Translation of non-capped mRNAs in a eukaryotic cell-free system: acceleration of initiation rate in the course of polysome formation.

Real-time monitoring of the translation of non-capped luciferase mRNA in a wheat germ cell-free system has been performed by continuous in situ measurement of the luminescence increase in the translation mixture. The phenomenon of acceleration of translation has been revealed. It has been shown that the acceleration is accompanied by the loading of translating polysomes with additional ribosomes, and thus is caused mainly by a rise in the initiation rate, rather than the stimulation of elongation or the involvement of additional mRNA molecules in translation. The acceleration requires a sufficient concentration of mRNA and depends on the sequence of the 5' untranslated region (UTR). It can be abolished by the addition of excess cap analog (m(7)GpppGm). As the acceleration does not depend on the preliminary translation of other mRNAs in the same extract, the conclusion has been made that the effect is not due to activation of the ribosome population or other components of the system during translation, but rather it is the consequence of intra-polysomal events. The acceleration observed is discussed in terms of the model of two overlapping initiation pathways in eukaryotic polysomes: translation of non-capped mRNAs starts with eIF4F-independent initiation at 5' UTR, and after the formation of sufficiently loaded polysomes, they rearrange in such a way that a mechanism of re-initiation of terminating ribosomes switches on. The eIF4F-mediated circularization of polysomes may be considered as a possible event that leads to the re-initiation switch and the resultant acceleration effect.