The ATP-dependent exonuclease V from Micrococcus Iysodekticus shows a Michaelian relation between steady-state velocity and the concentration of T7 DNA substrate. The K , (expressed as a mass concentration) does not change when the T7 DNA is broken into smaller fragments by a restriction enzyme. This is interpreted to mean that the predominant process by which the exonuclease-V -DNA complex breaks down is digestion of the entire DNA molecule rather than physical dissociation, in accord with the already known processive nature of degradation by this enzyme. The way that the V and K, towards DNA vary with ATP and ADP concentration suggests that enzyme-DNA complex is predominantly formed by reaction of DNA with an enzyme-ATP complex rather than with bare enzyme.Since Tsuda and Strauss first reported [ l ] the existence in Micrococcus lysodeikticus of an ATPdependent DNAase which is also a DNA-dependent ATPase, similar enzymes have been purified from at least eight bacterial species [2-131 so they are probably widely distributed in the bacterial world. The enzyme of this type from Escherichia coli having become known as exonuclease V, Wilcox and Smith proposed [21] that this term be used for the similar enzymes from other species, and we adopt this terminology here. These enzymes are very probably important in some function in genetic recombination and in recombinational repair of radiation damage. It has been demonstrated in four species that exonucleases V are coded by genes whose mutations give rise to deficiences in these processes [14-211. Exonucleases V are members of a wider class of enzymes, the nucleic-acid-dependent ATPases (reviewed in [27]) involved in many of the key processes in the molecular biology of the gene, such as replication, transcription, restriction, recombination and probably others.There is general agreement that the mode of action of exonucleases V is processive, that is a molecule of the enzyme attaches tightly to the end of a molecule of double-stranded DNA and moves along it, hydrolys-

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