Antagonistic role of vega1 and bozorok/dharma homeobox genes in organizer formation

During zebrafish development, zygotic gene expression initiated at the midb lastula transition converts maternal information on embryo polarity into a transcriptional read-out. Expression of a homeobox gene, vega1, is activate d at midblastula transition in all blastomeres, but is down-regulated dorsa lly before gastrulation. Ubiquitous expression of vega1 is maintained in bo zozok mutants, in which the dorsal-specific homeobox gene bozorok/dharma (b oz/dha) is disrupted and organizer formation is impaired. Vega1 inhibits ex pression of boz/dha and organizer-specific genes, and causes ventralization resulting in a headless phenotype. In contrast, VP16-vega1, a fusion inclu ding the Vega1 homeodomain and VP16 activation domain, elicits ectopic expr ession of organizer genes and suppresses several aspects of the boz mutant phenotype. We propose that boz/dha-dependent down-regulation of vega1 in th e dorsal region is an early essential step in organizer formation in zebraf ish.