The expression of rat homeobox-containing genes is developmentally regulated and tissue specific.

Seven rat homeobox-containing sequences have been isolated by screening a genomic library with a probe derived from a Drosophila antennapedia cDNA clone. The characterization of two of these homeobox-containing clones has been described (Falzon, M., Sanderson, N.D. and Chung, S. Y. (1987) Gene 54, 23-32). Sequence analysis of the remaining five homeobox regions reveals a 180 bp domain sharing 70-95% identity at the amino acid level with the homeodomain from the Drosophila antennapedia gene and with the homeodomains from other metazoan species. Genomic blot analysis shows that each of the homeobox-containing DNA segments is probably present in a single copy per haploid genome. Northern blot analysis of RNA transcripts indicates that the rat homeobox-containing sequences are expressed during embryogenesis and in newborn and adult tissues in a tissue-specific manner; RNA expression is predominantly detected in spinal cord and kidney. Moreover, the pattern of RNA transcripts observed is distinct for each homeobox sequence, indicating differential regulation. Among the seven rat homeobox-containing sequences, the flanking sequences of five of the clones have no obvious sequence similarity with previously published sequences of homeobox-containing genes from other species. Two of the rat clones have been identified as potential homologues to mouse homeobox-containing sequences. For both pairs, a high degree of amino acid conservation (greater than 95%) is observed within the homeodomain and its immediate flanking regions between the putative homologous genes in mouse and rat. This strengthens the assertion that some of the mammalian homeobox-containing genes encode highly conserved proteins and may serve important biological functions.