Spacer-length dependence of programmed -1 or -2 ribosomal frameshifting on a U(6)A heptamer supports a role for messenger RNA (mRNA) tension in frameshifting

Lin, Zhaoru; Gilbert, Robert J. C.; Brierley, Ian

An interesting question is whether À2 FS is more widely exploited in virus or cellular gene expression. Only one example of a À2 FS signal has been documented to date, involved in the expression of tail assembly chaperone genes in bacteriophage Mu (55, 56) . Here, tandem À2 slippage occurs on a GG GGG CGA with the anticodon of the A-site tRNA Arg ( 3 0 GCI 5 0 , where I in the wobble position is inosine) forming a more stable post-slippage contact with the mRNA in the À2 frame rather than the À1 frame. Another potential À2 FS signal in Trichomonas vaginalis virus 1 has been suggested (57) . In this virus, frameshifting most likely occurs on a conserved CC CUU UUU sequence, compatible with tandem À2 shifting. Examination of known viral À1 FS sites possessing a U 6 A or A 6 C slippery sequence, however, reveals that in most cases the spacing distances seem inappropriately long for efficient À2 FS. In HIV-1, where the stimulatory RNA forms immediately 3 0 of a U 6 A slippery sequence (58), a stop codon in the À2 reading frame is present directly downstream of the U 6 A heptamer and appears to be present in all isolates of HIV-1 (59, 60) . Any ribosomes entering the À2 reading frame would terminate immediately, generating a truncated Gag polyprotein lacking viral proteins p1 and p6. As yet, there is no evidence to suggest that such a species is expressed in HIV-1 infected cells. Frameshifting in the expression of mammalian ornithine decarboxylase antizyme has remarkably been shown to be +1 in mammals and fission yeast, yet À2 in budding yeast (61, 62) . Precise details of the mechanism of the À2 FS remain to be elucidated, but as observed by Matsufuji and colleagues, lengthening the spacing between the antizyme shift site and its pseudoknot by three bases increased +1 FS in yeast at the expense of À2 FS, supporting a link between spacer length and frameshift direction (61) . Intriguingly, replacing the 3 0 -stimulatory element that forms a component of the antizyme frameshifting signal with an annealed AON has been shown also to stimulate À1 FS when placed with zero spacing (50) . However, this À1 product is not seen with the natural antizyme frameshift signal. Conclusive evidence of a functional À2 FS signal in virus or cellular genes is therefore still awaited.