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Lethal transposition of Mud phages in Rec- strains of Salmonella typhimurium
R V Sonti1, D H Keating, J R Roth
1Department of Biology, University of Utah, Salt Lake City 84112.
Abstract:
Under several circumstances, the frequency with which Mud prophages form lysogens is apparently reduced in rec strains of Salmonella typhimurium. Lysogen formation by a MudI genome (37 kb) injected by a Mu virion is unaffected by a host rec mutation. However when the same MudI phage is injected by a phage P22 virion, lysogeny is reduced in a recA or recB mutant host. A host rec mutation reduces the lysogenization of mini-Mu phages injected by either Mu or P22 virions. When lysogen frequency is reduced by a host rec mutation, the surviving lysogens show an increased probability of carrying a deletion adjacent to the Mud insertion site. We propose that the rec effects seen are due to a failure of conservative Mu transposition. Replicative Mud transposition from a linear fragment causes a break in the host chromosome with a Mu prophage at both broken ends. These breaks are lethal unless repaired; repair can be achieved by Rec functions acting on the repeated Mu sequences or by secondary transposition events. In a normal Mu infection, the initial transposition from the injected fragment is conservative and does not break the chromosome. To account for the conditions under which rec effects are seen, we propose that conservative transposition of Mu depends on a protein that must be injected with the DNA. This protein can be injected by Mu but not by P22 virions. Injection or function of the protein may depend on its association with a particular Mu DNA sequence that is present and properly positioned in Mu capsids containing full-sized Mu or MudI genomes; this sequence may be lacking or abnormally positioned in the mini-Mud phages tested.
Insights
Bacterial DNA repair mechanisms (Rec functions) are crucial for Salmonella typhimurium lysogen formation. Rec mutations impair Mud phage lysogeny, especially when injected by P22 virions, suggesting a failure in conservative transposition.
Area of Science:
- Microbiology
- Molecular Biology
- Bacteriology
Background:
- Mud prophages are mobile genetic elements in Salmonella typhimurium.
- Bacterial Rec functions are involved in DNA repair and recombination.
- Lysogen formation is a key step in bacteriophage life cycles.
Purpose of the Study:
- To investigate the role of host Rec functions in Mud prophage lysogenization.
- To determine how different phage injection mechanisms affect lysogeny in rec mutant strains.
- To elucidate the mechanism underlying Rec-dependent effects on Mud transposition.
Main Methods:
- Comparative analysis of Mud prophage lysogenization frequencies in wild-type and rec mutant Salmonella typhimurium strains.
- Utilizing different phage injection systems (Mu and P22 virions) for MudI and mini-Mu phages.
- Characterization of chromosomal deletions in surviving lysogens.
Main Results:
- Host rec mutations reduce lysogen formation for certain Mud phages, particularly when injected by P22 virions.
- Mini-Mu phages show reduced lysogenization in rec mutants regardless of the injecting phage.
- Lysogens surviving rec-mediated reduction exhibit increased deletions near the Mud insertion site.
Conclusions:
- Rec functions are essential for efficient conservative transposition of Mu-based phages.
- A phage-encoded protein, likely injected by Mu but not P22 virions, is critical for conservative transposition.
- Failure in conservative transposition due to missing or improperly positioned Mu DNA sequences leads to chromosomal breaks and reduced lysogeny.