The biosynthesis of DNA by insects. I. The utilization of deoxycytidine by developing adults of the Cecropia silkworm

Abstract

The pathway of deoxycytidine utilization was investigated in developing pupae of Hyalophora cecropia. The following intermediates were identified in a single assay system starting with deoxycytidine: deoxyuridine, deoxyuridine monophosphate, thymidine monophosphate, thymidine diphosphate, and thymidine triphosphate. The existence of this pathway was confirmed in several ways in addition to the detection of the actual intermediates. First, injection of [ 14C]deoxycytidine or [ 14C]deoxyuridine into developing pupae resulted in the localization of the label in the thymine moiety of DNA and no significant radioactivity was detected in the cytosine moiety. Second, omission of various components from the complete assay system produced a predictable loss of certain intermediates. Third, the activity of other enzymes which might compete with this pathway were not detected or detected in trace amounts, e.g., deoxycytidine monophosphate deaminase, deoxycytidine kinase, thymidine synthetase, deoxyuridine monophosphate kinase and deoxycytidine degradative enzymes. The enzyme which converts deoxyuridine monophosphate to deoxythymidine monophosphate (thymidylate synthetase) was not detected in diapausing or injured diapausing pupae, but was present in significant amounts in developing pupae. The injection of [ 14C]thymidine into developing pupae also resulted in the association of the radioactivity with DNA-thymine, but little or no thymidine kinase activity could be detected in the extracts.