Figure 3. chordin Induction by Activin Requires De Novo Protein Synthesis.

(A) Temporal pattern of chordin expression. Northern blot analysis was performed with total RNA (7.5 µg) from various stages of early Xenopus embryos. Full-length chordin (chd) or gsc (gsc) cDNA were used as probes. 18S RNA stained with ethidium bromide is shown below as a loading control. The chordin transcript was first detected at stage 9.5, 1 hr before the onset of gastrulation. The accumulation of zygotic gsc RNA was detected earlier, at stage 9, 2 hr before gastrulation, as previously described (Cho et al., 1991). Maternal transcripts (E, egg) are present in the case of gsc but not in that of chordin, even after longer exposure.

(B) CHX inhibits activation of the chordin gene by activin. Animal caps (stage 8) were treated with 30 ng/ml activin for 2.5 hr (corresponding to stage 10) in the presence or absence of a protein synthesis inhibitor CHX (5 µg/ml) (Rosa, 1989; Cho et al., 1991). Total RNA (10 µg) was loaded in this Northern blot. Lane 1, untreated control; lane 2, CHX alone; lane 3, activin alone; lane 4, activin in the presence of CHX. Note that while chordin (chd) induction is inhibited by blocking protein synthesis, the induction of gsc and of noggin (nog) is somewhat increased. This indicates that while noggin and gsc are primary response genes to activin treatment, chordin is a secondary response gene.