3, 4 OR MORE - THE TRANSLATIONAL STOP SIGNAL AT LENGTH

Translational stop signals are defined in the genetic code as UAA, UAG and UGA, although the mechanism of their decoding via protein factors is clearly different from that of the other codons, There are strong biases in the upstream and downstream nucleotides surrounding stop cod ons. Experimental tests have shown that termination-signal strength is strongly influenced by the identity of the nucleotide immediately dow nstream of the codon (+4), with a correlation between the strength of this four-base signal and its occurrence at termination sites, The +4 nucleotide and other biases downstream of the stop codon may reflect s ites of contact between the release factor and the mRNA, whereas upstr eam biases may be due to coding restrictions, with the release factor perhaps recognizing the final tRNA and the last two amino acids of the polypeptide undergoing synthesis. This means that the translational s top signal is probably larger than the triplet codon, but its exact le ngth will be clearer when it is known which nucleotides are in direct contact with the release factor. Ultimately it will be defined exactly when a crystal structure of the release factor with its recognition s ubstrate becomes available.