Nucleotide sequence of the gene for the b subunit of human factor XIII.

Factor XIII (Mr 320,000) is a blood coagulation factor that stabilizes and strengthens the fibrin clot. It circulates in blood as a tetramer composed of two a subunits (Mr 75,000 each) and two b subunits (Mr 80,000 each). The b subunit consists of 641 amino acids and includes 10 tandem repeats of 60 amino acids known as GP-I structures, short consensus repeats (SCR), or sushi domains. In the present study, the human gene for the b subunit has been isolated from three different genomic libraries prepared in lambda phage. Fifteen independent phage with inserts coding for the entire gene were isolated and characterized by restriction mapping, Southern blotting, and DNA sequencing. The gene was found to be 28 kilobases in length and consisted of 12 exons (I-XII) separated by 11 intervening sequences. The leader sequence was encoded by exon I, while the carbonyl-terminal region of the protein was encoded by exon XII. Exons II-XI each coded for a single sushi domain, suggesting that the gene evolved through exon shuffling and duplication. The 12 exons in the gene ranged in size from 64 to 222 base pairs, while the introns ranged in size from 87 to 9970 nucleotides and made up 92% of the gene. The introns contained four Alu repetitive sequences, one each in introns A, E, I, and J. A fifth Alu repeat was present in the flanking 3' end of the gene. Two partial KpnI repeats were also found in the introns, including one in intron I and one in intron J. The KpnI repeat in intron J was 89% homologous to a sequence of approximately 2200 nucleotides flanking the gene coding for human beta globin and approximately 3800 nucleotides from the L1 insertion present in the gene for human factor VIII. Intron H also contained an "O" family repeat, while two potential regions for Z-DNA were identified within introns G and J. One nucleotide change was found in the coding region of the gene when its sequence was compared to that of the cDNA. This difference, however, did not result in a change in the amino acid sequence of the protein.