Functional Conversion of CPD and (6-4) Photolyases by Mutation

Area of Science:

• Molecular Biology
• Biochemistry
• Evolutionary Biology

Background:

• Sunlight's UV radiation causes DNA damage via cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts [(6-4) PP].
• Photolyase (PHR) enzymes repair this DNA damage using near-UV/blue light, initiated by electron transfer from a flavin adenine dinucleotide chromophore.
• PHR enzymes exhibit high functional selectivity, with distinct CPD PHR and (6-4) PHR variants.

Purpose of the Study:

• To investigate the potential for functional conversion between CPD and (6-4) PHR enzymes.
• To understand the structural and mechanistic basis for the substrate specificity of PHR enzymes.

Main Methods:

• Site-directed mutagenesis was used to create mutant (6-4) PHR and CPD PHR enzymes.
• Enzyme activity assays were performed to assess the repair of CPD and (6-4) PP photoproducts.
• Difference Fourier transform infrared spectroscopy and quantum chemical/molecular mechanical calculations were employed to analyze structural changes and reaction mechanisms.

Main Results:

• A triple mutant of (6-4) PHR demonstrated the ability to repair CPD photoproducts, albeit with a 10-fold lower efficiency compared to wild-type CPD PHR.
• Spectroscopic analysis indicated that the triple mutant acquired substrate binding but lacked optimized conformational changes for efficient DNA repair.
• Conversely, CPD PHR with multiple mutations (up to 11) failed to repair (6-4) photoproducts, revealing an asymmetric functional conversion.

Conclusions:

• The functional conversion suggests that CPD PHR might be evolutionarily ancestral to the photolyase family.
• The observed asymmetry points towards a more intricate repair mechanism for (6-4) photoproducts compared to CPDs.
• Understanding PHR enzyme specificity provides insights into DNA repair pathways and protein evolution.