Translational control at the level of initiation between mRNAs for pre‐existing proteins and a 22‐kDa heat‐shock protein of Chlamydomonas by small cytosolic RNAs

Small cytosolic RNAs (scRNAs) from human placenta inhibit translation of poly(A)‐rich RNA from Chlamydomonas in the wheat germ cell‐free system. The major exception is the mRNA for a nuclear‐coded 22‐kDa chloroplast heat‐shock protein whose translation is much less affected. Evidence is presented which suggests that scRNAs do not directly interact with the mRNAs but with a factor of the wheat germ system instead. It has been found that run‐off translation of polyribosomes is not impaired by scRNAs whereas the formation of initiation complexes in vitro, again with the exception of those of the mRNA for the 22‐kDa heat‐shock protein, is heavily affected. From this evidence we conclude that scRNAs interfere with the action of one or more of the wheat germ initiation factors and that the translation of the mRNA for the 22‐kDa heat‐shock protein is much less dependent upon this (these) factor(s).