Specific suppressor tRNAs reverse nonsense-mediated regulation. (A) Constructs A and C are described in the Fig. 1 legend. Construct J contains a UAG nonsense codon in place of the UAA nonsense in construct C but is otherwise identical. (B and C ) Northern blot analysis of total cellular RNA isolated from HeLa cells co-transfected with the constructs (4 μg) shown in A, along with plasmids encoding UAA-, UAG-, and UGA-specific suppressor tRNAs (12 μg). As with Fig. 1, neomycin mRNA levels were used as a measure of transfection efficiency, and similar loading of RNA in all lanes was demonstrated by methylene blue staining of 18S and 28S rRNA (data not shown), performed as described (42).