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

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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.