DISTINCT REGULATORY CONTROL OF THE BRACHYURY GENE IN AXIAL AND NONAXIAL MESODERM SUGGESTS SEPARATION OF MESODERM LINEAGES EARLY IN MOUSE GASTRULATION

Brachyury is required for the normal extension of the anteroposterior axis during mouse embryogenesis. A transgene comprising sequences from -500 to +150 relative to the start of Brachyury transcription, and th e reporter gene lacZ, recapitulates some, but not all elements of Brac hyury expression. beta-Galactosidase expression is seen in the primiti ve streak from 6.5 d.p.c. but there is no detectable reporter expressi on in the node or notochord. Thus, the regulatory sequences required f or the expression of Brachyury in the cells traversing the primitive s treak are distinct from those required for the initiation of expressio n in the node. This suggests that different or additional signals are involved in activation of Brachyury in the node and notochord than tho se inducing Brachyury in the primitive streak. Additionally, the data suggest the possibility that axial and non-axial mesoderm are distinct from the earliest stages of Brachyury expression.