The cDNA for the β-subunit of human chorionic gonadotropin suggests evolution of a gene by readthrough into the 3′-untranslated region
The cDNA for the β-subunit of human chorionic gonadotropin suggests evolution of a gene by readthrough into the 3′-untranslated region
复制标题
人绒毛膜促性腺激素 β 亚基的 cDNA 表明基因通过读入 3-非翻译区而进化
作者：
J. Fiddes;H. Goodman
克隆了编码人绒毛膜促性腺激素(HCG)β亚基145个氨基酸的579个碱基的全长基因，并测定了其全序列。从核苷酸序列中可以识别出一个由20个氨基酸组成的疏水前序列。已知β-亚基的氨基酸序列与其他糖蛋白激素LH、FSH和TSH的β-亚基的氨基酸序列相关，但β-亚基的独特之处在于它含有大约30个氨基酸的C末端延伸，这在其他三种激素中没有相应的对应。对βHCG基因核苷酸序列的分析表明，这种延伸可能是由于祖先类似β的基因终止密码子的丢失，使得以前的3‘-非翻译区的大部分现在编码蛋白质。HCG的β-亚基终止于密码子UAA，在AAUAAA序列中，UAA位于Poly(A)之前16个碱基。该序列被认为是参与多聚腺苷酸化或加工的识别信号，因此在该基因中具有双重作用，既有编码功能，也有调节功能。
A 579-base pair approximately full-length cDNA which codes for the 145-amino acid long β-subunit of human chorionic gonadotropin (HCG) has been cloned in the plasmid vector pBR322 and its complete nucleotide sequence determined. A hydrophobic presequence of 20 amino acids can be identified from the nucleotide sequence. The amino acid sequence of the β-subunit is known to be related to those of the β-subunits of the other glycoprotein hormones LH, FSH and TSH, but the β-subunit of HCG is unique in that it contains a C-terminal extension of about 30 amino acids which has no homologous counterpart in the other three hormones. Analysis of the βHCG cDNA nucleotide sequence suggests that this extension may have arisen by the loss of the termination codon of an ancestral β-like gene so that most of what was previously the 3′-untranslated region now codes for protein. The β-subunit of HCG terminates with the codon UAA located 16 bases before the poly (A) in the sequence AAUAAA. This sequence is believed to be a recognition signal involved in either polyadenylation or processing and therefore has a dual role in this gene, serving both a coding and a regulatory function.