Singh U N
Department of Biophysics, University of Delhi, India.
J Theor Biol. 1996 Mar 21;179(2):147-59. doi: 10.1006/jtbi.1996.0055.
Assembly of polyribosomes is the ubiquitous manifestation of the activation of translational machinery in cells. This is accompanied by a concomitant appearance of mRNAs either from an activation of pre-existing ribo-nucleoprotein particles (RNP) or from newly synthesized transcripts. The size-distributions of polyribosomes as inferred from their sedimentation profiles are essentially defined by various kinetic parameters responsible for continuous flux of ribosomes due to their recycling between the "free" and the "bound" states. In the theoretical analysis presented here the time-evolution of polyribosomes and their sustenance in the steady state is considered as a Markovian process with the states of mRNAs defined as the number of ribosomes attached. Expressions for the elements of one-step transition matrix as a function of kinetic parameters related to the initiation, translocation and the release of ribosomes and the size of mRNA are derived. The latter is shown to be relevant only in so far as it sets an upper limit to the size of polyribosomes. Implications of transient arrest of ribosomes at the initiation site as envisaged in the signal hypothesis for the synthesis of secretory proteins and the putative role of receptor-mediated post-translational transport of proteins have been examined. Results of computer simulations on model systems have focussed attention on the diverse types of profiles that may appear under varying physiological conditions.
多核糖体的组装是细胞中翻译机制激活的普遍表现。这伴随着mRNA的同时出现，这些mRNA要么来自预先存在的核糖核蛋白颗粒（RNP）的激活，要么来自新合成的转录本。从沉降曲线推断出的多核糖体的大小分布基本上由各种动力学参数决定，这些参数负责核糖体由于在“游离”和“结合”状态之间循环而产生的连续通量。在这里提出的理论分析中，多核糖体的时间演化及其在稳态下的维持被视为一个马尔可夫过程，其中mRNA的状态被定义为附着的核糖体数量。推导了一步转移矩阵元素作为与核糖体起始、易位和释放以及mRNA大小相关的动力学参数的函数的表达式。结果表明，后者仅在为多核糖体大小设定上限方面具有相关性。已经研究了信号假说中设想的核糖体在起始位点的瞬时停滞对分泌蛋白合成的影响以及受体介导的蛋白质翻译后转运的假定作用。模型系统的计算机模拟结果将注意力集中在不同生理条件下可能出现的各种类型的曲线。
