A single ancestral gene of the human LIM domain oncogene family LMO in Drosophila: characterization of the Drosophila Dlmo gene.

Members of the human TTG/RBTN family, now renamed 'LMO' for LIM-only proteins, encode proteins with two tandem copies of a LIM motif. There are three members of this family, two have been isolated at the sites of chromosomal translocations in T-cell leukaemia. The function of the LIM motifs is at present unknown. We found that the LMO-2 gene is highly conserved between mammals, Drosophila and yeast. As a first step to obtain a model system for studying the function of the LIM motifs we have isolated the Drosophila homologue Dlmo. In contrast to mammals Drosophila appears to have only one lmo gene. A 2087 bp cDNA clone was isolated from a larval cDNA library, encoding a protein of 266 amino acids. A second transcript with an alternative 5' end was identified in RNA from embryos. The Drosophila lmo protein consists of two tandem copies of the conserved LIM domain characteristic of the human LMO family and an extended amino and carboxy terminus, which is not present in the human proteins. The amino acid sequence similarity with human LMO-1 and LMO-2 in LIM 1 is 79% and 69% and in LIM-2 90% and 60%, respectively. In addition a short stretch of 25 nucleotides with a homology of 83% between LMO-2 and Dlmo is found in the 3' UTR. Dlmo, like LMO-1, has an intron after the second LIM encoding region, which is not present in LMO-2. It is expressed maternally and at a high level in early embryogenesis as well as in adults. Interestingly we observed that the Dlmo protein is immunologically related to LMO-2 and can be detected by immunohistochemistry in early cellular blastoderm embryos. The gene was localised to a genetically well characterized region (17C on the X chromosome) opening the way for identification of mutations.