DNA synthesis and activities of pyrimidine-synthesizing enzymes in the silk gland of Bombyx mori

Abstract Synthesis of DNA in the silk gland of Bombyx mori was investigated by studying incorporation of DNA precursors and determining activities of related enzymes in the developmental stage from the fourth apolysis to the middle fifth instar. Uptake of 32 Pi (for 10 hr) into pyrimidine nucleotides of DNA was higher than that into purine nucleotides of DNA. 3 H-Thymidine and 14 C-orotic acid were found to be active precursors for DNA and, seemingly, the incorporation of the former was more active than the latter. Activities of thymidine kinase and thymidylate kinase exhibited their highest levels on day 4 of the instar, and aspartate transcarbamylase showed a peak on the fifth day. Administration of ecdysterone (10 or 20 μg) into a larva resulted in decreased incorporation (by 30 or 60 per cent, respectively) of 3 H-thymidine into DNA fraction from intracellular pools. Uptake of 32 Pi into the fractions of the gland fell by 50 per cent after administration of the hormone (10 μg). A study of daily changes of incorporation of 3 H-thymidine into the DNA fraction demonstrated that its maximum rate was on the fourth day, in accordance with the appearance of peaks of the above-mentioned enzymes. The implications of these observations are discussed in relation to the known features of development of the insect during the period.