RSEB, A CHROMOSOMAL LOCUS THAT AFFECTS THE STABILITY OF A TEMPERATURE-SPECIFIC SURFACE PROTEIN MESSENGER-RNA IN TETRAHYMENA-THERMOPHILA

In Tetrahymena thermophila, the expression of a temperature-specific s urface protein known as SerH3 is primarily controlled by a temperature -dependent change in the stability of the mRNA that encodes this prote in. At 30-degrees-C the SerH3 mRNA displays a half-life of 60 minutes while at 40-degrees-C the half-life decreases to only 3 minutes. We us ed a Tetrahymena mutant cell line (rseB) defective in expression of Se rH3 at 30-degrees-C to explore the mechanisms involved in temperature- dependent mRNA stability. The results of in vitro nuclear run-off assa ys and Northern and slot blot analysis of cytoplasmic and nuclear RNAs show that the rseB locus encodes a temperature-sensitive product that has no effect on SerH3 gene transcription or the steady-state levels of SerH3 nuclear RNA. However, the product of this locus does have a d ramatic effect on cytoplasmic levels of the SerH3 mRNA at 30-degrees-C , indicating that SerH3 gene expression is affected post-transcription ally within the cytoplasm. To explore the possibility that the rseB lo cus controls SerH3 mRNA stability we developed an in vitro mRNA decay assay. This assay successfully duplicates the differential decay of th e SerH3 mRNA observed in wild-type cells grown at different temperatur es. The apparent half-life of the SerH3 mRNA in cytoplasmic extracts d erived from cells grown at 30-degrees-C is approximately 45 minutes wh ile in cytoplasmic extracts derived from cells grown at 40-degrees-C i t is only 6 minutes. When similar experiments are performed using extr acts prepared from the Tetrahymena rseB cell line, we find that the Se rH3 mRNA is only stable in extract prepared from cells grown under con ditions in which the mRNA accumulates to detectable levels in the cyto plasm. These results indicate that the product of the rseB locus is a trans-acting cytoplasmic factor that exerts its effect on SerH3 gene e xpression by regulating SerH3 mRNA stability.