Structure and developmental expression of Ikaros in the Mexican axolotl

The Ikaros family of transcription factors plays an essential role in hematopoiesis. We report here the structure of cDNA clones encoding two Ikaros isoforms, Ikl and Ik2, in the Mexican axolotl. The Ik1 cDNA sequence is very similar to that of the rainbow trout, chicken, and mammalian Ik1 sequences. However, a 96 base pair region which encodes the first N-terminal zing finger (F1) is lacking from axolotl Ik1, both in clones from a cDNA library and clones isolated from direct polymerase chain reaction products. A region corresponding to exon 3 is completely absent from the axolotl Ik2 sequence and thus the Ik1 and Ik2 isoforms possess the same number of zinc finger motifs. The structure of these five CC-HH motifs is very well conserved in the axolotl, including the structural deviations from its amino acid consensus composition which are identical in all species analyzed to date. The axolotl Ik1 3' untranslated region sequence is very long (2538 bp) and contains two UA-rich motifs known as instability determinants and which could play a role in mRNA translational efficiency. Ikaros transcripts are first detected in the ventral blood island of stage 36 embryos, about 24 h before the first heartbeats (late tailbud stage), and then in the major lymphopoietic organs of the developing larvae. In situ hybridization demonstrates that Ikaros transcripts are abundant at the periphery of the thymus lobes, in the presumptive site of early thymocyte differentiation.