Mild temperature shock affects transcription of yeast ribosomal protein genes as well as the stability of their mRNAs

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Abstract
Shifting the temperature of a yeast culture from 23° to 36°C results in a sudden and severe (>85%) decline in the cellular levels of ribosomal protein (rp-)mRNAs. Recovery during continued growth at 36°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 evide 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° to 36° 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° to 30°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° → 36°C shift. This enhanced breakdown is caused directly or indirectly by a factor whose synthesis is induced by the heat shock treatment.