SYNOPSIS. Growth hormone, prolactin and chorionic somatomammotropin (placental lactogen) area family of hormones that are related by function, immunochemistry and structure. Because of the structural similarities between these hormones, it was proposed that the corresponding genes were derived from a common precursor gene by duplication and sequence divergence. Comparisons of the mRNA sequences and chromosomal genes for these hormones from several species provide additional support for the model of their common ancestry and indications of how the precursor genewas formed. The diversification of these three genes has involved changes in codon choices thataffect the overall G-C content of the genes, alterations in the sizes of introns with conservedexon-intron boundaries and concerted evolutionary mechanisms with duplicated growth hormone andhorionic somatomammotropin genes in humans. The precursor gene appears to have evolved by the fourfold duplication of one exon element and the separate insertion of an exon encoding a different protein domain. Finally, there also appears to have been the separate insertion of sequences containing a promoter element and a potential glucocorticoid regulatory element.
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