Chemical Synthesis of a Gene Encoding the Human Complement Fragment C5a and Its Expression in Escherichia coli

A gene coding for the C5a fragment of the fifth component of human complement has been chemically synthesized, cloned, and expressed in Escherichia coli. The 253-base-pair gene fragment was built through a two-step enzymic assembly of 16 oligonucleotides, the average length of each being 32 residues. The oligonucleotides were synthesized by using the phosphoramidite method. The gene was cloned in a pBR322-derivative plasmid downstream from the lac uppromoter mutant, UV5-D. The expression of C5a was detected and measured by immunoassay and a radioligand binding assay. C5a from E. coli was comparable to C5a purified from human serum in inhibiting binding of human125I-labeled C5a to its putative receptor on polymorphonuclear leukocytes. Studies of smooth muscle contraction in isolated guinea pig ileum showed that the recombinant C5a was biologically active and produced cross-tachyphylaxis with human serum-derived C5a. The results demonstrate the feasibility of expressing C5a anaphylatoxin in bacteria and provide a system for mutagenesis of the C5a protein.