Structure of the Drosophila melanogaster annexin X gene.

The annexin X gene was cloned in the P1 recombinant phage carrying a genomic sequence of approximately 70 kb long. This DNA fragment encompasses at least two annexin X copies and several 7.8-kb tandem units represented by an anonymous sequence fused to the 3' truncated part of the annexin X gene. The proteins of annexin family contain a variable amino-terminal domain and a core domain; the latter includes four structurally conserved repeats that presumably arose as a result of duplications. The annexin X gene of Drosophila is about 2 kb long and contains four exons. Exon 1 encodes four amino-terminal amino acids, exon 2 encodes the remaining part of the amino-terminal domain and the three conserved repeats, and exon 3 and exon 4 encode the fourth repeat. The positions of introns 2 and 3 are strictly conserved with respect to both the amino acid position and codon phase as compared to introns 10 and 12 of the fourth repeat in vertebrate annexin genes. We propose the existence of a primordial annexin coding structure comprising at least two introns whose duplications during evolution have been followed by the loss of ancient introns in the first three repeats of Drosophila and vertebrates. Acquisition of new introns in vertebrates is supposed taking into account that exon borders are not found at homologous locations in four repeats of a given vertebrate annexin. Transcription of the annexin gene was detected in embryonic cell cultures. No profound effects of ecdysterone on the annexin X message content in cell cultures were observed.