THE ZEBRAFISH HOMEOBOX GENE HOX-2.2 - TRANSCRIPTION UNIT, POTENTIAL REGULATORY REGIONS AND INSITU LOCALIZATION OF TRANSCRIPTS

The data presented in this report strongly suggest that the genome of the zebrafish, Brachydanio rerio, has a homeobox cluster which is equivalent to the murine Hox-2 locus. In support of this conclusion, we have found two closely linked zebrafish genes which are true homologues of the mouse Hox-2.1 and Hox-2.2 genes. Here we describe structural and functional properties of the zabrafish Hox-2.2 homologue hox-2.2. Furthermore, we have identified another zebrafish gene related to hox-2.2 which appears to correspond to the Hox-6.1 gene of the murine Hox-3 locus. In order to characterize the zebrafish hox-2.2 gene we have determined the genomic DNA sequence of a 3.4 kb SalI fragment. This revealed that the hox-2.2 transcription unit encodes a putative protein of 228 amino acids. The homeodomains of the murine Hox-2.2 and the zebrafish hox-2.2 proteins are almost identical and extensive sequence identity exists in other regions of the two proteins, which share 160 (70%) of the amino acid residues. Also in terms of expression, strong similarities were observed relative to the murine Hox-2.2 gene. Transcripts derived from zebrafish hox-2.2 start to accumulate when somite formation is initiated. Later in the development these transcripts are detected mainly in the central nervous system. Reminiscent of Hox-2.2, the rostral boundary of zebrafish hox-2.2 expression is located in the posterior of the hindbrain. Notably, untranslated regions of the hox-2.2 gene contain several short sequences closely related to a known homeodomain recognition sequence.