Organization and transcriptional mechanisms of the zebrafish otp1 gene. A, otp1 mRNA is alternatively spliced and encodes two forms of the protein. On the right are shown the amino acid residues of the short and the long form of Otp1, respectively. The solid arrowhead indicates the position of the 304-nucleotide (nt) intron involved in the alternative splicing. The deleted 39-nt region of the otp1 mRNA resulting in the short form of the protein is shown by a thin solid line. The additional donor site triggering the 39-nt deletion is also indicated (ADS). The open arrowhead points to the position of the second intron. The asterisk indicates the stop codon at the end of the ORF. The AATAAA polyadenylation site and the TTATTTATT motif that mediates mRNA degradation are double-underlined. The thin dashed line marks the homeodomain, the thick solid line marks the conserved 12 amino acids downstream of the homeodomain and the bold dotted line marks the OAR domain. B, Alternative splicing pattern of the otp1 nuclear RNA. The sequences of the donor (DS) and the acceptor sites (AS) of the 304-nt intron are shown, together with the flanking regions of the coding sequence. The alternative donor site (ADS) located in the upstream exon that determines the deletion of the 39-nt region (solid line) is also indicated. The sequence has been submitted to the GenBank/EMBL database under accession number AF071496.