Genomic structure of Chlamydomonas caltractin. Evidence for intron insertion suggests a probable genealogy for the EF-hand superfamily of proteins.

A clone containing the gene locus for Chlamydomonas caltractin, a 20,000 Mr calcium-binding protein that is a member of the EF-hand superfamily of calcium-modulated proteins, was isolated and the structural organization of the gene was determined. The intron-exon organization was resolved by direct comparison of the genomic sequence with a caltractin cDNA. The promoter region does not contain the typical TATA or CCAAT boxes, but the sequences at the splice junctions are similar to those of other eukaryotes. The positions of the six introns in the caltractin gene do not typically define unit structures, nor do they coincide with those in genes for other members of the EF-hand superfamily. An analysis of exon sequences at the splice junctions in the genes of this multigene family was undertaken; evidence was obtained that supports the hypothesis that introns arose at proto-splice sites. A probable evolutionary history for the EF-hand superfamily based on intron insertion is offered.