An
amino acid starvation-induced
mRNA, increased up to 4-fold in the absence of
amino acids, was identified previously as rat 60 S subunit ribosomal
protein L17. The data presented here demonstrate that among
ribosomal proteins, L17, as well as the smaller subunit ribosomal
protein S25, are uniquely regulated by
amino acid deprivation of cells; the increase in L17 and S25
mRNA content in response to substrate
starvation is not shared by the 11 other
ribosomal proteins tested. When rat Fao
hepatoma cells were incubated in the presence of
actinomycin D, the L17
mRNA level decayed below the basal level, regardless of medium
amino acid content, and nuclear run-off assays confirmed that nutrient
starvation leads to enhanced transcription of the
L17 ribosomal protein gene. Likewise, the induction of S25
mRNA also was prevented in the presence of
actinomycin D. Furthermore, the induction of L17 and S25
mRNA content was blocked by
cycloheximide, demonstrating the requirement for a newly synthesized
protein in the signaling pathway. Northern analysis with
RNA isolated from cytoplasmic, polysomal, and nuclear enriched fractions indicated that the
starvation-dependent increase in the S25 and L17 mRNAs is retained within the nucleus and not is available for translation.
Amino acid refeeding of the cells caused the translocation of the stored nuclear mRNAs to the polysomal fraction.