Molecular cloning and characterization of the gene coding for human complement protein factor B

Four cosmid clones, each with an average insert size of 40 kilobase pairs and containing the factor B gene, were isolated from a human genomic DNA library. The clones were identified by hybridization with a 515-base-pair cDNA probe isolated by using a unique 17-base synthetic oligonucleotide probe from a human liver cDNA library. The cosmid clones were characterized by restriction endonuclease digestion and Southern blotting, and a partial restriction map of the DNA represented in the cosmids was constructed. The Bb portion of the factor B gene is about 4 kb in length. DNA sequence analysis has resulted in the determination of 3.3 kb of sequence at the 3' end of the gene. This region codes for amino acids 87-505 of Bb and includes the whole of the serine proteinase domain of the protein. The three active site residues of histidine, aspartic acid, and serine found at positions 267, 317, and 440 of the Bb sequence, respectively, lie on separate exons. Other functional regions within the serine proteinase domain are separated also by intervening sequences.