5S-RIBOSOMAL-RNA IS INVOLVED IN FIDELITY OF TRANSLATIONAL READING FRAME

Chromosomal mutants (maintenance of frame = mof) in which the efficien cy of -1 ribosomal frameshifting is increased can be isolated using co nstructs in which lacZ expression is dependent upon a -1 shift of read ing frame. We isolate a new mof mutation, mof9, in Saccharomyces cerev isiae 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::LEU2 and rDNA::URA3) also display the Mof(-) phenotype and are also complemented by single and multi-copy 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 frameshi fting is greatest when the lacZ reporter gene is expressed on a high c opy, episomal vector. No differences were found in 5 S rRNA copy numbe r or electrophoretic mobilities in mof9 strains. Both mof9 and rDNA::L EU2 increase the efficiency of +1 frameshifting as well but have no ef fect on readthrough of UAG or UAA termination codons, indicating that not ail translational specificity is affected. These data suggest a ro le for 5 S rRNA in the maintenance of frame in translation.