J. Biol. Chem., Vol. 266, Issue 12, 7469-7480, 04, 1991
Concerted evolution led to high expression of a prosimian primate delta globin gene locus
DA Tagle, JL Slightom, RT Jones and M Goodman
Department of Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201.
The delta globin gene in simian primates is either weakly expressed (in
hominoids and New World monkeys) or silent (in Old World monkeys). In
prosimian primates, however, an unequal homologous crossover between the
psi eta and delta loci of lemurs produced a hybrid psi eta delta pseudogene
locus, whereas in tarsier the delta locus encodes a beta- type chain found
in 18% of adult tarsier hemoglobin molecules. In the present study, the
nucleotide and amino acid sequences of the galago delta and beta globin
genes and their encoded peptides were determined, and evidence is provided
showing that the galago delta locus encodes a beta-type chain (beta 2)
found in 40% of the galago fetal and postnatal hemoglobin molecules,
whereas the beta locus encodes the remaining 60% of the beta-type chain
(beta 1). Galago beta 1 and beta 2 chains differ from each other by only
one amino acid residue. The homology between the galago delta and beta loci
extends from 800 base pairs 5' of the proximal CCAAT element to near the
end of exon 3 as a result of a recombination event in which beta sequence
replaced delta sequence. After this initial recombination event, concerted
evolution between the loci continued over their conserved coding, intron 1,
and promoter regions but failed to occur between the two loci in their
intron 2 and distal 5'-flanking sequences where the two loci have now
diverged by 20%. Calculations based on this divergence value and on a rate
of noncoding sequence evolution of 4.2 x 10(-9) to 5.5 x 10(-9)
substitutions/site/year for the lorisiform lineage to galago yielded a date
of 18-24 million years ago for the initial recombination event. The fact
that the promoter sequences of the galago delta locus are the same as that
of the galago beta locus may account for the high level of expression of
the galago delta gene.
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