No abstract

We have cloned two novel homeobox genes which are the mouse (Lbx1) and human (LBX1) homologs of the Drosophila lady bird genes. They are highly related not only within the coding region but also in 5' and 3' untranslated regions. Several amino acid residues inside and around the homeodomain, have been conserved between the mammalian Lbx genes and their Drosophila counterparts. The mouse Lbx1 gene is located on chromosome 19 (region D) and the human LBX1 gene maps to the related q24 region of chromosome 10, known as a breakpoint region in translocations t(7;10) and t(10;14) involved in T-cell leukemias. Thus, LBX1 and the protooncogene HOX11 map to a common chromosomal region, as do their Drosophila counterparts, the lady bird and 93Bal genes. The mouse Lbx1 gene is specifically expressed during embryogenesis. From 10.5 days of gestation, Lbx1 expression is detected in the central nervous system and some developing muscles. In the CNS, Lbx1 transcripts are expressed in the dorsal part of the mantle layer of the spinal cord and hindbrain, up to a sharp boundary within the developing metencephalon. Thus, Lbx1 may be inolved in spinal cord and hindbrain differentiation and/or patterning, and its restricted expression pattern could depend upon evolutionarily conserved inductive signals involving some mammalian Wnt and Pax genes, as is the case for Drosophila lady bird genes and wingless or gooseberry.

We examined the chromatin organization of -3 kb of DNA in the 5' -end flanking region of the ovalbumin gene in chicken erythrocyte and oviduct cell nuclei. With specific DNA probes and an indirect end-labeling technique, we analysed the pattern of the DNA fragments obtained after micrococcal nuclease digestion and generated comparative maps of the nuclease cuts. This region of the chicken genome displays a "typical" chromatin arrangement in erythrocyte nuclei, with nucleosomes apparently positioned at random. In contrast, in oviduct nuclei, the same region has an "altered" chromatin structure, and lacks a typical nucleosomal array. The existence of specifically positioned proteins and of alterations in the DNA secondary structure in this region of the oviduct chromatin is suggested by comparison of the nuclease cleavage maps which reveals specific changes: disappearance of nuclease cuts present in "naked" and erythrocyte chromatin DNAs, and appearance of new cuts absent from these DNAs.