Vik D P, Amiguet P, Moffat G J, Fey M, Amiguet-Barras F, Wetsel R A, Tack B F
Department of Immunology, Scripps Clinic and Research Foundation, La Jolla, California 92037.
Biochemistry. 1991 Jan 29;30(4):1080-5. doi: 10.1021/bi00218a029.
The third component of human complement (C3) is a key molecule in the activation of the complement cascade. C3 cDNA fragments were used to identify seven cosmid clones that covered all but 1 kilobase pair (kb) of the C3 gene. The remainder of the gene was cloned by using the polymerase chain reaction. These clones were used to identify the intron/exon boundaries and to map the gene. The C3 gene is 42 kb in length and comprises 41 exons ranging in size from 52 to 213 base pairs (bp). The transcription start site was identified by primer extension, and approximately 1 kb of DNA upstream of this site was sequenced. Putative TATA and CAAT boxes were identified along with a number of regions that shared homology with known regulatory sequences. These include responsive elements for interferon-gamma, interleukin-6, nuclear factor kappa B, estrogen, glucocorticoids and thyroid hormone. Several of these agents have been shown to affect C3 synthesis and mRNA levels. The sizes of the exons in C3 were compared to those of C4 and alpha 2-macroglobulin (alpha 2M). Thirty-nine of 41 exons in C4 were found to be of similar size to the analogous ones in C3, and two-thirds of those in alpha 2M were also similarly sized, supporting the hypothesis that these genes arose from a common ancestor.
人类补体的第三成分（C3）是补体级联反应激活中的关键分子。利用C3互补DNA（cDNA）片段鉴定出7个黏粒克隆，这些克隆覆盖了C3基因除1千碱基对（kb）以外的所有区域。该基因的其余部分通过聚合酶链反应进行克隆。这些克隆用于鉴定内含子/外显子边界并绘制基因图谱。C3基因长度为42 kb，由41个外显子组成，大小从52到213碱基对（bp）不等。通过引物延伸确定转录起始位点，并对该位点上游约1 kb的DNA进行测序。鉴定出了推定的TATA盒和CAAT盒以及一些与已知调控序列具有同源性的区域。这些区域包括γ干扰素、白细胞介素-6、核因子κB、雌激素、糖皮质激素和甲状腺激素的反应元件。其中几种因子已被证明会影响C3的合成和信使核糖核酸（mRNA）水平。将C3中外显子的大小与C4和α2巨球蛋白（α2M）的外显子大小进行了比较。发现C4的41个外显子中有39个与C3中类似的外显子大小相似，α2M中的三分之二外显子大小也相似，这支持了这些基因起源于共同祖先的假说。
