The Xenopus laevis globin gene family: chromosomal arrangement and gene structure.

Clones containing nine different larval and adult globin genes have been isolated from two genomic libraries of Xenopus laevis. They encompass three distinct DNA regions: a 70 kb region containing five genes in the order 5'-alpha Lla-alpha Llb-alpha Al-beta Al-beta Lla-3', all with the same transcriptional polarity; a 40 kb region with three genes, 5'-alpha Llla-alpha Lllb-alpha All-3', again with the same polarity; and a 10 kb segment comprising the beta Llla gene only. The beta All gene has not been found in our libraries. Genetic analysis has revealed two more beta L genes (beta Lllb, beta Lllb). Globin-like sequences have also been isolated, but have not been further characterized. The X. laevis globin gene family thus consists of 12 genes arranged in two clusters, each containing larval and adult alpha and beta genes in a unique, "symmetrical" arrangement. Electron microscopic analysis has revealed that Xenopus globin genes also comprise three exons and two introns, but differ from the globin genes of higher vertebrates in that the introns are much larger.