Comparison of the G4 and φX174 phage genomes by electron microscopy

The order of G4 Haemophilus influenzae Rd (HindII) DNA fragments was mapped by electron microscopy. G4 HindII DNA fragments were annealed to φX174 viral singles-tranded DNA circles and found to partially hybridize. Eight to eighteen percent of the φX174 DNA circles had a G4 DNA fragment attached, depending upon the fragment and the time of annealing. The G4 HindII restriction enzyme cleavage map was aligned with the φX174 restriction enzyme cleavage map by analyzing under the electron microscope partial heteroduplex molecules consisting of φX174 single-stranded viral DNA circles annealed with φX174 Arthrobacter luteus (AluI) DNA fragments as markers and G4 HindII DNA fragments. These results indicate that the gene order of the bacteriophages G4 and φX174 is the same. Despite the high frequency of φX174/G4 heteroduplex DNA molecules observed under the electron microscope, heterologous marker rescue experiments with G4 DNA fragments and φX174 mutant viral DNA strand circles were not successful, except in the case of a φX174 amber mutation in gene E (am3)