CHARACTERIZATION OF CMIX, A CHICKEN HOMEOBOX GENE-RELATED TO THE XENOPUS GENE MIX.1

Members of the TGF beta, Wnt and FGF families act in concert to induce and pattern the mesoderm of gastrulating embryos. Downstream effecter s for these growth factors include homeobox proteins, which also feed back to activate and repress upstream signaling pathways (e.g. Fainsod , A., Steinbeisser, H., De Robertis, E.M. 1994. On the function of BMP -4 in patterning the marginal zone of the Xenopus embryo. EMBO J. 13, 5015-5025; Carnac, G., Kodjabachian, L., Gurdon, J.B., Lemaire, P. 199 6. The homeobox gene Siamois is a target of the Wnt dorsalization path way and triggers organizer activity in the absence of mesoderm. Develo pment 122, 3055-3065). As well as having interwoven upstream and downs tream regulatory pathways Mix.1, siamois and goosecoid, all paired-typ e homeobox genes, may physically interact with each other as heterodim ers to regulate dorsal-ventral polarity (Mead, P.E., Brivanlou, I.H., Kelley, C.M., Zon, L.I. 1996. BMP-4 responsive regulation of dorsal-ve ntral patterning by the homeobox protein Mix.1. Nature 382, 357-360). We report here a chicken paired-type homeobox gene, CMIX, with a homeo domain having 72% aa identity to its nearest homolog, Xenopus Mix. 1. CMIX is expressed in the epiblast of the posterior marginal zone of ea rly chick embryos, and later along the entire anterior-posterior axis of the primitive streak in cells of the medial ectoderm, in nascent me soderm, but not in endoderm, Coincident with formation of prechordal m esoderm, CMIX mRNA levels rapidly decline throughout the embryo. (C) 1 998 Elsevier Science Ireland Ltd. All rights reserved.