cDNA and amino-acid sequences and organization of the gene encoding the Bβ subunit of fibrinogen from Xenopus laevis

Fibrinogen, the major blood-clotting protein, is made up of three chains, Aα, Bβ and γ, which are synthesized and secreted by the liver. In this communication, we describe the complete cDNA sequence, deduced amino acid (aa) sequence and organization of the gene encoding the Bβ subunit of fibrinogen from Xenopus laevis (Xl). The cDNA representing the predominant form of the Bβ mRNA comprises 2390 nucleotides (nt), with an open reading frame of 1467 nt coding for a 488-aa protein. The percent identity between Xl Bβ and that of other animals ranges from 50% for lamprey to 66% for human. The Xl Bβ gene consists of nine exons, one more than found in the human gene. The exon/intron boundaries in the frog and human Bβ genes are in exactly conserved positions, except for junctions in the highly variable fibrinopeptide-encoding regions. Three of the exon/intron boundaries in the Xl Bβ gene are also analogous to ones in Aα and γ genes of other species, supporting the notion of a close evolutionary relationship between the genes for all three subunits. This analysis of Bβ from an amphibian provides the first complete description of the arrangement of exons and introns in any fibrinogen subunit gene from a non mammal and gives insight into the most highly conserved aspects of fibrinogen protein structure and gene organization.