Homeodomain Position 54 Specifies Transcriptional versus Translational

Hahn and Jäckle, 1996) of Drosophila as well as their Bicoid (BCD), the anterior determinant of Drosophila, vertebrate homologs (Blumberg et al., 1991; Simeone controls embryonic gene expression by transcriptional et al., 1993). activation and translational repression. Both functions Previous results have shown that the lysine at position require the homeodomain (HD), which recognizes DNA 50 of the BCD HD is not only necessary for DNA recognimotifs at target gene enhancers and a specific setion (Hanes and Brent, 1989; Treisman et al., 1989) but quence interval in the 39 untranslated region of caudal also for the binding to a discrete target sequence in the (cad) mRNA. Here we show that the BCD HD is a nu39 untranslated region (39UTR) of cad mRNA, termed the cleic acid–binding unit. Its helix III contains an argiBCD-binding region (BBR) (Dubnau and Struhl, 1996; nine-rich motif (ARM), similar to the RNA-binding doRivera-Pomar et al., 1996; Chan and Struhl, 1997), promain of the HIV-1 protein REV, needed for both RNA viding a link between the DNA and RNA binding properand DNA recognition. Replacement of arginine 54, ties of BCD. Here we show that the helix III of the BCD within this motif, alters the RNA but not the DNA bindHD contains a region similar to the RNA-binding argiing properties of the HD. Corresponding BCD mutants nine-rich motif (ARM) of the HIV-1 protein REV (Sodroski fail to repress cad mRNA translation, whereas the tranet al., 1986; Tan et al., 1993; Tan and Frankel, 1995; scriptional target genes are still activated. Battiste et al., 1996). We investigated whether mutations