Heat-shock protein synthesis in Chlamydomonas reinhardi. Translational control at the level of initiation of a poly(A)-rich-RNA coded 22-KDa protein in a cell-free system

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Abstract
The effect of temperature on the in vitro translation of control and heat-shock poly(A)-rich RNA, obtained from Chlamydomonas reinhardi cells, incubated for 2 h at 25°C and 42°C respectively, was studied using the wheat-germ translation system. Incubation of the cells at 42°C induces the synthesis of RNAs coding for several heat-shock proteins, including a 22-kDa major polypeptide as well as several proteins of 45–94 kDa, as demonstrated by run-off translation of polyribosomes isolated from intact cells. However, the high-molecularmass heat-shock proteins are poorly translated in the wheat-germ system. The poly(A)-rich RNA coding for the 22-kDa heat-induced polypeptide has an apparent sedimentation coefficient higher than that expected from the molecular mass of its translation product, and was preferentially translated in vitro at temperatures above 31°C as compared with pre-existing RNAs. Raising the temperature of translation, slightly inhibited (10%) the run-off translation of polyribosomes isolated from intact cells. However, when initiation was carried out in vitro for a short time at increasing temperatures and translation continued at 25°C in the presence of aurintricarboxylic acid, the 22-kDa heat-shock polypeptides was preferentially translated. Aurintricarboxylic acid did not significantly inhibit incorporation of [35S]methionine when added to polyribosomes isolated from control or heat-shocked cells. From the above data we conclude that the translation of the 22-kDa heat-shock protein is controlled in vitro at the initiation level.