The activity of UK is usually higher than that of UPRT and thus uridine is more efficiently salvaged to UMP than is uracil (e.g. Kanamori-Fukuda et al. 1981; Ashihara et al., 2000). UK has been partially purified from higher plants and its enzymatic properties have been well characterized (Deng and Ives, 1972 and 1975). However, there have been no reports of the isolation of UK coding sequences or genes. Deoxycytidine kinase (EC 2.7.1.74) and thymidine kinase (EC 2.7.1.21) are also present in plants, but details of their activities have yet to be described.