IDENTIFICATION OF THE SERRATIA ENDONUCLEASE DIMER - STRUCTURAL BASIS AND IMPLICATIONS FOR CATALYSIS

The Serratia endonuclease is an extracellularly secreted enzyme capabl e of cleaving both single- and double-stranded forms of DNA and RNA. I t is the first member of a large class of related and usually dimeric endonucleases for which a structure is known. Using X-ray crystallogra phy, the structure of monomer of this enzyme was reported by us previo usly (Miller MD et al., 1994, Nature Struct Biol I:461-468). We now co nfirm the dimeric nature of this enzyme through light-scattering exper iments and identify the physiologic dimer interface through crystal pa cking analysis. This dimerization occurs through an isologous twofold interaction localized to the carboxy-terminal subdomain of the enzyme. The dimer is a prolate ellipsoid with dimensions 30 Angstrom x 35 Ang strom x 90 Angstrom. The dimer inter face is flat and contains four sa lt links, several hydrogen bonds, and nonpolar interactions. Buried wa ter is prominent in this interface and it includes an unusual ''cubic' ' water cluster. The position of the two active sites in the dimer sug gests that they can act independently in their cleavage of DNA, bur ha ve a geometrical advantage in attacking substrate relative to the mono mer.