FIG. 6.

Model for the role of Tll in the regulation of gene expression at the posterior pole of the embryo. tll expression is first activated in a posterior cap (shadowed) as a result of localized Torso signaling. This activation is indirect and occurs through relief of repression: tll is silenced in middle regions of the embryo by a repressor complex (R) that is inhibited by the Torso signal at the pole, thus permitting localized tll activation by generally distributed factors. Subsequently, the Tll product acts as a repressor of central gap genes such as kni and gt and is also required for activation of secondary targets such as hb, byn, and fkh (dashed arrows). The latter function is again indirect and involves repression of kni and probably other intermediaries that mediate repression of hb, byn, and fkh. A similar model applies for the initial activation of hkb in response to Torso signaling, and the Hkb protein has been shown to act predominantly as a negative transcriptional regulator (see Discussion). Thus, repressive interactions define the regulatory network controlling terminus-specific gene expression at the posterior of the embryo.