DIFFERENTIAL ACTIVATION OF XENOPUS HOMEO BOX GENES BY MESODERM-INDUCING GROWTH-FACTORS AND RETINOIC ACID

What is the nature of positional information during embryogenesis? By using Xenopus homeo box genes as anteroposterior (A-P) markers, we confirm the findings of others that mesoderm-inducing growth factors and retinoic acid (RA) can provide positional information along the axis of the body. Xenopus tissue culture-mesoderm-inducing factor (XTC-MIF) selectively activates an anteriorly expressed homeo box gene (X1Hbox 1), while basic fibroblast growth factor (bFGF) activates selectively a posteriorly expressed homeo box gene (X1Hbox 6). RA activates expression of the posterior gene X1Hbox 6, but not of X1Hbox 1. This activation, however, requires exposure to growth factors. The data suggest that growth factors and RA may cooperate with each other to provide positional information in vertebrates.