uridine residues in the "constant uridine" position suggest that both of the internal "U turn" hydrogen bonds predicted by the X-ray crystal structure are necessary for maximal ribosome binding.