Identification of a novel mouse Iroquois homeobox gene, Irx5, and chromosomal localisation of all members of the mouse Iroquois gene family.

The Drosophila genes of the Iroquois-Complex encode homeodomain containing transcription factors that positively regulate the activity of certain proneural Achaete/Scute-C (AS-C) genes during the formation of external sensory organs (J. L. Gomez-Skarmeta and J. Modolell, EMBO J 17:181-190, 1996). Previously, we have identified three highly-related genes of the mouse Iroquois gene family that exert specific expression patterns in the central nervous system (A. Bosse et al., Mech Dev 69:169-181, 1997). In the present paper, we report the identification of a novel member of the Iroquois gene family, Irx5, that shows a restricted spatio/temporal expression during early mouse embryogenesis, distinct from the expression of Irx1-3. An extensive sequence analysis of 20 Iroquois-like genes from seven organisms reveals a high conservation of the homeodomain. Phylogenetic tree reconstruction showed a clustering of the members of the Iroquois gene family into groups of orthologous genes. Together, with the data obtained from the chromosomal mapping analysis, the results indicate that these genes have appeared in vertebrates during evolution as a result of gene duplication.