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Transformation of restriction endonuclease phenotype in Streptococcus pneumoniae
Abstract:
The genetic basis of the unique restriction endonuclease DpnI, that cleaves only at a methylated sequence, 5'-GmeATC-3', and of the complementary endonuclease DpnII, which cleaves at the same sequence when it is not methylated, was investigated. Different strains of Streptococcus pneumoniae isolated from patients contained either DpnI (two isolates) or DpnII (six isolates). The latter strains also contained DNA methylated at the 5'-GATC-3' sequence. A restrictable bacteriophage, HB-3, was used to characterize the various strains and to select for transformants. One laboratory strain contained neither DpnI nor Dpn II. It was probably derived from a DpnI-containing strain, and its DNA was not methylated at 5'-GATC-3'. Cells of this strain were transformed to the DpnI restriction phenotype by DNA from a DpnI-containing strain and to the DpnII restriction phenotype by DNA from a DpnII-containing strain. Neither cross-transformation, that is, transformation to one phenotype by DNA from a strain of the other phenotype, nor spontaneous conversion was observed. Extracts of transformants to the new restriction phenotype were shown to contain the corresponding endonuclease.
Insights
The genetic basis of restriction enzymes DpnI and DpnII in Streptococcus pneumoniae was studied. Transformation experiments confirmed that specific DNA sequences determine whether DpnI or DpnII activity is present.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Restriction-modification systems are crucial for bacterial defense and genome regulation.
- DpnI and DpnII are complementary restriction endonucleases recognizing the 5'-GATC-3' sequence, differing in their methylation sensitivity.
- Understanding the genetic basis of these enzymes is key to their function in bacterial strains.
Purpose of the Study:
- To investigate the genetic basis of DpnI and DpnII restriction endonuclease activity in Streptococcus pneumoniae.
- To determine the inheritance and expression of restriction phenotypes in different bacterial strains.
Main Methods:
- Isolation and characterization of Streptococcus pneumoniae strains with varying DpnI and DpnII activity.
- Use of a characterized bacteriophage (HB-3) for strain typing and selection of transformants.
- Transformation experiments using DNA from DpnI- or DpnII-containing strains to modify a DpnI/DpnII-negative strain.
Main Results:
- Streptococcus pneumoniae strains were found to possess either DpnI or DpnII activity, correlating with DNA methylation status.
- A laboratory strain lacking both enzymes was successfully transformed to exhibit either DpnI or DpnII restriction phenotype using homologous DNA.
- Cross-transformation between DpnI and DpnII phenotypes and spontaneous conversion were not observed, indicating specific genetic determinants.
Conclusions:
- The restriction phenotype (DpnI or DpnII) in Streptococcus pneumoniae is genetically determined and can be transferred via DNA transformation.
- The study elucidates the genetic basis for differential DNA methylation and restriction by DpnI and DpnII endonucleases.