The expression pattern of the mafB/kr gene in birds and mice reveals that the kreisler phenotype does not represent a null mutant.

The recessive mouse mutation kreisler affects hindbrain segmentation and inner ear development in homozygous mice. The mouse gene affected by the mutation was found to encode a basic domain leucine-zipper (bZIP)-type transcription factor of the Maf-family named kr (Cordes, S.P. and Barsh, G.S. (1994) Cell 79, 1025-1034). The avian bZIP transcription factor mafB, which shows high homology to kr, has been identified as an interaction partner of c-Ets 1 (Sieweke, M.H., Tekotte, M.H., Frampton, J. and Graf, T. (1996) Cell 85, 49-60). Here we demonstrate by Southern blot analysis that mafB is the avian homologue of kr, and present a detailed pattern of its expression during avian and murine embryonic development. Consistent with the kreisler phenotype, mafB is expressed in avians in the tissues which are affected by the mouse mutation: rhombomeres 5 and 6 (r5 and r6) and the neural crest derived from these rhombomeres. However, our analysis reveals a variety of additional expression sites: mafB/kr expression persists in vestibular and acoustic nuclei and is also observed in differentiating neurons of the spinal cord and brain stem. Restricted expression sites are found in the mesonephros, the perichondrium, and in the hemopoietic system. Since these expression sites are conserved between mouse and chicken we reexamined homozygous kreisler mice for unrevealed phenotypes in the hemopoietic system. However, peritoneal macrophages from homozygous kreisler mice were found to be functionally normal and still expressed mafB/kr. Other adult tissues examined from homozygous kreisler mice had also not lost mafB/kr expression. Our results thus indicate that the kreisler mutation involves a tissue specific gene inactivation and suggest additional roles for mafB/kr in later developmental and differentiation processes that are not revealed by the mutation.