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 anticodons5,6. 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.
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