EVOLUTIONARY HISTORY OF INTRONS IN A MULTIDOMAIN GLOBIN GENE

The Artemia hemoglobin contains two subunits that are similar or diffe rent chains of nine globin domains. The domains are ancestrally relate d and are presumed to be derived from copies of an original single-dom ain parent gene. Since the gene copies have remained in the same envir onment for several hundred million years they provide an excellent mod el for the investigation of intron stability. The cDNA for one of the two types of nine-domain subunit (domains T1-T9) has been sequenced. C omparison with the corresponding genomic DNA reveals a total of 17 int radomain introns. Fourteen of the introns are in locations on the prot ein that are conventional in globins of other species. In eight of the nine domains an intron corresponds to the B helix, amino acid B12, fo llowing the second nucleotide (phase 2), and in six domains a G-helix intron is located between G6 and G7 (phase 0). The consistency of this pattern is supportive of the introns having been inherited from a sin gle-domain parent gene. The remaining three intron are in unconvention al locations. Two occur in the F helix, either in amino acid F3 (phase 1) in domain T3, or between F2 and F3 (phase 0) in domain T6. The two F introns strengthen an interpretation of intron inheritance since gl obin F introns are rare, and in domains T3 and T6 they replace rather than supplement the conventional G introns, as though displacement fro m G to F occurred before that part of the gene became duplicated. It i s inferred that one of the F introns subsequently moved by one nucleot ide. Similarly, the third unconventional intron location is the G intr on in domain T4 which is in G6, phase 2, one nucleotide earlier than t he other G introns. Domain T4 is also unusual in lacking a B intron. T he pattern of introns in the Artemia globin gene supports a concept of general positional stability but the exceptions, where introns have m oved out of reading frame, or have moved by several codons, or have be en deleted, suggest that intron displacements can occur after inherita nce from an ancient source.