Abstract
Nonsense mutations in capsid genes F and G of phages S13 and phi X174 decreased the expression of genes downstream. These polar effects were suppressed by ultraviolet irradiation of the host before infection. Activities of the downstream genes were restored to between 30 and 95% of their normal levels, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the viral proteins. The effect of ultraviolet irradiation may be to suppress rhodependent termination of transcription. The polarity of one mutation was poorly suppressed by ultraviolet irradiation.