Aminoglycoside antibiotics in Escherichia coli and yeast can cause ribosomes to read through stop codons during translation. This can result in the phenotypic suppression of nonsense mutations. We show here for the first time that the aminoglycosides G-418 and paromomycin have similar effects in monkey (COS-7) cells in vivo. Suppression of an amber mutation (TAG) by aminoglycosides can restore the activity of a mutant gene transfected into COS-7 cells to almost 20% of wild type levels.

An evaluation has been made of the E. coli beta-galactosidase (beta-gal) gene for use as a reporter gene in mammalian cells in culture. We have adopted a histochemical procedure which enables identification of those cells within a population that express the introduced bacterial gene. Data is presented concerning the sensitivity of the histochemical method relative to an immunological method of detection. It has been found that several clonal cell lines generated after transfection of human 293 cells with a Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter-beta-gal construction are mosaic for expression of the introduced mini-gene. Furthermore, after treatment of these clonal cell lines with the nucleoside analog 5-aza-cytidine (5-aza-C), an increase in production of beta-gal under control of this promoter element was observed.

We have compared the suppression of nonsense mutations by aminoglycoside antibiotics in Escherichia coli and in human 293 cells. Six nonsense alleles of the chloramphenicol acetyl transferase (cat) gene, in the vector pRSVcat, were suppressed by growth in G418 and paromomycin. Readthrough at UAG, UAA and UGA codons was monitored with enzyme assays for chloramphenicol acetyl transferase (CAT), in stably transformed bacteria and during transient expression from the same plasmid in human 293 tissue culture cells. We have found significant differences in the degree of suppression amongst three UAG codons and two UAA codons in different mRNA contexts. However, the pattern of these effects are not the same in the two organisms. Our data suggest that context effects of nonsense suppression may operate under different rules in E. coli and human cells.

We describe the generation of an amber mutation in the chloramphenicol acetyltransferase (cat) gene of the mammalian cell transfection vector pRSVcat (Gorman et.al. (1982), Proc.Natl.Acad.Sci. 79 6777-6791). We have demonstrated the in vivo suppression of this amber mutation in monkey and human cells by co-transfection with a synthetic Xenopus suppressor tRNATyr under the control of the late SV40 promoter. The vector, pRSVcatamb38, may be used to quantitate amber suppression in various mammalian cells.

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