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Shifting the temperature of a yeast culture from 23 degrees to 36 degrees C results in a sudden and severe (greater than 85%) decline in the cellular levels of ribosomal protein (rp-)mRNAs. Recovery during continued growth at 36 degrees C occurs within 1 h. The use of hybrid genes carrying different portions of the region upstream of the gene coding for ribosomal protein L25 revealed that this characteristic, coordinate temperature shock phenomenon does not depend on the presence of specific upstream DNA sequences. Analysis of a heterologous gene carrying a synthetic UASrpg (upstream activation site of yeast ribosomal protein genes) provided conclusive evidence that the rp-characteristic, transient heat shock response is not mediated through the UASrpg elements. The addition of the transcription inhibitor 1,10-phenantroline prior to a 23 degrees to 36 degrees C heat shock inhibited the severe decline of the rp-mRNA levels. The latter observation indicates that transcription is required for the rp-gene- specific response to heat shock. A milder temperature shift, from 23 degrees to 30 degrees C, gave rise to a two-fold decrease in mRNA levels for all genes studied, both ribosomal and non-ribosomal. Together, these results indicate that a temperature shift causes a temporary general transcriptional arrest in yeast cells, resulting in an over-all decrease in mRNA levels. In addition, an enhanced nucleolytic break-down of pre-existing rp-mRNAs accounts for the dramatic drop in the steady state amounts of these mRNAs observed upon a 23 degrees----36 degrees C shift. This enhanced breakdown is caused directly or indirectly by a factor whose synthesis is induced by the heat shock treatment.
Herruer et al. (Fri,) studied this question.