Heat Shock Disrupts Cap and Poly(A) Tail Function during Translation and Increases mRNA Stability of Introduced Reporter mRNA

Abstract

and lnstitute of Mutagenesis and Differentiation, Consiglio Nazionale delle Richerche, via Svezia 1 O, 561 24 Pisa, ltaly (L.P.)l h e effect of heat shock on translational efficiency and message stability of a reporter mRNA was examined in carrot (Daucus carofa). Heat shock of short duration resulted in an increase in protein yield, whereas repression was observed following extended exposure t o the stress. Regardless of the duration of the heat shock, a loss in the function of the 5' cap [m7G(5')ppp(5')N, where N represents any nucleotide] and the 3' poly(A) tail, two regulatory elements that work in concert to establish an efficient leve1 of translation, was observed. This apparent paradox was resolved upon examination of the mRNA half-life following thermal stress, i n which increases up to 1 O-fold were observed. Message stability increased as a function of the severity of the heat shock so that following a mild to moderate stress the increase in message stability more than compensated for the reduction in cap and poly(A) tail function. Following a severe heat shock, the increased mRNA halflife was not sufficient t o overcome the virtual loss in cap and poly(A) tail function. No stimulation of protein synthesis was observed following a heat shock in Chinese hamster ovary cells, data suggesting that the heat-induced increases in mRNA stability may be unique to the heat-shock response i n plants.

Both 5[prime] and 3[prime] Sequences of Maize adh1 mRNA Are Required for Enhanced Translation under Low-Oxygen Conditions

“…Our demonstration that PLLUC(A),, mRNA is stable but poorly translated in hypoxic protoplasts may reflect the posttranscriptional regulation of the adenine nt translocator protein mRNA, which is maintained in hypoxic roots but poorly loaded onto polyribosomes. Translational repression of PLLUC(A),, was also observed in response to heat shock in carrot protoplasts; however, in contrast to what we observed with hypoxia, heat shock significantly increased the half-Iife of PLLUC(A),, (Gallie et al, 1995).…”

“…The efficient translation of a chimeric mRNA construct in heat-shocked maize and tobacco protoplasts was conferred by the 5'-UTR of the ksp70 gene of maize (Pitto et al, 1992). Gallie et al (1995) presented evidence indicating that translational repression of normal cellular mRNAs in heatshocked cells may involve the disruption of interactions behveen the 5' cap and the 3' poly(A) tail. How the leader sequence of ksp70 mRNA allows it to escape translational repression has not been elucidated.…”

eIF4G Functionally Differs from eIFiso4G in Promoting Internal Initiation, Cap-independent Translation, and Translation of Structured mRNAs

“…The level of eIF4G (but not eIF4E) also increases following a heat shock as a function of the severity of the stress (59). Heat stress results in a loss of cap function and a preferential increase in the translation of uncapped mRNAs to a level equal that of capped mRNAs (60). These in vivo observations are similar to the loss-of-cap function and a preferential increase in the translation of uncapped mRNAs following an increase in eIF4F concentration (Table IV).…”

Heat Shock Inhibits Release of the Signal Recognition Particle from the Endoplasmic Reticulum in Barley Aleurone Layers

“…Perhaps components of the secretory systems in the various tissues may have fundamental differences that result in different sensitivities to heat shock. In electroporated carrot (Daucus carota) protoplasts, heat shock increased stability of the reporter mRNA (56).…”