The unique structure of the Paramecium caudatum hemoglobin gene: the presence of one intron in the middle of the coding region.

The gene encoding hemoglobin (Hb) in the ciliated protist, Paramecium caudatum, has been cloned and sequenced. A single Hb locus was detected on an isolated genomic fragment with a size of approx. 15 kbp. This gene contained 443 bp. The 5' transcriptional regulatory region contained TATA box- and CAAT box-like sequences which resemble the structure of the promoter region in higher eukaryotic genes. The amino acid sequence deduced from the gene agreed precisely with that of the major Hb component. It was an unusual structure, much smaller than Hbs from many other organisms. Furthermore, all of the untranslated regions in this gene were also small. Particularly, one intervening sequence (intron) being only 23 bp was shorter than any other introns reported for eukaryotic genes encoding protein. The position of the intron in the P. caudatum Hb gene corresponded closely to that of the second intron which had been reported only for plant Hb genes. Possible routes for the evolution of Hb genes were discussed.