雞的CD 8α基因Flanking Region結構之研究

Abstract

CD8分子是一種由T細胞表現至細胞表面的醣蛋白，由α和β兩條勝?鏈以Heterodimer結構存在。其所扮演的角色為與抗原呈現細胞（Antigen Prensting Cell）中的MHC Classl I 分子結合，及參予Cytotoxic T Cell本身的活化。 本實驗主要目的為定出CD8 α基因在雞的genome上的結構以幫助瞭解此基因在演化上可能形成的過程，並希望藉此能找到與CD8 α基因表現上有關的調控位元或機轉。目前我們的研究結果得到一與演化有關的結果。我們由目前定出的CD8 α基因，CD8 α上游約15 kb的序列，進入GCG搜尋，發現不少基因片段和人跟老鼠的Ig kLight Chain極為相似。同時在我們定出15 kb的序列中，6046?6712和部分CD8 α structure gene與Ig k Light Chain 有極高相似性。CD8 α與Ig k Light Chain皆為Immunoglobulin Superfamily的一員。在這段序列中我們找到 Immunoglobulin形成Floding時所必須的Cysteine，這意味著這段基因有某個Conserve的特殊結構為 Immunoglobulin所使用。 雞至今仍未發現有Ig k Light Chain的存在，在人跟老鼠Ig k Light Chain和CD8分子基因位於一條染色體上，如果雞有Ig k LightChain ，這些和Ig k LightChain相似的基因片段可能就是Ig k Light Chain的一部份，或其前身。人類和老鼠則為增加免疫球蛋白的變異性，而演化出Ig k Light Chain來。雞則可能因為屬於較低等的動物、對免疫球蛋白變異性需求不這麼高，所以沒有Ig k Light Chain演化出。 在這段約15kb的序列中我們還找到兩段Reverse Repeat的DNA片段，我們將這兩段基因送入GCG搜尋發現其中有一段序列Immunoglobulin Fc Receptor的基因很像。至於這兩段DNA片段所代表的意義還不是很清楚，有待進一步的研究。

CD8 molecules are cell-surface glycoproteins that participate in both T cell development and antigen recognition by T cells . CD8 exists as a disulfide-linked heterodimer of the α and β chains . The sequence encoding chicken genomic CD8 α gene was proviously determined in this laboratory . In order to futher understand the mechanism involved in regulating the expression of CD8 αgene , we extended our studies by determining the flanking sequences surrounding the CD8 α coding region. We found some DNA fragments upstream of the chicken CD8 α gene displaying squence similarity to the coding region of CD8 α gene and Ig k light chain genes of human and mouse. A block of DNA in the 15 kb region was also found to be a partial duplicate of the CD8 α gene . Till now no Ig k light chain is documented to be expressed in the chicken . In human and mouse , CD8 α gene and Ig k light chain are located in the same chromosome . If chickens do have Ig k light chain ,these DNA fragment with sequences similar to the Ig K light chain might be part of the pseudogenes of the chicken Ig k V gene members. More experiments are needed to determine whether there is (pseudo-) Ig k loci present in chickens . Two different inverted repeats were found in the 15 kb region . One of them showed certain degree of sequence similarity to Ig Fc receptor . Again , more experiments are needed to help us understanding the meanings of these DNA fragments .