Thymidine-phosphorylating Activity in gamma-Plantlets: Separation of Onset of Activity from Deoxyribonucleic Acid Synthesis.

Thymidine-phosphorylating capacity, mediated by thymidine kinase or a nucleoside phosphotransferase, is closely correlated with DNA synthesis and cell division in a wide variety of bacterial, animal, and plant systems (4, 5, 11, 14, 17, 22). In cells that are not undergoing DNA synthesis and cell division, these enzymes are usually low or undetectable. Conversely, systems undergoing rapid DNA synthesis and cell division, such as regenerating rat liver and exponentially growing HeLa cells, have high capacities for thymidine phosphorylation (3, 6, 17). Those organisms that lack thymidine-phosphorylating enzymes are unable to incorporate thymidine into their DNA (9). In higher plants, thymidine-phosphorylating enzymes may be involved in pyrimidine nucleotide metabolism during germination and early seedling growth (1, 10, 19, 21). Our primary purpose in this paper is to test the hypothesis that the control mechanisms for onset and development of thymidine-phosphorylating enzyme activities are separable from DNA synthesis. The results are based upon a study of thymidine phosphorylating enzyme activities in -y-plantlets and unirradiated controls. /-Plantlets are seedlings growing without DNA synthesis and cell division after /-irradiation of dry wheat grains (12). Despite high doses received before sowing, they fail to show certain radiation effects that typically result in physiological injury and, apart from the absence of cell division, are therefore remarkably normal (12). In addition to possible implications for organismal development, the study reported here is relevant to the usefulness of y-plantlets for two reasons. (a) Since thymidine-phosphorylating capacity is sensitive to general physiological insults, including ionizing irradiation (17), any perturbations in its development would indicate possible injury resulting from the irradiation treatment used to produce y-plantlets; and (b) a frequently used method of checking for the absence of DNA synthesis in y/-plantlets involves checking for the absence of thymidine incorporation (13) and therefore presupposes the capacity to phosphorylate thymidine.