Initiation reactions in the mRNA-dependent reticulocyte lysate.

Reticulocyte lysates depleted of mRNA by digestion with micrococcal nuclease still show an unexpectedly high rate of formation of 80 S initiation complexes. Formation of these complexes is sensitive to all inhibitors of the normal protein synthesis initiation process tested. Such lysates contain high concentrations of mRNA fragments which can be utilized for initiation, with which exogenous mRNA must compete. As a consequence of this competition, mRNAs that are weak initiators may be translated poorly by this system even at low exogenous mRNA concentrations.