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.

Genetic evolution of bacteriophage. I. Hybrids between unrelated bacteriophages P22 and Fels 2

A new bacteriophage species, designated F22, was isolated from phage P22 stocks grown on Salmonella typhimurium Q1 lysogenic for Fels 2 at a frequency of less than 10(-11). P22 has a very short tail with a hexagonal base plate and six spikes. Phage Fels 2 is morphologically similar to E. coli T-even phages, having a long tail with a contractile sheath and carrying no genetic region related to P22. Phage F22 is morphologically and serologically indistinguishable from Fels 2, but carries the c(c(1), c(2), and c(3)) markers of P22. The color markers h(21), g, and m(3) of P22 do not appear in F22. Thus, F22 is evidently a recombinant between the unrelated bacteriophages P22 and Fels 2. The recombination between unrelated bacteriophages could play an important role in the evolution of bacteriophages.

Chelating agent shock of bacteriophage T5

When two strains of phage T5 (heat-susceptible form T5st(+) and its heat-resistant mutant T5st) were placed in solutions containing various high concentrations of chelating agents (sodium citrate and ethylenediaminetetraacetic acid) at room temperature, they could be effectively inactivated by rapid dilution in distilled water of relatively low temperatures (2 to 37 C). This phenomenon has been termed "chelating agent shock" (CAS). The susceptibility of phage T5 to CAS increased with an increase in the concentration of chelating agents and with an increase in temperature of the water used for rapid dilution. Under any given condition, T5st(+) was much more sensitive to CAS than was T5st. Phage T5 was protected against inactivation by the addition of monovalent or divalent metal salts, but not by the addition of nonionic solutes, to the shocking water prior to CAS treatment. This finding is compatible with the view that cations combined with the phage protein are removed by the chelating agent, although no metal ion has been identified in the phage protein. Alternatively, since the chelating agents used are polyanions, they may bind relatively tightly to the protein subunits in the head of T5, thereby distorting the structure of the phage head. Rapid dilution of these distorted particles could lead to loss of phage DNA. No evidence for recovery of phage activity could be obtained by the addition of metal salts to the inactivated phage after CAS. The morphological properties of phage inactivated by CAS are similar to those of heat-inactivated T5 phage. Electron micrographs showed that most of the phage particles consisted of empty head membranes; some of the particles had lost their tails. Both heritable and nonheritable resistance to heat was accompanied by resistance to CAS in phage T5. The sensitive element detected by each test seemed to be the same.

Inactivation by nitrogen mustard of single- and double-stranded DNA and RNA bacteriophages

Nitrogen mustard inactivates bacteriophage containing single-stranded DNA and RNA as well as double-stranded DNA. Inactivation may occur by intrastrand cross-linkage in DNA or RNA as well as by interstrand crosslinkage between complementary strands of DNA.