A mutation in the R body-coding sequence destroys expression of the killer trait in P. tetraurelia

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• Caedibacter taeniospiralis is a bacterial endosymbiont of Paramecium.
• Wild-type strains produce refractile (R) bodies and exhibit a killer trait against sensitive paramecia.
• The killer trait is mediated by toxins associated with R bodies.

Purpose of the Study:

• To characterize a mutant strain of Caedibacter taeniospiralis 169.
• To investigate the role of refractile (R) bodies in the killer trait.
• To determine the genetic basis of the mutation affecting R body production.

Main Methods:

• Isolation and characterization of a mutant Caedibacter taeniospiralis strain.
• Plasmid DNA analysis to identify the mutation.
• Genetic mapping of the transposon insertion site.

Main Results:

• A mutant strain (Caedibacter taeniospiralis 169) was identified that does not produce refractile (R) bodies.
• This mutant strain does not kill sensitive paramecia.
• The mutation resulted from the insertion of a 7.5-kilobase transposon-like element into the R body-coding region of plasmid pKAP169.
• Despite lacking R bodies and killer activity, the mutant strain conferred resistance to its host against wild-type Caedibacter taeniospiralis.

Conclusions:

• Refractile (R) body synthesis is essential for the expression of the killer trait in Caedibacter taeniospiralis.
• The insertion of a transposon-like element disrupts R body production and consequently the killer phenotype.
• Host resistance can be conferred independently of R body production and direct killing.