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Virulent mutants of bacteriophage p22.I. Isolation and genetic analysis
Abstract:
Mutants of phage P22 which form plaques on a P22 lysogen have been isolated. These virulent mutants have been classified into three groups. (i) VirA mutants arise spontaneously in wild-type stocks and form very small turbid plaques on a P22 lysogen. The single mutation responsible for VirA virulence maps near the mnt locus, one of the immunity regions of phage P22. (ii) VirB mutants do not arise spontaneously and have been isolated only from mutagenized P22 stocks. VirB mutants form small, clear plaques on a P22 lysogen. One of the VirB mutants, virB-3, was analyzed in detail. The virB-3 mutant is comprised of two mutations: K5, which maps within the c(2) gene, and Vx, which maps in the region between the c(2) and c(3) genes. Phages carrying either the K5 or Vx mutation are not virulent in themselves but mutate to VirB virulence at a frequency of 10(-5) to 10(-6). It is concluded that K5 and Vx are mutations at specific sites which together confer the ability to undergo phage development in the presence of repressor. (iii) VirC mutants are defined by a large clear plaque morphology when plated on a P22 lysogen. VirC mutants are comprised of the determinants of both VirA and VirB virulence.
Insights
Researchers identified three types of virulent mutants in bacteriophage P22, including VirA, VirB, and VirC. These mutants exhibit distinct plaque morphologies on P22 lysogens, indicating different mechanisms for overcoming phage immunity.
Area of Science:
- Bacteriophage genetics
- Molecular biology
- Microbial genetics
Background:
- Bacteriophage P22 is a model system for studying viral replication and lysogeny.
- Lysogens are bacterial cells infected with a temperate bacteriophage, which typically prevents superinfection by other phages of the same type.
Purpose of the Study:
- To isolate and characterize virulent mutants of bacteriophage P22 that can form plaques on a P22 lysogen.
- To understand the genetic basis of virulence in bacteriophage P22 and its ability to overcome host immunity.
Main Methods:
- Isolation of spontaneous and mutagenized virulent mutants of bacteriophage P22.
- Plaque morphology analysis on P22 lysogenic lawns.
- Genetic mapping of mutations conferring virulence.
Main Results:
- Three classes of virulent mutants (VirA, VirB, VirC) were identified based on plaque morphology.
- VirA mutants arise spontaneously and map near the P22 mnt locus.
- VirB mutants require specific mutations (K5 and Vx) in combination to confer virulence, suggesting a complex genetic interaction.
Conclusions:
- Virulence in bacteriophage P22 can arise from mutations in different genetic loci.
- VirB virulence results from the combined effect of two distinct mutations, indicating a synergistic interaction.
- VirC mutants appear to possess determinants for both VirA and VirB virulence.