Comparative Study of Gene Structure for the Human liver-microsoml Type and Erythrocyte-soluble Type of Cytochrome <b_5> .

肝细胞色素b_5&gt；是一种膜蛋白，由包埋在微粒体内的亲水区和疏水区组成。红细胞中的细胞色素&lt；b5&gt；以可溶部分形式存在，其分子结构与肝酶的亲水区相似。这两种形式的细胞色素在不同的组织中发挥着不同的功能。(1)测定了人、猪和牛红细胞细胞色素b_5和gt_1的氨基酸全序列，以及肝脏蛋白质N-末端和连接区的序列。在红细胞细胞色素b5和gt的97个氨基酸残基中，有1~96个氨基酸残基与同种肝脏蛋白的氨基酸残基相同。然而，97个残基(C端残基)在两种类型中是不同的，这表明这两种形式的细胞色素lt；b5和gt；是由两个不同但密切相关的mRNAs编码的。(2)构建了兔肝细胞色素-lt；b_5&gt；基因的编码区，并对其进行了克隆和测序。该基因全长727个核苷酸。5‘非翻译区包含22个碱基，其次是402个碱基的开放阅读框，286个碱基的3’非翻译区和一个Poly(A)。N端氨基酸残基5‘端为ATG，表明该细胞色素不含疏水信号肽。(3)以兔细胞色素c DNA为探针，克隆了人细胞色素b_5&gt；基因组基因。该基因的测序目前正在进行中。人和兔细胞色素b5和gt的氨基酸残基同源性在90%以上，但在可比氨基酸残基的第三位密码子上只有不到50%的碱基完全相同。

Liver cytochrome <b_5> is a membrane protein consisting of a hydrophilic catalitic region and a hydrophobic region buried in microsomes. Cytochrome <b_5> in erythrocytes is present in soluble fraction and is similar in molecular structure to the hydrophilic domain of the liver enzyme. The two forms of the cytochrome play different functions in different tissues. (1) We have determined the complete amino acid sequence for human, porcine and bovine erythrocyte cytochrome <b_5> , and the sequences of the N-terminal region and joint region of the liver proteins. Of the 97 amino acid residues of erythrocyte cytochrome <b_5> , residues 1-96 were identical with those of the liver proteins of the same species. However, residue 97 (C-terminal residue) was different between the two types, indicating that the two forms of cytochrome <b_5> are encoded by two different but closely related mRNAs. (2) The cDNA encoding rabbit liver cytochrome <b_5> was constructed from mRNA, isolated and sequenced. The cDNA was found to be 727 nucleotides in length. The 5' untranslated region containd 22 bases and was followed by 402 bases of an open reading frame, 286 bases of 3' untranslated region and a poly (A). The 5' side of N-terminal amino acid residue was ATG indicating that the cytochrome had no hydrophobic signal peptide. (3) Using the cDNA of rabbit cytochrome <b_5> as a probe, genomic gene for the human cytochrome <b_5> has been isolated. The sequencing of the gene is now under progress. Although the homology in amino acid residues between human and rabbit cytochrome <b_5> is more than 90%, only less than 50% of the bases at the 3rd podition of the codons of the comparable amino acid residues were identical.