Using an in vitro prokaryotic termination assay, it was demonstrated that sequences neighbouring UA are recognized by RF-1 and stimulate cleavage of ribosome-bound f-met-tRNA_fmet. The ability of UA to signal release depends upon the nature of nucleotides adjacent both 3' and 5' to this sequence. RF-1 exhibits different specificity when potential termination sequences are covalently linked to AUG within the same polynucleotide, as in mRNA. Under these circumstances, within certain base context, (1) UA functions as a termination signal, (2) UA-containing terminator signals can be read out of the AUG-aligned reading frame and (3) RF-1 competes with aminoacyl-tRNA for sequence UUA. Another factor has been discovered, which partially corrects the specificity of RF-1. This factor (designated Specificity Factor) appears to be a protein, or a protein-containing component, and enhances RF-1-mediated termination caused by UAA but inhibits termination caused by UA. The factors known to participate in protein synthesis are not responsible for conferring specificity to the RF-1-mediated termination reaction. For this reason, it is believed that the Specificity Factor may be a new protein. / Thesis / Master of Science (MSc)

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