Putrescine and Spermidine Biosynthesis in the Development of Normal and Anucleolate Mutants of Xenopus laevis

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Abstract
Ornithine decarboxylase (EC 4.1.1.17), the enzyme that catalyzes the synthesis of putrescine from ornithine, increases dramatically in developing Xenopus embryos. Between the 2-cell stage and early blastula stage, activity increases 10-fold, and in swimming tadpoles, the enzyme activity is 100-fold higher than that present in either unfertilized eggs or 2-cell embryos. S-adenosyl-L-methionine decarboxylase, an enzyme that catalyzes spermidine synthesis from putrescine and S-adenosyl-L-methionine, increases 40-fold in activity during the development of Xenopus, but does not increase in activity prior to gastrulation. Concomitant with these enzyme changes, putrescine and spermidine concentrations are elevated during the development of Xenopus embryos. Maximal accumulations are present in the swimming tadpole and correspond to maximal enzyme activities. Anucleolate-mutant embryos of Xenopus, which do not synthesize new ribosomes, have no detectable S-adenosyl-L-methionine decarboxylase activity and do not accumulate spermidine after gastrulation. Ornithine decarboxylase activity is depressed in these mutants and putrescine accumulation is decreased also. The activity of some dehydrogenases that increase in Xenopus embryos after gastrulation show normal increases in the anucleolate mutants. Thus, the synthesis of putrescine and spermidine in embryos correlates with the onset of ribosomal-RNA synthesis and the formation of a viable nucleolus in the embryonic cell.