The ribosome filter redux

Abstract

The ribosome filter hypothesis postulates that ribosomes are not simply translation machines but also function as regulatory elements that differentially affect or filter the translation of particular mRNAs. On the basis of new information, we take the opportunity here to review the ribosome filter hypothesis, suggest specific mechanisms of action, and discuss recent examples from the literature that support it.

Figure 1

Schematic representation of ribosomal filtering. 40S ribosomal subunits are indicated as large ovals. Subunit A contains two mRNA-binding sites indicated as green and red bars. The red binding site is masked in subunit B and the green binding site is masked in subunit C. The mRNA population shows three types of mRNAs: (#1) lacks mRNA-elements that function as ribosomal binding sites, (#2) contains an mRNA-element (green bar) that enhances translation initiation when it binds to the green binding site in 40S subunits, and (#3) contains an mRNA-element (red bar) that can block translation initiation when it binds to the red binding site the 40S subunits. The amount of protein expressed by the various mRNA-ribosomal subunit combinations are represented by the size of the black bands. Translation of mRNA #1 occurs by a cap-dependent mechanism and is translated with the same relative efficiency by ribosomal subunits A, B, or C. Translation of mRNA #2 is enhanced when translation involves ribosomal subunits A or B, in which the green binding site is accessible. Translation of mRNA #3 is inhibited when translation involves ribosomal subunits A or C, in which the red binding site is accessible.