Coupled Expression of The Genes Encoding The Constituents of The Glycine Cleavage System in Chicken

利用特异性抗体从鸡肝脏cDNA表达文库中分离出几个编码甘氨酸裂解系统组成部分甘氨酸脱羧酶的cDNA克隆。重叠的cDNA克隆均匀编码在相同的开放阅读框中，对酶的部分初级结构进行编码，并确定了3490碱基长的甘氨酸脱羧酶cDNA序列。用免疫筛选法克隆了鸡h蛋白的前体cDNA，编码了鸡h蛋白840碱基序列中164个氨基酸残基的前体。利用这两个cDNA片段，我们可以分析甘氨酸脱羧酶和h蛋白基因的表达模式。甘氨酸脱羧酶mRNA丰度随酶系统在肝、肾和脑中的含量呈平行变化，显示了整个反应的特定活性，比例为35:11:1。相比之下，在反应中失活的心脏、脾脏和骨骼肌线粒体中含有大量但少量的活性h蛋白及其mRNA，而甘氨酸脱羧酶及其mRNA未被发现，表明酶系统的组织特异性分布主要由甘氨酸脱羧酶基因的表达决定。我们将其定义为h蛋白基因的基础表达。排除基础表达，甘氨酸脱羧酶和h蛋白基因的相对转录效率似乎是相等的，甘氨酸脱羧酶和h蛋白mrna在肝脏、肾脏和大脑中保持等摩尔水平，而不考虑基因表达量的差异。这表明，甘氨酸裂解活性的大小是由调节甘氨酸裂解系统组分生物合成的协调机制决定的，而甘氨酸脱羧酶基因的表达在间质起源的细胞中受到抑制。

several clones of cDNA encoding qlycine decarboxylase, a constituent of the glycine cleavage system, were isolated from chicken liver cDNA expression libraries with a specific antibody. The overlapping cDNA clones evenly coded in an identical and open reading frame for the partial primary structures of the enzyme, and sequence of the 3,490 base long glycine decarboxylse cDNA was determined. H-protein cDNA was also cloned by immunoscreening, and this encoded the precursor from of chicken h-protein of 164 amino acid residues within the 840 base long cDNA sequence. Using both cDNA fragments we could analyze the mode of expression of the glycine decarboxylase and h-protein genes.Glycine decarboxylase mRNA aboundance varied in parallel with the content of the enzyme system in liver, kidney, and brain, which reveal specific activities of the overall reaction at a tatio of 35:11:1. in contrast, heart, spleen, and skeletal muscle mitochondria inactive in the reaction contained significant but small amounts of active H-protein and its mRNAs, whereas glycine decarboxylase and its mRNA were not found, indicationg that tissue-specific distribution of the enzyme system is primarily determined by expression of the hlycine decarboxylase gene. We defined them as the basal expression of the H-protein gene. Excluding the basal expression, relative efficiencies of run-off transcription on the glycine decarboxylase and H-protein genes appeared to be equal and the equimolar level of glycine decarboxylase and H-protein mRNAs are maintaianed in liver, kidney, and brain, irrespective of the difference in the amounts of expression of the genes. It is suggested that the magnitude of the glycine cleavage activity is specified by the coordinate mechanism which resides in regulation for biosynthesis of the components of the glycine cleavage system, except that the glycine decarboxylase gene expression alone is repressed in the cells of mesenchymal origin.