Physarum polycephalum L., a naturally synchronized myxomycophyta, was demonstrated to contain a cyclin B1-like protein by Western blot and immunoelectron microscopy. The content and subcellular location of the protein varied during the cell cycle. The cyclin B1-like protein was first detected in the plasmodia of S phase while it did not appear in the nuclei until late G2 phase. The content of the protein in both the plasmodia and nuclei rose gradually onwards, peaked at metaphase and disappeared abruptly at ana-telophase. The protein was found to be distributed in both the cytoplasm and nuclei in late G2 phase and metaphase. In nuclei, the protein was mainly located in the chromosomal and nucleolar areas. The results suggest that the cyclin B1-like protein of P. polycephalum begins to be synthesized at S phase, enters the nuclei at late G2 phase, accumulates in both cytoplasm and nuclei onwards and breaks down at ana-telophase. The results also suggest that the cyclin B1-like protein acts as a cytoplasmic-nuclear protein during certain phases of the cell cycle.
DNA的大部分区域通过包装成特殊的染色质结构而失去活性称为染色质沉默。这些特殊的染色质结构在维持染色体结构稳定和基因调控中起重要作用。有实验表明,沉默染色质的组蛋白H3和H4的的氨基末端尾部相对于基因组的其他区域是低乙酰化的。组蛋白去乙酰化酶SIR2(silent information regulator2)是参与染色质沉默的一种重要的蛋白质。SIR2具有两种相关联的酶活性,组蛋白去乙酰化酶活性和NAD高能骨架的断裂活性,并在酶反应过程中产生一种新的产物氧代乙酰基ADP核糖基(O-acetyl-ADP-ribose)。SIR2的组蛋白去乙酰化酶活性为研究SIR2与沉默染色质的组蛋白低乙酰化状态的关系提供了直接证据。而SIR2的这两种酶活性的关系也表明,组蛋白去乙酰化酶活性不是SIR2惟一的功能。SIR2的NAD水解酶活性和O-acetyl-ADP-ribose的合成过程也可能是染色质沉默机制所必需的。
