The small and large ribosomal subunits depend on each other for stability and accumulation

The 1:1 balance between the numbers of large and small ribosomal subunits can be disturbed by mutations that inhibit the assembly of one of the subunits. We have investigated if the cell has mechanisms to counteract an imbalance of subunits. We show that accumulation of 40S subunits stops after abrogating 60S assembly. In contrast, cessation of the 40S pathways does not prevent 60S accumulation, but does, however, lead to fragmentation of the 25S rRNA in 60S subunits. Even though 40S subunits do not accumulate in the absence of 60S assembly, 40S assembly continues, indicating that excess 40S is degraded after assembly. Thus, the cell expends substantial resources in a futile attempt to increase the number of functional 80S ribosomes. We propose that a mechanism for preventing the accumulation of 40S subunits in excess over 60S subunits may have evolved to prevent mRNAs from being tied up in 40S-mRNA translation initiation complexes that cannot be turned into productive translation complexes because of the deficit of 60S subunits.