DNA Packaging Specificity in the λ-Like Phages: Gifsy-1

Area of Science:

• Virology
• Molecular Biology
• Genetics

Background:

• DNA viruses require specific mechanisms to recognize and package their genomes into new virions, distinguishing viral DNA from host DNA.
• λ-like bacteriophages infecting Escherichia coli serve as valuable models for studying large DNA virus genome packaging.
• Gifsy-1, a Salmonella phage related to λ, provides an opportunity to investigate DNA packaging specificity differences.

Purpose of the Study:

• To compare the DNA packaging specificity of the Salmonella phage Gifsy-1 with closely related phages λ, 21, and N15.
• To elucidate the molecular basis for differential DNA packaging efficiencies observed among these phages.
• To explore the evolutionary implications of observed packaging specificities in the context of their host bacteria.

Main Methods:

• In vivo packaging assays were performed to assess the efficiency of different phages packaging heterologous and homologous DNA.
• Bioinformatic analysis of database sequences of enteric prophages was conducted to determine the distribution of specific DNA packaging determinants.
• Comparative analysis of phage DNA packaging systems, including helix-turn-helix motifs and packaging sites (cosB).

Main Results:

• Gifsy-1 and phage λ exhibit similar DNA packaging specificities, with Gifsy-1 phage packaging λ DNA at ~50% efficiency and λ phage packaging Gifsy-1 DNA at ~30% efficiency.
• Phage 21 demonstrated very low packaging efficiencies for λ, N15, and Gifsy-1 DNAs (0.01%, 0.01%, and 1%, respectively).
• Database analysis indicated that Gifsy-1's packaging determinants are primarily found in Salmonella, while λ's specificity is rare in Salmonella, suggesting host-specific adaptation.

Conclusions:

• Gifsy-1 and λ likely diverged from a common ancestor, with their distinct packaging specificities reflecting adaptation to different host environments.
• A proposed incompatibility between the phage 21 helix-turn-helix motif and the cosB site may explain its inability to efficiently package Gifsy-1 DNA.
• The observed differences in DNA packaging specificity among these phages highlight the evolutionary pressures shaping viral genome packaging and host adaptation.