Phenotypic suppression of opal mutants by L-tryptophan

L-tryptophan (codon UGG) at high concentrations, usually 20 mM, phenotypically suppressed all four phage T4 opal (codon UGA) mutants tested. The suppression was incomplete. With three of the opal mutants, H301, H317, and H340, the suppression resulted in marked increases in plaque size and clarity in the restrictive host Escherichia coli strain B. With the very tight opal mutant H429, the suppression was discerned only by a 1,200-fold increase in phage yield from restrictive host cells in the chemically defined medium. Comparisons of L-tryptophan with seven other amino acids showed that only L-tryptophan provided a high magnitude of phenotypic suppression. The possibility that phenotypic suppression by L-tryptophan can be used in a screen for opal mutants of viruses of species without known opal suppressor mutations is discussed.