Contribution of internal initiation to translation of cellular mRNAs containing IRESs.

A broad range of cellular stresses lead to the inhibition of translation. Despite this, some cellular mRNAs are selectively translated under these conditions. It is widely supposed that cap-independent internal initiation may maintain efficient translation of particular cellular mRNAs under a variety of stresses and other special conditions when cap-dependent protein synthesis is impaired. However, in spite of a large number of reports focused on the investigation of the regulation of IRES (internal ribosome entry site) activity in different tissues and under various stresses, only rarely is the real efficiency of IRES-driven translation in comparison with cap-dependent translation evaluated. When precisely measured, the efficiencies of candidate IRESs in most cases appeared to be very low and not sufficient to compensate for the reduction of cap-dependent initiation under stresses. The usually low efficiency of internal initiation of translation is inconsistent with postulated biological roles of IRESs.