To investigate the relative contribution of DS-epi1 and DS-epi2 to epimerase activity in the embryo, we used morpholino oligonucleotides (MOs) against Dse and Dsel (Fig. 2A). Following microinjection into the animal pole of embryos (hereafter called animal injection), Dse- and Dsel-MOs blocked 89% and 12% of the endogenous DS epimerase activity, respectively, at stage 25 (Fig. 2B). The Dse-MO substantially suppressed the epimerase activity induced by injected Dse mRNA at stage 12, whereas a standard control MO and a Dsel-MO had no effect (Fig. 2C). In contrast, the Dse-MO did not block the epimerase activity of the injected Dse* mRNA, which contains seven point mutations in the MO target sequence; this finding demonstrates the specificity of the Dse-MO knockdown. The data suggest that DS-epi1 is the main contributor to DS biosynthesis in the early Xenopus embryo.
