| Abstract: | The metabolism of 2-14C]thymine and 2-14C]thymidine in the cotyledons and embryonic axes of black gram (Phaseolus mungo) seedlings was investigated. Both 2-14C]thymine and 2-14C]thymidine degraded extensively into 14C]CO2. The rate of release of 14C]CO2 from 2-14C]thymine was much greater than that from 2-14C]thymidine. Radioactivity from both precursors was also observed beta-ureidoisobutyric acid. This indicated that thymine was degraded by the reductive pathway of pyrimidine degradation. Small amounts of 2-14C]thymine and 2-14C]thymidine were salvaged for deoxyribonucleotide and DNA synthesis. The highest incorporation of 2-14C]thymine and 2-14C]thymidine into the DNA fraction was observed in 24 hour-old cotyledons where net DNA synthesis was not observed. These precursors seem to be utilised for DNA synthesis of organelles of the cotyledonary cells, probably mitochondria. In embronic axes, 2-14C]thymine is more effectively salvaged for DNA synthesis than 2-14C]thymine. The incorporation rate increased during the early phase of germination and attained its maximum at 48 h after which it decreased. No thymidine kinase activity was detected in either cotyledons or in the embryonic axes. Thymidine salvage seems to be catalysed by nucleoside phosphotransferase which is present both in the cotyledons and in the embryonic axes. This suggests that, in contrast to other pyrimidine and purine bases and nucleosides, no specific salvage system for thymine and thymidine is present in black gram seedlings. |
