J Mol Evol (1996) 42:641-647 jou..ALo, MOLECULAR [EVOLUTION © Springer-Verlag New York Inc. 1996 Evolutionary History of Introns in a Multidomain Globin Gene Anna M. Jellie, Warren P. Tate, Clive N.A. Trotman Department of Biochemistry, University of Otago, Box 56, Dunedin, New Zealand Received: 27 September 1995 / Accepted: 15 December 1995 Abstract. The Artemia hemoglobin contains two sub- units that are similar or different chains of nine globin domains. The domains are ancestrally related and are presumed to be derived from copies of an original single- domain parent gene. Since the gene copies have re- mained in the same environment for several hundred million years they provide an excellent model for the investigation of intron stability. The cDNA for one of the two types of nine-domain subunit (domains T1-T9) has been sequenced. Compari- son with the corresponding genomic DNA reveals a total of 17 intradomain introns. Fourteen of the introns are in locations on the protein that are conventional in globins of other species. In eight of the nine domains an intron corresponds to the B helix, amino acid B12, following 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 in- trons having been inherited from a single-domain parent gene. The remaining three introns are in unconventional 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 inter- pretation of intron inheritance since globin F introns are rare, and in domains T3 and T6 they replace rather than supplement the conventional G introns, as though dis- placement from G to F occurred before that part of the gene became duplicated. It is inferred that one of the F introns subsequently moved by one nucleotide. Simi- larly, the third unconventional intron location is the G intron in domain T4 which is in G6, phase 2, one nucleo- Correspondence to: C.N.A. Trotman tide earlier than the other G introns. Domain T4 is also unusual in lacking a B intron. The pattern of introns in the Artemia globin gene supports a concept of general positional stability but the exceptions, where introns have moved out of reading frame, or have moved by several codons, or have been deleted, suggest that intron displacements can occur after inheritance from an an- cient source. Key words: Protein evolution --Artemia -- Intron -- Exon -- Hemoglobin Introduction In the debate about the possible "early" or "late" origin of introns (Roger and Doolittle 1993; Dibb 1993; Hurst 1994) the hemoglobin gene is a well-studied model adopted widely in support of both sides of the argument. Hemoglobin introns, where present, are usually found in one or more of only three positions in terms of the trans- lated protein: corresponding to amino acid residue B 12 (B helix, residue 12); in a somewhat variable E helix location; and between G6 and G7. This consistency lends some support to the introns- early idea of each intron having been inherited continu- ously since a single primordial origin. Not all hemoglo- bins have all three introns, and any missing ones, according to the introns-early theory, have been deleted at some ancestral stage. The origin of those remaining could have been coincident with an event in which pri- mordial minigenes, coding for compact modules of pro- tein now represented by exons, became linked together to encode more complex proteins while untranslated DNA flanking the minigenes survived as inU'ons. Since it is
