IDENTIFICATION OF THE CIS-ELEMENTS MEDIATING THE AUTOGENOUS CONTROL OF RIBOSOMAL-PROTEIN L2 MESSENGER-RNA STABILITY IN YEAST

The ribosomal protein L2 (rpL2) of Saccharomyces cerevisiae regulates the accumulation of its own mRNA by a feedback mechanism. An RNA seque nce is responsible for this control, initially characterized as a 360 nucleotide-long region, localized at the 5' end of the transcript, Thi s region, fused to an unrelated coding sequence, is able to down-regul ate the accumulation of the chimeric transcript when increased levels of rpL2 are induced in the cell, The target regulatory region also res ponds to regulation when inserted inside an intron, demonstrating that the control process can take place inside the nucleus, Deletion analy sis from the 5' and 3' borders have restricted the responsive region t o approximately 200 nt, The insertion of a poly-G cassette downstream of the regulatory region allowed the identification of truncated 3' cu t-off poly(A)(+) RNA molecules, The parallel identification of cut-off molecules containing the 5' portion of the transcript allowed us to d educe that the truncated products originate by endonucleolytic cleavag e, Altogether, these results are consistent with a mechanism by which the presence of excess amounts of rpL2 in the cell triggers its own mR NA to a degradative pathway; this involves an initial endonucleolytic cleavage that is followed by exonucleolytic trimming, Such a regulator y mechanism shows interesting analogies with the translational regulat ion of r-proteins in Escherichia coli.