| Translation normally occurs in a linear fashion reading nonoverlapping nucleotide triplets. Alternatively, some genes require translation in two overlapping reading frames. The ability of the ribosome to change to an alternative reading frame is called frameshifting. Frameshifting occurs at a specific site in the mRNA sequence, which usually allows the tRNA(s) bound to the ribosome to repair in the new reading frame. Frameshifting can be stimulated by both cis- and trans-acting elements. Frameshift stimulatory elements are thought to function by either pausing the ribosome at the frameshift site, decreasing the normal accuracy of the translating ribosome, or by aiding in the repositioning of the message into the new frame.; A +1 frameshift is required for synthesis of functional antizyme protein. Antizyme frameshifting occurs at the site UCC UGA. The termination codon of the shift site is thought to pause the ribosome. Pausing at the shift site is likely aided by a 3' pseudoknot structure. The pseudoknot likely stimulates frameshifting by additional unknown mechanisms beyond the ribosomal pause. A 50 nucleotide sequence 5' of the shift site on antizyme mRNA is also stimulatory, though the mechanism is unclear. Finally, antizyme frameshifting is stimulated by polyamines. Antizyme is a key negative regulator of polyamine biosynthesis, and therefore the frameshift event acts as a sensor and regulator of cellular polyamine levels.; Very little is known about how polyamines stimulate antizyme frameshifting. Analysis presented here demonstrates that polyamine stimulation of frameshifting requires a termination codon as part of the shift site. Polyamines appear to interfere with the termination process increasing the likelihood of the frameshift event. High levels of polyamine stimulation require the 5' element. In contrast, the pseudoknot stimulates frameshifting in a polyamine independent manner. The assay developed to studying polyamine stimulation of antizyme frameshifting has many potential uses. It can be used to screen libraries of small molecules for their ability to stimulate frameshifting. A diverse array of polyamine analogs was shown to stimulate the antizyme frameshifting. The assay may also be useful in discovering a tmRNA-like activity, which targets aberrant proteins for degradation in eukaryotes. |
