Alteration of the ATG start codon of the A* protein of bacteriophage [phi]X174 into an ATT codon yields a viable phage indicating that A* protein is not essential for [phi]X174 reproduction

Bacteriophage X174 gene A encodes two proteins: the gene A protein and the smaller A* protein, which is synthesized from a translational start signal within the A gene in the same reading frame as the gene A protein. The gene A protein is involved in initiation, elongation and termination of rolling circle DNA replication. The role of the A* protein in the life cycle of X174, however, is unknown. Using oligonucleotide-directed mutagenesis a viable Xl74 mutant was constructed in which the ATG start codon of the A* protein was changed into an ATT codon. This mutant, X-4499T, does not synthesize A* protein. The burst size of X-4499T amounted to 50% of that of wild type XI74. This indicates that A* protein, although advantageous for phage reproduction, is not essential during the life cycle of bacteriophage X174