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

Hoxb1 has several rhombomere-specific roles in hindbrain development and may contribute to development of a small number of tissues outside the hindbrain. To gain insight into the regulation of late Hoxb1 expression in these structures, several regions of the Hoxb1 locus were systematically evaluated for their ability to control late Hoxb1 expression in transgenic mouse embryos. This was achieved by progressive enlargement of the portion of the Hoxb1 locus used to control reporter gene expression. Unexpectedly, the previously identified rhombomere 4 (r4) enhancer and r3/r5 repressor that were thought to be sufficient for r4 restriction of Hoxb1 produced continuous expression throughout much of the length of the central nervous system (CNS) and in several structures outside the nervous system. However, adjacent regions of the Hoxb1 locus, in combination with the r4 enhancer and r3/r5 repressor were capable of restricting most expression to r4 in the hindbrain, and to gut epithelium/mesoderm, caudal spinal cord and caudal paraxial mesoderm outside the hindbrain. Expression that occurred outside r4 in the brains of the majority of founder embryos was confined to scattered cells in specific regions. These cells may have arisen in r4 and then migrated into adjacent brain regions as rhombomere lineage restrictions dissipated. Alternatively, these dispersed stained cells may have originated outside r4 implying that Hoxb1 cannot be accurately regulated in a consistent manner outside the Hox complex.