Synthesis of trypanosome hsp70 mRNA is resistant to disruption of trans-splicing by heat shock

Synthesis of mRNA in trypanosomes involves an apparent trans-splicing reaction whereby a common 39-nucleotide mini-exon sequence is joined to the protein-coding exon of a mRNA precursor. We have previously shown (Muhich, M. L., and Boothroyd, J. C. (1988) Mol. Cell. Biol. 8, 3837-3846) that the trans-splicing pathway of Trypanosoma brucei is sensitive to disruption by severe heat shock. Here we demonstrate that the synthesis of heat shock protein 70 (hsp 70) mRNA in T. brucei is apparently resistant to the heat-induced disruption of splicing. The 5'-ends of hsp 70 mRNAs are shown to be identical for molecules synthesized at either normal or heat shock temperatures, and in both cases, the 5'-terminal mini-exon sequence is present. These results suggest that T. brucei has evolved a mechanism which directly compensates for the deleterious effects of heat shock on trans-splicing, one which allows for the continued mini-exon-dependent trans-splicing of selected pre-mRNAs.     

The groESL operon of Agrobacterium tumefaciens: evidence for heat shock-dependent mRNA cleavage.

The heat shock response of the groESL operon of Agrobacterium tumefaciens was studied at the RNA level. The operon was found to be activated under heat shock conditions and transcribed as a polycistronic mRNA that contains the groES and groEL genes. After activation, the polycistronic mRNA appeared to be cleaved between the groES and groEL genes and formed two monocistronic mRNAs. The groES cleavage product appeared to be unstable and subjected to degradation, while the groEL cleavage product appeared to be stable and became the major mRNA representing the groESL operon after long periods of growth at a high temperature. The polycistronic mRNA containing the groES and groEL genes was the major mRNA representing the groESL operon at a low temperature, and it reappeared when the cells were returned to the lower growth temperature after heat shock induction. These findings indicate that the cleavage event is part of the heat shock regulation of the groESL operon in A. tumefaciens.