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Transition of the R factor R12 in Proteus mirabilis
Abstract:
When Proteus mirabilis harboring the R factor R12 (a round of replication mutant of the R factor NR1) is cultured in medium containing streptomycin there can be an amplification in the number of copies of r-determinants per cell and the formation of enlarged polygenic R factors containing repeated sequences of r-determinants as well as polygenic molecules consisting of repeated sequences of r-determinants. This phenomenon has been referred to as the "transition." When transitioned cells are then cultured in drug-free medium, within a few generations two distinct density species of R factor deoxyribonucleic acid (DNA) are observed in a CsCl density gradient: a 1.712 g/ml band of covalently closed circular R factor DNA consisting of one transfer factor (RTF-TC) plus one r-determinant and a 1.718 g/ml band consisting of repeated sequences of r-determinants. The RTF-TC component of the R factor appears to control the replication of all the R factor DNA which is attached to it. In the autonomous state, however, polygenic sequences of r-determinants do not appear to replicate under the same control mechanism as when they are attached to an RTF-TC.
Insights
Proteus mirabilis undergoes a "transition" in streptomycin, amplifying r-determinants and forming enlarged R factors. Subsequent culture in drug-free media reveals distinct R factor DNA species, indicating differential replication control.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- R factors are plasmids conferring antibiotic resistance.
- R12 is a mutant R factor of NR1, affecting replication.
- Streptomycin induces changes in R factor structure and copy number.
Purpose of the Study:
- To investigate the phenomenon of R factor amplification and structural changes in Proteus mirabilis.
- To analyze the behavior of R factor DNA after induced transition and subsequent culture in drug-free conditions.
Main Methods:
- Culturing Proteus mirabilis with R factor R12 in streptomycin-containing medium.
- Inducing the
- transition
- phenomenon.
- Analyzing R factor DNA species using CsCl density gradients in drug-free medium.
Main Results:
- Amplification of r-determinant copies and formation of enlarged, repetitive R factors during transition.
- Separation into two distinct R factor DNA species (1.712 g/ml and 1.718 g/ml) after culturing in drug-free medium.
- Identification of a 1.712 g/ml band as covalently closed circular R factor DNA (RTF-TC + r-determinant) and a 1.718 g/ml band as repeated r-determinant sequences.
Conclusions:
- The RTF-TC component of R factors regulates the replication of attached DNA.
- Autonomous polygenic r-determinant sequences replicate independently of the RTF-TC control mechanism.