STRUCTURAL, FUNCTIONAL, AND EVOLUTIONARY RELATIONSHIPS BETWEEN LAMBDA-EXONUCLEASE AND THE TYPE-II RESTRICTION ENDONUCLEASES

lambda-exonuclease participates in DNA recombination and repair. It bi nds a free end of double-stranded DNA and degrades one strand in the 5 ' to 3' direction. The primary sequence does not appear to be related to any other protein, but the crystal structure shows part of lambda-e xonuclease to be similar to the type II restriction endonucleases PvuI I and EcoRV, There is also a weaker correspondence with EcoRI, BamHI, and Cfr101. The structure comparisons not only suggest that these enzy mes all share a similar catalytic mechanism and a common structural an cestor but also provide strong evidence that the toroidal structure of lambda-exonuclease encircles its DNA substrate during hydrolysis.