Method for Restricting Incorporation of Radioactivity from 3 H-Thymidine into Deoxyribonucleic Acid Only During Outgrowth of Spores of Bacillus cereus T

When heat-activated spores of Bacillus cereus T (thy(-)) were germinated and grown in medium containing (3)H-thymidine, a significant amount of radioactivity was incorporated into ribonucleic acid and deoxyribonucleic acid (DNA). A method was developed to restrict the incorporation of radioactivity from (3)H-thymidine into DNA only. This was accomplished by labeling the cells with (3)H-thymidine in the presence of 2 mg of 2-deoxyadenosine per ml, 250 mug each of uracil, cytosine, and guanosine per ml, and 500 mug of adenosine per ml. Under these conditions, 97% of the radioactivity incorporated into cold trichloroacetic acid-insoluble material was associated with DNA only. In the absence of these compounds, DNA contained only 72% of the total radioactivity incorporated into cold acid-insoluble material.