NUCLEOTIDE-SEQUENCE OF THE GENOME OF THE BACTERIOPHAGE ALPHA-3 - INTERRELATIONSHIP OF THE GENOME STRUCTURE AND THE GENE-PRODUCTS WITH THOSE OF THE PHAGES, PHI-X174, G4 AND PHI-K

The complete nucleotide sequence of the genome of the circular single-stranded DNA (isometric) phage alpha-3 has been determined and compared with that of the related phages phi-X174 and G4. The alpha-3 genome consists of 6087 nucleotides, which is 701 nucleotides longer than the nucleotide sequence of the phi-X174 genome and 510 nucleotides more than that of the G4 genome. The results demonstrated that the three phage species have 11 homologous genes (A, A*, B, C, K, D, E, J, F, G and H), the order of which is fundamentally identical, suggesting that they have evolved from a common ancestor. The sequence of some genes and untranslated intergenic regions, however, differs significantly from phage to phage: for example, the degree of amino acid sequence homology of the gene product is averaged at 47.7% between alpha-3 and phi-X174 and 46.9% between alpha-3 and G4, and alpha-3 has a remarkable longer intergenic region composed of 758 nucleotides between the genes H and A compared with the counterparts of phi-X174 and G4. Meanwhile, in vivo experiments of genetic complementation showed that alpha-3 can use none of the gene products of phi-X174 and G4, whereas the related phage phi-KA can rescue alpha-3 nonsense mutants of the genes B, C, D and J. These sequencing and in vivo rescue results indicated that alpha-3 is closely related to phi-KA, but distantly remote from phi-X174 or G4, and supported an evolutional hypothesis which has been so far proposed that the isometric phages are classified into three main groups: the generic representatives are phi-X174, G4 and alpha-3.