The levels of nonsense suppression related to Itt1p overexpression. Suppression efficiency was determined in the strain 1A-H8-B2 (itt1::LEU2) and its derivative, 1A-H8-B2-R (itt1::LEU2 SUP35-C). The strains carried either one of the plasmids YCplac22-ITT1 (centromeric ITT1), YEplac112-ITT1 (multicopy ITT1) or pITT1-SUP45 (multicopy plasmid with both ITT1 and SUP45), in combinations with pUKC815, pUKC817, pUKC818 or pUKC819. The levels of readthrough of UAA (a), UAG (b) or UGA (c) nonsense codons were determined as described in Materials and Methods. Values are the mean of three independent assays ± SD. The expression status of ITT1 and other relevant genes is shown below the graph: Single, ITT1 expressed from centromeric plasmid; Multi, expression of ITT1 or SUP45 from multicopy plasmids; C, chromosomal SUP35-C deletion allele; WT, chromosomal SUP35 or SUP45 wild-type alleles.