Polyribosome dynamics: size-distribution as a function of attachment, translocation and release of ribosomes

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.