Heat Shock Disrupts Cap and Poly(A) Tail Function during Translation and lncreases

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 to 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 to 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 in plants.