Effects of addition of derived 40 S subunits on translation rate and polysome profile of the reticulocyte lysate.

We have investigated the way in which the addition of exogenous 40 S subunits to a reinitiating cell-free translation system, prepared from reticulocytes, may affect translational parameters of the system. The disturbance of the system's subunit stoichiometry resulted in the following changes in the ribosome profile: (1) rapid exhaustion of the pool of native 60 S subunits; (2) appearance of humps on the peaks of the polysome profile, which probably represent unusually long-lived [40 S. polysomal] complexes; (3) at higher doses of exogenous particles, the amount of polysomes decreased. This latter effect reflected a corresponding decrease in the overall translation (i.e. initiation) rate. The phenomena are interpreted as follows: exogenous 40 S subunits combine with 60 S subunits, forming idle 80 S ribosomes. The shortage of 60 S subunits delays the utilization of [40 S. polysomal] complexes, which is compensated for by a pool increase of these complexes. At high 40 S subunit doses this compensatory mechanism fails, and the 60 S shortage begins to determine the overall translation rate. The observations underline that the various translational parameters of the lysate function in an optimally concerted manner, so that only small amounts of derived 40 S subunits are tolerated by the system for analysis.