Isolation, subunit structure and properties of the ATP-dependent deoxyribonuclease of Bacillus subtilis. State of the protein in a mutant devoid of activity

Abstract

A prodcedure was developed for the purification of the ATP-dependent deoxyribonuclease of Bacillus subtilis 168. It comprises ammonium sulphate fractionation, Sephadex gel filtration, DEAE-cellulose chromatography and gel electrophoresis on a discontinuous polyacrylamide gradient. The enzyme has been obtained in a homogeneous state. Its molecular weight was estimated to be 270000 by disc electrophoresis. Dodecylsulfate-polyacrylamide gel electrophoresis showed the presence of five nonidentical subunits of the following molecular weights: 81000, 70000, 62000, 52500 and 42500. These values give 308000 as the molecular weight of the native enzyme. The pH optimum of the purified enzyme is 9.6. The optimal concentrations of Mg2+ and ATP for exonuclease activity on native B. subtilis DNA were determined. ATP-requirement for hydrolysis of single-stranded DNA is less strigent. The enzyme also possesses high DNA-dependent ATPase activity. The purification procedure was applied to extracts of a mutant devoid of activity for this enzyme (strain GSY 1290). A protein was isolated which is very similar to the active DNAase as regards electrophoretic mobility, reaction with specific antisera and size of four of the subunits. One subunit is missing (Mr 70000) and is replaced by a smaller polypeptide (Mr 565000). The latter results suggest that the mutant is affected in the genetic locus coding for the 70000-Mr subunit.