Protein synthesis directed by an amber coat-protein mutant of the RNA phage MS2

Synthesis of viral coat protein or of a coat-protein fragment was examined in various host cells infected with a mutant strain of the RNA phage MS2. The mutant (MU9) has an amber nonsense mutation near the middle of the coat protein gene. Apparently the mutation has no polar effect. When protein synthesis is directed by MU9 under the non-suppressing condition, it would be expected that a peptide fragment of coat protein would be produced by premature termination of peptide synthesis at the site of the amber mutation. However, little coat protein or coat protein fragment was produced in MU9-infected non-suppressor cells; but the coat protein fragment was indeed synthesized in an in vitro protein-synthesizing system when MU9 RNA was used as messenger. The suppression of MU9 was examined by analyzing the production of intact coat protein under suppressing conditions in vivo and in vitro. Two suppressor hosts ( Su-I and Su-III) were used which are known to suppress efficiently amber mutations in the alkaline phosphatase gene of Escherichia coli and the head protein gene of the bacteriophage T4D. It was found that Su-I was a good suppressor of MU9 both in vivo and in vitro, whereas Su-III was a very poor suppressor in vivo and a reasonably good suppressor (about half as good as Su-I) in vitro. These results are interpreted to indicate that although viral RNA is synthesized vigorously in MU9-infected non-suppressor or suppressor ( Su-III) cells, such RNA is incapable of directing the active synthesis of coat protein or coat-protein fragment. It is suggested that the synthesis of functional coat protein is a prerequisite for the preferential synthesis of viral coat protein.