Biogenesis of polysomes and transport of messenger RNA in yeast

The study of polysomal formation in the ts-136 thermo-sensitive yeast mutant indicates that ribosomes are assembled with an mRNA, which is in a structure tightly bound to membranes, and are then released into the soluble cytoplasmic fraction. This has been observed by studying the recruitment of ribosomes when glucose is added to glucose-starved cells and when transport of mRNA is permitted by shifting the mutant to the permissive temperature. A soluble cytoplasm and a fraction becoming soluble after sodium deoxycholate treatment of a rapidly sedimenting structure have been characterized. The former contains the majority of polysomes, free 80-S monomers and almost all of the ribosomal subunits. The latter fraction is composed of bound 80-S monomers and polysomes, but lacks ribosomal subunits. Treatment of the rapidly sedimenting structure with pancreatic ribonuclease produces the release of 80-S monomers, with EDTA the release of an equal proportion of both ribosomal subunits, and with sodium deoxycholate the release of 80-S monomers and polysomes. These findings are consistent with the assumption that bound ribosomes are assembled with an mRNA which is tightly bound to this rapidly sedimenting structure, presumably membranes. From the operational viewpoint this fraction is called the "membrane." During the process of polysomal formation ribosomes are recruited more rapidly in the "membranes" than in the soluble cytoplasm. Since "membranes" do not accumulate polysomes and contain only a small fraction of the total amount of ribosomes, the result is consistent with the assumption that either there is a higher turnover of bound versus free polysomes or bound polysomes are the precursors of free polysomes. The latter assumption is more likely since we have shown previously (a) that in yeast, transport is coupled with the translation of bound mRNA, (b) that this mRNA is tightly bound to a structure which sediments very rapidly and becomes soluble only after sodium deoxycholate treatment, and (c) when cycloheximide is added during the recruitment of ribosomes there is accumulation of membrane-bound ribosomes.