Substrate DNA and cofactor regulate the activities of a multi-functional restriction-modification enzyme, BcgI

Area of Science:

• Molecular Biology
• Enzymology

Background:

• The BcgI system is a bifunctional protein complex with DNA cleavage and methylation activities.
• Regulation of competing activities depends on DNA substrates and cofactors like Mg2+ and S-adenosylmethionine (AdoMet).

Purpose of the Study:

• To investigate the relationship between AdoMet binding and the DNA cleavage/methylation activities of BcgI.
• To understand the role of conserved residues in the AdoMet binding pocket of the A subunit.

Main Methods:

• Site-directed mutagenesis was used to create specific mutations in the BcgI A subunit.
• Purification and in vitro testing of mutant BcgI proteins to assess DNA cleavage, methylation, and AdoMet binding activities.

Main Results:

• Most mutations in the AdoMet binding pocket abolished both methylation and cleavage, indicating AdoMet binding is a common early step.
• The Y439A mutation specifically abolished methylation activity while retaining AdoMet binding and DNA cleavage activity.

Conclusions:

• AdoMet binding is essential for both BcgI DNA cleavage and methylation.
• Specific residues, like Y439, play distinct roles in regulating methylation activity independently of cleavage and AdoMet binding.