The previously identified r3 /r5 repressor may require the cooperation of additional negative elements for rhombomere restriction of Hoxb1

Hoxb1 has several rhombomere-specific roles in hindbrain development and ma y contribute to development of a small number of tissues outside the hindbr ain. To gain insight into the regulation of late Hoxb1 expression in these structures, several regions of the Hoxb1 locus were systematically evaluate d for their ability to control late Hoxb1 expression in transgenic mouse em bryos. This was achieved by progressive enlargement of the portion of the H oxb1 locus used to control reporter gene expression. Unexpectedly, the prev iously identified rhombomere 4 (r4) enhancer and r3/r5 repressor that were thought to be sufficient for r4 restriction of Hoxb1 produced continuous ex pression throughout much of the length of the central nervous system (CNS) and in several structures outside the nervous system. However, adjacent reg ions of the Hoxb1 locus, in combination with the r4 enhancer and r3/r5 repr essor were capable of restricting most expression to r4 in the hindbrain, a nd to gut epithelium/mesoderm, caudal spinal cord and caudal paraxial mesod erm outside the hindbrain. Expression that occurred outside r4 in the brain s of the majority of founder embryos was confined to scattered cells in spe cific regions. These cells may have arisen in r4 and then migrated into adj acent brain regions as rhombomere lineage restrictions dissipated. Alternat ively, these dispersed stained cells may have originated outside r4 implyin g that Hoxb1 cannot be accurately regulated in a consistent manner outside the Hox complex. (C) 2000 Elsevier Science B.V. All rights reserved.