Comparative DNA Sequence Analysis of the G4 and ϕX174 Genomes

Abstract

G4 was isolated in 1974 (Godson 1974b) and was quickly shown to be different from ϕX174 in some aspects of its physiology and DNA replication (see below). At this time DNA sequencing methods employing ϕX DNA as a model system were being developed in this laboratory, which resulted in the determination of the entire 5386-nucleotide sequence of the ϕX genome (Sanger et al. 1977b; an updated version of this sequence is given in this volume in Appendix I). A natural next step was to apply these methods to a closely related virus such as G4 and to compare the sequences.¹ We will present the results of this comparison after first reviewing the biological differences between G4 and ϕX.

Biological Differences and Similarities between G4 and ϕX
G4 has the same host range as ϕX; it forms plaques on Escherichia coli C strains but not on K or B strains (Godson 1974b). Its growth is restricted by the E. coli K (Eco K) and B (Eco B) restriction systems, and it forms plaques with low efficiency on E. coli K and B spheroplasts and on some K/C hybrid cells (L. Dumas, pers. comm.). Growth of G4 is also restricted in cells containing the plasmid RY13 because G4 has a single EcoRI cleavage site (Godson and Boyer 1974). Multiplication of ϕX is not restricted in any of these systems.

The G4 genome is 5577 nucleotides long, compared with 5386 for ϕX, and it has a different G + T content. This causes...