REVERSAL OF MUTANT PHENOTYPES BY 5-FLUOROURACID: AN APPROACH TO NUCLEOTIDE SEQUENCES IN MESSENGER-RNA

Genetic information in DNA is apparently expressed via transcription into RNA messengers [1-5,10] which in turn act as the templates for protein synthesis. Thus, incorporation of base analogues into messenger-RNA could lead to errors in the reading of the message into an amino acid sequence. The effect would be an alteration of phenotype without a permanent change in the DNA genotype. A promising analogue for this purpose is 5-fluorouracil (5FU), which is readily incorporated into RNA, mostly in place of uracil [6]. Modification of proteins by 5FU has been reported by Naono and Gros [7] and by Bussard et al. [8] who found that the enzymes alkaline phosphatase and β-galactosidase synthesized in the presence of the analogue are abnormal. 
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\nA very sensitive method for the detection of induced errors in the translation of genetic information is to begin with a mutant that is defective in some function so that errors causing the appearance of a small amount of activity can be easily observed. This approach has been successful [9] using rII mutants of phage T4, which are ordinarily unable to grow on strain K of E. coli. Addition of 5FU after infection of the cell can partially reverse the phage mutant phenotype, leading to active development of the phage. The response is highly specific, occurring only with certain rnI mutants and not others within the same cistron. Since, after phage infection, synthesis of ribosomal RNA and S-RNA is almost totally arrested [10], it is plausible that the effect of 5FU on phage mutants is due to its incorporation into messenger-RNA.