The Deoxyribonucleases of Escherichia coli

Abstract

INTRODUCTION
The organism that has been most extensively characterized with regard to its DNases is undoubtedly Escherichia coli. The number and complexity of the enzymes that have been identified to date are impressive and were certainly unexpected. To obtain an overview of the dynamics of DNA metabolism in this or other organisms, a familiarity with the general features of these enzymes is necessary. It is the purpose of this section to list these enzymes and to summarize some highlights of their properties. More in-depth discussions of particular enzymes can be found elsewhere in this volume, in the specific references cited below and in several other reviews and monographs (Lehman 1963Lehman 1971; Cantoni and Davies 1966; Privat de Garilhe 1967), and various chapters in Boyer (1981).

EXONUCLEASES
The exonucleases of E. coli have been very recently reviewed by Weiss (1981) with emphasis upon exonucleases I, III, IV, VII, and VIII.

Exonuclease I
Exonuclease I is a monomeric protein of M_r about 72,000 (Mackay and Linn 1974), though active oligomeric forms have been described. The enzyme hydrolyzes single-strand substrates about 40,000-fold faster than duplex DNA. It acts in a 3′→5′ direction, releasing nucleoside 5′-phosphates in a processive manner, but leaves the 5′-terminal dinucleotide intact. A free 3′-hydroxyl terminus is required for activity (Lehman and Nussbaum 1964).

The sbcB gene at 44 minutes on the E. coli genetic map specifies exonuclease I (Bachmann and Low 1980). Mutants lacking the enzyme appear to be perfectly normal, but in the presence of recB and/or recC...