The zebrafish homeobox gene hox‐2.2: transcription unit, potential regulatory regions and in situ 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 zebrafish 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 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 region of the hindbrain. Notably, untranslated regions of the hox‐2.2 gene contain several short sequences closely related to a known homeodomain recognition sequence.