Genetic evolution of bacteriophage. II. Physical length of the homologous region in a hybrid between serologically and morphologically unrelated phages.

P22 and P221 DNA were labeled with (3)H-thymidine and hybridized with P22 DNA, P221 DNA, or Fels 1 DNA immobilized on Millipore membrane filters. With this technique, 31% of the P22 genome was computed to be homologous to the P221 genome. Since the molecular weight of P22 DNA is 2.65 x 10(7) daltons, the homologous region between P22 and P221 should be equivalent to about 8.2 x 10(6) daltons and contain 13,300 nucleotide pairs or about 4400 triplets. Moreover, 46% of the P221 genome was computed to be homologous to the P22 genome. Therefore, the molecular weight of P221 DNA was calculated to be about 1.80 x 10(7) daltons. Sucrose gradient centrifugation showed that the molecular weight of P221 DNA was 2.01 x 10(7) daltons. Thus, the P221 genome is about 25 to 30% smaller than the P22 genome. Further-more, 54% of the P221 genome was found to be homologous to the Fels 1 genome. However, the labeled P22 DNA hybridized with Fels 1 DNA was about 2% of the labeled P22 DNA hybridized with P22 DNA.