Purification, crystallization and preliminary X-ray analysis of the HsdR subunit of the EcoR124I endonuclease from Escherichia coli

Functional coupling of duplex translocation to DNA cleavage in a type I restriction enzyme

Type I restriction-modification enzymes are multifunctional heteromeric complexes with DNA cleavage and ATP-dependent DNA translocation activities located on motor subunit HsdR. Functional coupling of DNA cleavage and translocation is a hallmark of the Type I restriction systems that is consistent with their proposed role in horizontal gene transfer. DNA cleavage occurs at nonspecific sites distant from the cognate recognition sequence, apparently triggered by stalled translocation. The X-ray crystal structure of the complete HsdR subunit from E. coli plasmid R124 suggested that the triggering mechanism involves interdomain contacts mediated by ATP. In the present work, in vivo and in vitro activity assays and crystal structures of three mutants of EcoR124I HsdR designed to probe this mechanism are reported. The results indicate that interdomain engagement via ATP is indeed responsible for signal transmission between the endonuclease and helicase domains of the motor subunit. A previously identified sequence motif that is shared by the RecB nucleases and some Type I endonucleases is implicated in signaling.

Type I restriction enzymes and their relatives

Type I restriction enzymes (REases) are large pentameric proteins with separate restriction (R), methylation (M) and DNA sequence-recognition (S) subunits. They were the first REases to be discovered and purified, but unlike the enormously useful Type II REases, they have yet to find a place in the enzymatic toolbox of molecular biologists. Type I enzymes have been difficult to characterize, but this is changing as genome analysis reveals their genes, and methylome analysis reveals their recognition sequences. Several Type I REases have been studied in detail and what has been learned about them invites greater attention. In this article, we discuss aspects of the biochemistry, biology and regulation of Type I REases, and of the mechanisms that bacteriophages and plasmids have evolved to evade them. Type I REases have a remarkable ability to change sequence specificity by domain shuffling and rearrangements. We summarize the classic experiments and observations that led to this discovery, and we discuss how this ability depends on the modular organizations of the enzymes and of their S subunits. Finally, we describe examples of Type II restriction-modification systems that have features in common with Type I enzymes, with emphasis on the varied Type IIG enzymes.

Crystallization and preliminary X-ray diffraction analysis of the HsdR subunit of a putative type I restriction enzyme from Vibrio vulnificus YJ016

Type I restriction enzymes are multimeric proteins that consist of three subunits. The HsdS and HsdM subunits form a complex protein that shows methyltransferase activity, while the HsdR subunit functions as an endonuclease as well as as a translocase. Of these three subunits, no structural information on the HsdR subunit is yet available. The putative HsdR gene from Vibrio vulnificus YJ016 (HsdR_Vv) was cloned and expressed and the expressed protein HsdR_Vv was purified. HsdR_Vv was crystallized from 8%(w/v) polyethylene glycol 3350, 0.15 M ammonium chloride, 0.1 M HEPES pH 7.5 and 2 mM beta-mercaptoethanol. Diffraction data were collected to 2.60 A resolution using synchrotron radiation. The crystal belongs to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 71.01, b = 89.04, c = 113.66 A. With one HsdR_Vv molecule in the asymmetric unit, the Matthews coefficient was 2.14 A(3) Da(-1) and the solvent content was 42%.