Restriction-modification system with methyl-inhibited base excision and abasic-site cleavage activities

Area of Science:

• Molecular Biology
• Epigenetics
• Biochemistry

Background:

• Restriction-modification (R-M) systems are crucial for distinguishing self from non-self DNA via epigenetic modifications.
• Typically, restriction enzymes cleave DNA lacking methylation marks, hydrolyzing phosphodiester bonds.
• The PabI superfamily of restriction enzymes possesses a unique half-pipe fold structure.

Purpose of the Study:

• To investigate the enzymatic activities of the R.PabI restriction enzyme beyond canonical DNA hydrolysis.
• To elucidate the mechanism of DNA modification and cleavage by R.PabI.
• To understand the implications of R.PabI's unique activities on DNA transformation and epigenetic regulation.

Main Methods:

• Characterization of R.PabI's DNA glycosylase activity, specifically adenine base excision.
• Analysis of the enzyme's AP (apurinic/apyrimidinic) lyase activity at abasic sites.
• Use of NaBH4 reduction to trap covalent DNA-R.PabI reaction intermediates.
• Assessment of the impact of base excision and strand breaks on DNA transformation efficiency in vitro.

Main Results:

• R.PabI exhibits DNA glycosylase activity, excising an adenine base from the 5'-GTAC recognition sequence.
• The enzyme possesses secondary AP lyase activity, generating atypical strand breaks at abasic sites.
• Base excision by R.PabI impairs DNA transformation even without strand breakage, demonstrating a novel restriction mechanism.
• Methylation of the target adenine base inhibits R.PabI's base excision activity.

Conclusions:

• R.PabI represents a novel class of restriction enzymes with dual DNA glycosylase and AP lyase activities.
• The base excision mechanism, independent of strand breakage, contributes to DNA restriction and epigenetic control.
• These findings bridge the understanding of genetic and epigenetic mechanisms across prokaryotes and eukaryotes.