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Characterization of a unique methyl-specific restriction system in Streptomyces avermitilis
1Exploratory Microbiology and Genetics, Merck Sharp & Dohme Research Laboratories, Rahway, New Jersey 07065.
Abstract:
Streptomyces avermitilis contains a unique restriction system that restricts plasmid DNA containing N6-methyladenine or 5-methylcytosine. Shuttle vectors isolated from Escherichia coli RR1 or plasmids isolated from modification-proficient Streptomyces spp. cannot be directly introduced into S. avermitilis. This restriction barrier can be overcome by first transferring plasmids into Streptomyces lividans or a modification-deficient E. coli strain and then into S. avermitilis. The transformation frequency was reduced greater than 1,000-fold when plasmid DNA was modified by dam or TaqI methylases to contain N6-methyladenine or by AluI, HhaI, HphI methylases to contain 5-methylcytosine. Methyl-specific restriction appears to be common in Streptomyces spp., since either N6-methyladenine-specific or 5-methylcytosine-specific restriction was observed in seven of nine strains tested.
Insights
Streptomyces avermitilis possesses a unique DNA restriction system targeting N6-methyladenine and 5-methylcytosine. This barrier can be bypassed by intermediate host transfers, enabling successful plasmid introduction into S. avermitilis.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Streptomyces avermitilis exhibits a unique restriction system.
- This system targets modified DNA bases, specifically N6-methyladenine and 5-methylcytosine.
- Standard shuttle vectors from E. coli or modified Streptomyces plasmids cannot be directly transformed into S. avermitilis.
Purpose of the Study:
- To characterize the unique restriction system in Streptomyces avermitilis.
- To identify methods for overcoming the restriction barrier for efficient plasmid transformation.
- To investigate the prevalence of methyl-specific restriction in other Streptomyces species.
Main Methods:
- Plasmid transformation experiments were conducted using modified and unmodified DNA.
- DNA modification was performed using specific methylases (dam, TaqI, AluI, HhaI, HphI).
- Transformation efficiency was assessed across different host strains, including S. avermitilis, S. lividans, and E. coli.
Main Results:
- Transformation frequency decreased over 1,000-fold when plasmid DNA contained N6-methyladenine or 5-methylcytosine.
- A two-step transformation strategy involving intermediate hosts (S. lividans or modification-deficient E. coli) successfully bypassed the restriction barrier.
- Methyl-specific restriction targeting N6-methyladenine or 5-methylcytosine was observed in seven out of nine tested Streptomyces strains.
Conclusions:
- Streptomyces avermitilis possesses a potent methyl-specific DNA restriction system.
- Intermediate host transformation is an effective strategy to overcome this restriction.
- Methyl-specific restriction is a common phenomenon in the Streptomyces genus, impacting DNA manipulation.