Bacteriophage ϕX174 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 ϕX174, however, is unknown. Using oligonucleotide-directed mutagenesis a viable ϕXl74 mutant was constructed in which the ATG start codon of the A* protein was changed into an ATT codon. This mutant, ϕX-4499T, does not synthesize A* protein. The burst size of ϕX-4499T amounted to 50% of that of wild type ϕXI74. This indicates that A* protein, although advantageous for phage reproduction, is not essential during the life cycle of bacteriophage ϕX174.
