Regulation of HSP70 synthesis by messenger RNA degradation.

When Drosophila cells are heat shocked, hsp70 messenger RNA (mRNA) is stable and is translated at high efficiencies. During recovery from heat shock, hsp70 synthesis is repressed and its messenger RNA (mRNA) is degraded in a highly regulated fashion. Dramatic differences in the timing of repression and degradation are observed after heat treatments of different severities. The 3' untranslated region (UTR) of the hsp70 mRNA was sufficient to transfer this regulated degradation to heterologous mRNAs. Altering the translational efficiency of the message or changing its natural translation-termination site did not alter its pattern of regulation, although in some cases it changed the absolute rate of degradation. We have previously shown that hsp70 mRNA is very unstable when it is expressed at normal growth temperatures (from a metallothionein promoter). We report here that the 3' untranslated region of the hsp70 mRNA is responsible for this instability as well. We postulate that a mechanism for degrading hsp70 mRNA pre-exists in Drosophila cells, that it is inactivated by heat shock and that it is the reactivation of this mechanism that is responsible for hsp70 repression during recovery. This degradation system may be the same as that used by other unstable mRNAs.     

An alternative non-tyrosine protein kinase product of the c-src gene in chicken skeletal muscle.

While the c-src locus is expressed as a 4.0-kilobase (kb) mRNA coding for pp60c-src in various chicken tissues, including embryonic muscle, it is expressed as a novel 3.0-kb mRNA in adult skeletal muscle. We have analyzed the primary structure of this alternatively transcribed and spliced c-src mRNA. The sequence revealed three open reading frames, with the previously defined c-src exons 1 through 5 or 6 comprising the third, on the 3' untranslated region of this 3-kb mRNA. The exons coding for the tyrosine kinase domain of pp60c-src were excluded. On the 5' side, 2 kb of sequence upstream from the previously defined exon 1 of the c-src gene was included in this mRNA. The start site for the 3-kb mRNA probably lies downstream of that for the 4-kb mRNA. The first reading frame of the 3.0-kb mRNA, called sur (for src upstream region), encoded a 24-kilodalton (kDa) protein product rich in cysteine and proline residues. In vitro analysis indicated that the 24-kDa sur protein was membrane associated. Antibodies to sur protein detected in vivo a 24-kDa muscle-specific protein which was developmentally regulated and corresponded to the switch from the 4-kb to the 3-kb c-src mRNA. A striking kinetic pattern of appearance of sur protein and disappearance of pp60c-src suggests that the expression of these two proteins is inversely related.