Structural aspects of the human C5 gene: Intron/exon organization, 5′-flanking region features, and characterization of two truncated cDNA clones

Human C5 cDNA fragments were used to identify five overlapping cosmid clones that spanned the entire C5 gene. Partial sequencing and Southern analysis of the clones were performed to identify intron/exon boundaries and to map intron size. The human C5 gene is 79 kilobases in length and is comprised of 41 exons. Comparison of C5 with the homologous family members C3 and C4 revealed striking similarities in exon size and number. Less, although significant similarities were also observed with the family member α₂-macroglobulin. The transcriptional start site for the C5 gene was observed as a doublet at positions 29 and 28 nucleotides upstream of the ATG start codon. The 5′-flanking region of the gene contains sequences homologous with several known responsive elements, including interferon, interleukin-6, glucocorticoid, estrogen, NF-_κB, and HNF-1. Two previously identified truncated cDNAs, pHC5A and pHC5B, contain 21 and 16 exons, respectively. The last exon in pHCSA, designated exon 2 1a, is a product of alternative splicing and is not present in the major full-length transcript. Truncation of pHC5A is the result of an alternative polyadenylation signal located in exon 2 1a. In pHC5B, exon 16 is extended on the 3′ end by additional flanking genomic sequence that also contains an alternative polyadenylation signal.