Isolation and crystallization of λ exonuclease

Abstract λ Exonuclease is a deoxyribonuclease induced by bacteriophage λ. Mutations in the structural gene for the protein affect general recombination and indicate a possible function for the enzyme. A large scale isolation procedure was employed to purify enough enzyme from a heat-induced λ lysogen for X-ray crystallographic analysis. Analytical ultracentrifugation and SDS-polyacrylamide electrophoresis revealed that λ exonuclease is a tetramer with molecular mass 107,000 Da. Crystallization trials produced morphologically perfect crystals of a size suitable for X-ray diffraction studies. Cubic crystallographic symmetry was indicated by the lack of birefringence when the crystals were inspected with polarized light. X-ray precession photographs indicated that λ exonuclease crystallizes in a space group of P4 1 32, or its enantiomorph P4 3 32, with 24 tetramers in the unit cell of edge 210 A.