CAAT sites are required for the activation of the H-pulcherrimus Ars gene by Otx

A product of sea urchin homologues of the Drosophila orthodenticle gene, Hp OtxL has been implicated as a transcription activator of the aboral ectoder m-specific arylsulfatase (Ars) gene during early development of the sea urc hin embryo. Using an in vivo transactivation system, we present evidence th at HpOtxL activates the target gene by interacting with co-factors. Otx bin ding sites alone have little effect on the activity of an Ars promoter, but when both Otx binding sites and CAAT sequences are present in the enhancer region of Ars, the fragment shows a high enhancer activity. A gel mobility shift assay reveals that a nuclear protein binds to the CAAT sequences pre sent near the Otx binding sites in the enhancer region of Ars. The activati on domain of HpOtxL resides in the C terminal region between amino acids 21 8 and 238. The N-terminal region is responsible for the enhancement of tran sactivation of the Ars promoter, although the region itself does not functi on as an activation domain. HpOtxE, which possesses an N-terminal region di fferent from HpOtxL, does not activate the Ars promoter even in the presenc e of CAAT sequences. Together with previous findings, our results suggest t hat Otx regulates different genes by interacting with different cofactors i n sea urchin development.