Kauffman M G, Rose P A, Kelly T J
Department of Molecular Biology and Genetics, Johns Hopkins Medical School, Baltimore, Maryland 21205.
Oncogene. 1991 Aug;6(8):1427-35.
Thymidine kinase (TK) is a nucleotide salvage pathway enzyme whose activity is highly dependent on the growth state and cell cycle phase of a cell. Cells in the resting or quiescent (G0) phase express very low levels of TK mRNA and protein. When quiescent cells are stimulated to enter the cell cycle by the addition of serum, TK mRNA, activity and polypeptide increase coordinately after about 10-15 h, at the beginning of S phase. When growth-independent heterologous promoters are substituted for the natural TK promoter, TK mRNA can be expressed in quiescent cells. Despite the presence of TK mRNA in such G0 cells, there is little expression of TK polypeptide; the normal increase in enzyme at S phase is observed following serum stimulation. Deletion of the introns and 3' untranslated sequences does not affect the expression of the TK gene in serum stimulation experiments. In contrast, deletion of the C-terminal 40 amino acids or fusion of a small segment of a beta-galactosidase to the C-terminus overcomes the block to expression of the TK polypeptide in G0 cells. These C-terminal alterations are the same as those which lead to constitutive expression of TK during the cell cycle of proliferating cells, suggesting that mechanisms which control the levels of TK in cycling cells may also operate in quiescent cells.
胸苷激酶（TK）是一种核苷酸补救途径酶，其活性高度依赖于细胞的生长状态和细胞周期阶段。处于静止或静息（G0）期的细胞表达极低水平的TK mRNA和蛋白质。当通过添加血清刺激静息细胞进入细胞周期时，在S期开始时约10 - 15小时后，TK mRNA、活性和多肽会协同增加。当用与生长无关的异源启动子替代天然TK启动子时，TK mRNA可在静息细胞中表达。尽管在这种G0细胞中存在TK mRNA，但TK多肽的表达很少；血清刺激后可观察到S期酶的正常增加。在血清刺激实验中，内含子和3'非翻译序列的缺失不影响TK基因的表达。相反，C末端40个氨基酸的缺失或一小段β-半乳糖苷酶与C末端的融合克服了G0细胞中TK多肽表达的障碍。这些C末端改变与增殖细胞在细胞周期中导致TK组成型表达的改变相同，这表明控制循环细胞中TK水平的机制可能也在静息细胞中起作用。
