Promoter activity of the zebrafish bhikhari retroelement requires an intact activin signaling pathway

We have investigated mesoderm induction in zebrafish employing the zebrafis h LTR-retroelement bhikhari (bik). bik elements are transcribed in all earl y mesendodermal cells. This expression pattern is generated by a promoter l ocated in the U3 region of the LTR. We show that bik is activated through t he activin/Vg1 signaling pathway in an immediate early fashion. This activa tion critically depends on a sequence motif that occurs among others also i n the Xenopus Mix2 activin response element (ARE). It has been shown that t he Mix2 ARE binds FAST-1, which complexes with Smad proteins to form a mult i-protein complex. We confirm that also the bik ARE can be bound by FAST-1 in vitro. In animal cap experiments we demonstrate that this binding site i s required for activin-induced transcriptional activation mediated by FAST and Smad-type proteins. (C) 1999 Elsevier Science ireland Ltd. All rights r eserved.