A nucleoside triphosphate-dependent deoxyribonucleic acid-breakdown system in Mycobacterium smegmatis, and the effect of iron limitation on the activity of this system.

1. The presence of a nucleoside triphosphate-dependent DNA-breakdown system was demonstrated in extracts of Mycobacterium smegmatis. Its activity was increased substantially by iron limitation, apparently after the fall in DNA content that took place under these conditions. A maximal activity of about 0.2mumole of deoxyribonucleotide/30min./mg. of protein was found in crude extracts. 2. After slight purification by streptomycin treatment, the enzyme showed maximal activity with undenatured DNA (K(m) approximately 200mug./ml.), ATP (K(m) approximately 1.2mm) or UTP, CTP and GTP giving lower activity and pyrophosphate giving none, and Mg(2+) ions (optimum concn. 12mm). The optimum pH was 8.5. 3. In the assay system there was proportionality between enzyme concentration and rate of reaction, but the rate fell off with time. 4. ATP was broken down in the reaction and monodeoxy-ribonucleotides were among the products, but the presence of some oligodeoxy-ribonucleotides was not excluded and the degree of phosphorylation of the primary products was uncertain.