Pyrimidine deoxyribonucleotide metabolism during maturation and germination of white spruce (Picea glauca) somatic embryos: Metabolic fate of C-labelled cytidine, deoxycytidine and thymidine

Abstract

Pyrimidine deoxyribonucleotide metabolism was investigated during maturation and germination of white spruce somatic embryos by following the metabolic fate of [2-14C]cytidine, [2-14C]deoxycytidine and [2-14C]thymidine. The de-novo pathway of deoxyribonucleotides was estimated indirectly, by the ability of the tissue to incorporate cytidine into DNA after conversion to dCTP. The salvage pathway was estimated by the utilization of labelled cytidine, deoxycytidine and thymidine for synthesis of deoxyribonucleotides and nucleic acids. Utilization of cytidine for DNA synthesis, via the de novo pathway, was always lower than that observed for RNA throughout the course of the experiment. Incorporation of cytidine into RNA was found to occur either directly, after conversion to CTP, mediated by the enzymes cytidine kinase, nucleoside monophosphate kinase and nucleoside diphosphate kinase, or indirectly, after conversion to UTP via uridine and UMP. Active incorporation of uridine into RNA of white spruce-cultured cells was demonstrated previously. Salvage of deoxycytidine and thymidine was operative in maturing and germinating white spruce somatic embryos, as label from both compounds was recovered in nucleotides and DNA. However, the utilization of these precursors by the cells was different. Salvage of deoxycytidine was always higher than that observed for thymidine, which was extensively catabolized to CO2 at all stages of embryo development.