Department of Molecular Biology and Genetics, The Johns Hopkins University, School of Medicine, Baltimore, Maryland 21205, USA.
BMC Biochem. 2010 Nov 16;11:45. doi: 10.1186/1471-2091-11-45.
Telomere function requires a highly conserved G rich 3'- overhang. This structure is formed by 5'-resection of the C-rich telomere strand. However, while many nucleases have been suggested to play a role in processing, it is not yet clear which nucleases carry out this 5'-resection.
We used biochemical purification to identify a sequence-dependent exonuclease activity in Tetrahymena thermophila cell extracts. The nuclease activity showed specificity for 5'-ends containing AA or AC sequences, unlike Exo1, which showed sequence-independent cleavage. The Tetrahymena nuclease was active on both phosphorylated and unphosphorylated substrates whereas Exo1 requires a 5'-phosphate for cleavage.
The specificities of the enzyme indicate that this novel Tetrahymena exonuclease is distinct from Exo1 and has properties required for 3'-overhang formations at telomeres.
端粒功能需要高度保守的富含 G 的 3'-突出端。这种结构是通过 C 丰富的端粒链的 5'-切除形成的。然而，尽管已经提出许多核酸酶在加工中发挥作用，但尚不清楚哪些核酸酶进行这种 5'-切除。
我们使用生化纯化方法在嗜热四膜虫细胞提取物中鉴定出一种序列依赖性外切酶活性。与序列非依赖性切割的 Exo1 不同，该核酸酶活性特异性识别含有 AA 或 AC 序列的 5'-末端。四膜虫核酸酶对磷酸化和非磷酸化底物均具有活性，而 Exo1 则需要 5'-磷酸基团才能进行切割。
酶的特异性表明，这种新型的嗜热四膜虫外切酶与 Exo1 不同，具有在端粒处形成 3'-突出端所需的特性。
