Degradation of hsp70 and other mRNAs in Drosophila via the 5' 3' pathway and its regulation by heat shock.

Two general pathways of mRNA decay have been characterized in yeast. Both start with deadenylation. The major pathway then proceeds via cap hydrolysis and 5'-exonucleolytic degradation whereas the minor pathway consists of 3'-exonucleolytic decay followed by hydrolysis of the remaining cap structure. In higher eukaryotes, these pathways of mRNA decay are believed to be conserved but have not been well characterized. We have investigated the decay of the hsp70 mRNA in Drosophila Schneider cells. As shown by the use of reporter constructs, rapid deadenylation of this mRNA is directed by its 3'-untranslated region. The main deadenylase is the CCR4.NOT complex; the PAN nuclease makes a lesser contribution. Heat shock prevents deadenylation not only of the hsp70 but also of bulk mRNA. A completely deadenylated capped hsp70 mRNA decay intermediate accumulates transiently and is degraded via cap hydrolysis and 5'-decay. Thus, decapping is a slow step in the degradation pathway. Cap hydrolysis is also inhibited during heat shock. Degradation of reporter RNAs from the 3'-end became detectable only upon inhibition of 5'-decay and thus represents a minor decay pathway. Because two reporter RNAs and at least two endogenous mRNAs were degraded primarily from the 5'-end with cap hydrolysis as a slow step, this pathway appears to be of general importance for mRNA decay in Drosophila.