In
bacteria, ribosomal protein synthesis is regulated by translational
repression. Regulatory r-proteins bind to their own mRNAs to prevent
translation and/or promote mRNA decay when they are not bound in
ribosomes. No equivalent mechanism has been found in yeast for
controlling r-protein abundance, although several r-proteins have been
shown to autoregulate their own synthesis. We identified an
autoregulatory system controlling the expression of SUF14 (RPS3),
which codes for ribosomal protein S3. Analyzing the effects of
increased gene dosage on mRNA accumulation revealed that expression
was regulated. We found that S3 inhibits its own synthesis by limiting
the accumulation of SUF14 mRNA, suggesting that regulation may be
mediated either through a change in the rate of mRNA transcription or
through a change in the rate of mRNA decay. A frameshift mutation in
SUF14 that blocks translation eliminated regulation, indicating that
S3 is required for regulation. Experiments with a fusion of the CUP1
promoter to the SUF14 ORF confirmed this conclusion and also indicated
that either the promoter and/or the 5'-UTR of SUF14 is required for
regulation. Using a SUF14placZ fusion, run-on transcription assays,
and estimates of mRNA half-life, our results suggest that the 5'-UTR
is necessary but not sufficient for regulation. Transcription appears
to play a minor role if any in regulation. S3-mediated changes in mRNA
decay rate may be the primary mechanism for regulation.
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