Chloroplast Development in 4-Thiouridine-Cultured Radish Seedlings VI. Anabolic Pathway of 4-Thiouridine

摘要

In germinating radish seeds, [U-14C]-4-thiouridine was converted to 4-thio-UMP, 4-thio-UDP, 4-thio-UTP, 4-thio-UDP glucose and 4-thiouracil, of which 4-thiouracil accounted for 60–85%. 4-Thio-UTP is incorporated into RNAs of radish seedlings [Shibata et al. (1980)FEBS Lett.119: 85]. These same metabolites were labeled following germination of radish seeds with [2-14C]-4-thiouracil. 4-Thiouridine was hydrolyzed by the uridine nucleosidase (EC 3.2.2.3) of radish seedlings as effectively as was uridine. The activity of uridine nucleosidase was increased by germination with 4-thiouridine. These results are a strong indication that 4-thiouridine is converted to 4-thiouracil, then to 4-thio-UMP by uracil phosphoribosyltransferase (EC 2.4.2.9). The alternative formation of 4-thio-UMP from 4-thiouridine by uridine kinase (EC 2.7.1.48) also was suggested. A possible mechanism which may cause inhibition of chloroplast biogenesis in 4-thiouridine-cultured seedlings is discusse