5S rRNA is involved in fidelity of translational reading frame

Abstract

Chromosomal mutants (maintenance of frame = mof) in which the efficiency of -1 ribosomal frameshifting is increased can be isolated using constructs in which lacZ expression is dependent upon a -1 shift of reading frame. We isolate a new mof mutation, mop, in Saccharomyces cereuisiae and show that it is complemented by both single and multi-copy 5 S rDNA clones. Two independent insertion mutations in the rDNA locus (rDNA::LEUZ and rDNA::URA3) also display the Mof-phenotype and are also complemented by single and multicopy 5 S rDNA clones. Mutant 5 S rRNAs expressed from a plasmid as 20-50% of total 5 S rRNA in a wild-type host also induced the Mof-phenotype. The increase in frameshifting is greatest when the lac2 reporter gene is expressed on a high copy, episomal vector. No differences were found in 5 S rRNA copy number or electrophoretic mobilities in mopstrains. Both m o p and rDNA::LEUZincrease the efficiency of +1 frameshifting as well but have no effect on readthrough of UAG or UAA termination codons, indicating that not all translational specificity is affected. These data suggest a role for 5 S rRNA in the maintenance of frame in translation.A LTHOUGH correct maintenance of frame is critical to ribosome function, a growing number of cases of directed alterations in translational reading frame have been identified, mostly in viruses (e.g., retroviruses, coronaviruses BAUGH 1993). The study of these ribosomal frameshifts is important both because of their critical role in animal and plant pathogens and because of the information they provide about the mechanisms by which reading frame is normally maintained.Ribosomal frameshifting in the -1 direction in retroviruses, (+) sRNA viruses and dsRNA viruses generally requires a special sequence, X XXY W Z (the 0-frame is indicated by spaces) called the "slippery site" (JACKS et al. 1988). Here the simultaneous slippage of ribosome-bound A-and P-site tRNAs by one base in the 5' direction still leaves their nonwobble bases correctly paired in the new reading frame. A second promoting element (JACKS et al. 1988), usually an RNA pseudoknot, is located immediately 3' to the slippery site (BRIERLEY et , TEN DAM et al. 1992. The RNA pseudoknot makes the ribosome pause over the slippery site (TU et al. 1992, SOMOCYI et al. 1993, increasing the probability of 5' ribosomal movement. The efficiency of -1 ribosomal frameshifting can be affected by the ability of the ribosome-bound tRNAs (especially the Asite tRNA) to un-pair from the 0-frame (DINMAN et al. , BRIERLEY et al. 1992, the ability of these tRNAs to re-pair to the -1 frame (JACKS et al. 1988), and the relative position of the RNA pseudoknot from the slippery site and its thermodynamic stability (BRIERLEY et al. 1989,1991; DINMAN and WICKNER 1992). The efficiency of -1 ribosomal frameshifting can also be affected by mutations in cellular gene products that presumably interact with these tRNA and mRNA factors WICKNER 1992, 1994). Changes in any of these components can be observed as changes in the effici...