The Jcojrnal Ob Biological Chemistry

Radioactive precursors are not incorporated into deoxyribonucleic acid in viva until 18 hours after partial hepatectomy (1,2). This lag period may represent the time required for the synthesis of some specific protein which enables DNA synthesis to occur (3). It is known that inhibitors of protein synthesis will inhibit the synthesis of bacteriophage DNA if they are added before DNA synthesis begins (4-8). It is therefore of interest to determine whether inhibitors of protein synthesis injected into rats during the first 18 hours after partial hepatectomy will inhibit DNA synthesis. Use of thymidine as a specific precursor of DNA is well established (3, 9-17). It is a particularly good precursor since, aside from phosphorylated derivatives and degradation products, it is incorporated specifically into DNA with negligible diversion of the radioactivity into RNA (9, 10, 12). This high degree of specificity has been extremely useful in studies of the organization and duplication of chromosomes (14) where tritium-labeled thymidine is incorporated only into the new DNA formed during chromosome division and not into DNA already present. In addition, in regenerating rat liver homogenates, tritium-labeled thymidine is incorporated into deoxypolynucleotide in diester linkages which are indistinguishable from the linkages of nonradioactive thymidine in natural DNA (15). All these findings suggest that thymidine incorporation into regenerating rat liver DNA in viva may be used to measure the rate of DNA synthesis. It is shown in this paper that ethionine and p-fluorophenylalanine and P-2-thienylalanine hydrochloride strongly inhibit the incorporation of thymidine into DNA in viva. Under certain conditions, chloramphenicol also inhibits incorporation.