In vivo DNA protection by relaxed-specificity SinI DNA methyltransferase variants

Area of Science:

• Molecular Biology
• Enzymology
• Genetics

Background:

• The SinI restriction-modification system involves a DNA methyltransferase and a restriction endonuclease.
• Wild-type SinI methyltransferase recognizes and methylates GG(A/T)CC sequences.
• Relaxed-specificity mutants methylate additional GG(G/C)CC sites at a lower efficiency.

Purpose of the Study:

• To evaluate the in vivo protective capacity of SinI methyltransferase mutants against the Sau96I restriction endonuclease.
• To assess the impact of controlled endonuclease expression on cell viability.
• To investigate the role of DNA ligase activity in enhancing cell survival.

Main Methods:

• Utilized Escherichia coli strains harboring recombinant plasmids with mutant SinI methyltransferase genes.
• Expressed Sau96I restriction endonuclease under the control of an inducible araBAD promoter.
• Assessed cell viability through plating efficiency assays under varying inducer concentrations.
• Analyzed plasmid DNA methylation status and quantified DNA ligase activity.

Main Results:

• Cells expressing N172S or V173L mutant methyltransferases showed increased plating efficiency compared to wild-type.
• This enhanced protection was insufficient for long-term survival due to incomplete methylation of GG(G/C)CC sites.
• Increased DNA ligase activity significantly improved the viability of cells co-expressing the V173L mutant and Sau96I endonuclease.

Conclusions:

• SinI methyltransferase mutants provide partial protection against cognate restriction enzymes in vivo.
• Incomplete methylation of alternative recognition sites limits the protective efficacy of these mutants.
• Enhancing DNA repair mechanisms, such as increasing DNA ligase activity, can improve cell survival in the presence of restriction enzymes.