THE EXON-INTRON STRUCTURE OF HUMAN LHX1 GENE

We have determined the genomic structure of the human LHX1 gene, a mem ber of the LIM/homeobox (Lhx) gene family. The transcript is assembled from five exons, which are separated by introns ranging in size from 93 nt to 2.3 kb. The two LIM domains are entirely contained in the fir st and second exons, respectively, while the homeodomain is split into exons three and four. This structure closely parallels the organizati on of other mouse and human Lhx genes whose genomic Structure is known . An exception is the mouse and human isl1 genes, whose homeodomain do es not contain introns. An intron at the same position also occurs in the Xlim1 gene as well as in other homeobox genes, such as evx1 and ev x2, suggesting that this intron insertion represents an ancestral even t, from which homeobox genes of different families originated. In this context, evolution of the Lhx gene family probably involved the shuff ling of this intron-containing homeobox in the proximity of a LIM-only gene, while Islet genes were formed either by the shuffling of an int ronless homeobox to the same LIM domain or, alternatively, by intron l oss during their evolution. (C) 1996 Academic Press. Inc.