The cellular level of yeast ribosomal protein L25 is controlled principally by rapid degradation of excess protein.

When the gene dosage for the primary rRNA-binding ribosomal protein L25 in yeast cells was raised about 50-fold, the level of mature L25 transcripts was found to increase almost proportionally. The plasmid-derived L25 transcripts were structurally indistinguishable from their genomic counterparts, freely entered polysomes in vivo and were fully translatable in a heterologous in vitro system. Nevertheless, pulse-labelling for periods varying from 3-20 min did not reveal a significant elevation of the intracellular level of L25-protein. When pulse-times were decreased to 10-45 s, however, we did detect a substantial overproduction of L25. We conclude that, despite the strong RNA-binding capacity of the protein, accumulation of L25 is not controlled by an autogenous (pre-)mRNA-targeted mechanism similar to that operating in bacteria, but rather by extremely rapid degradation of excess protein produced.