Evidence for a function for the UAA codon in vivo

THE three known and well characterized amber suppressors¹ are almost certainly transfer RNAs altered by single base substitutions in their anticodons, the altered anticodon being the complement to the amber codon (UAG)^1-6. Studies on the conversion of these amber suppressors to ochre suppressors, presumed still to be charged with the same amino-acids, showed that at least these three ochre suppressors are probably also transfer RNAs with altered anticodons^5,6. As a result of previous work, we had at our disposal cells containing no suppressor (su^-), a class 1 amber suppressor presumed to insert serine, and a class 1 ochre suppressor also presumed to insert serine. Cells containing a class 1 amber suppressor were derived from an su^- parent cell, probably by a single base change in the DNA information specifying the anticodon of a seryl-transfer RNA. Cells containing a class 1 ochre suppressor were derived from cells containing a class 1 amber suppressor by suppressor conversion⁵. The growth characteristics of su^- cells and cells containing a class 1 amber suppressor are always similar, and differ from those for cells containing a class 1 ochre suppressor. The latter cells usually have reduced growth rates, support the growth of a smaller number of amber mutants of T4 phage, and give rise to reduced burst sizes of T4⁺. These effects of the presence of an ochre suppressor could result from anticodon-codon (UUA-UAA) interactions if UAA codons are normally used as chain terminators during translation.