| Abstract: | Incorporation of the nucleic acid precursors, orotic acid, adenosine, thymidine, and uridine, was studied in various stages of intraerythrocytic Plasmodium knowlesi from infected rhesus monkeys. Incubation of the parasitized erythrocytes with the precursors was for 3 hr periods using a plasma-free culture medium. The samples containing primarily rings, early trophozoites, or late trophozoites incorporated orotic acid, adenosine, and uridine into RNA; however, these stages exhibited negligible or very low levels of incorporation of any of the precursors into DNA. The sample containing late trophozoite and schizont stages incorporated orotic acid, adenosine, and uridine into RNA, and orotic acid, adenosine, and very low levels of thymidine into DNA. These results indicate that DNA synthesis (the S phase of the cell cycle) occurs very close to the time of nuclear division, and that either the G1 or G2 phase is very short in P. knowlesi. It was also observed that adenosine and orotic acid, 2 precursors which are incorporated into both DNA and RNA, are utilized differently by the intraerythrocytic parasites. Incorporation of orotic acid into RNA and DNA and adenosine incorporation into DNA were continuous for the entire incubation period, whereas incorporation of adenosine into RNA was very low during the last 2 hr of each period. It was further demonstrated that the parasites utilized exogenous uridine for synthesis of RNA, and that the older parasite stages incorporated thymidine into DNA. |
