Fig. 4.

Outline of the ribosome concentration and ribosome filter models. In the ribosome concentration model, different cells maintain different translational capacities. Specific mRNAs are more or less sensitive to the number of available ribosomes because of different cis-elements contained within their 5′UTRs and 3′UTRs. Many such transcripts encode for proteins needed for cell fate determination. When the translation capacity of a cell falls below a required threshold, specific subsets of mRNAs may be more affected relative to others. In the ribosome filter model, ribosomes exhibit a certain heterogeneity in rRNA sequence, rRNA modifications, ribosomal protein composition and ribosomal protein post-translation modifications. These differences may give rise to specialization, whereby specific subtypes of ribosomes selectively target specific mRNAs for translation. Therefore, loss of specialized ribosomes would impact the translation of specific transcripts to a greater degree relative to the total mRNA pool within a cell.