Translation Regulation and RNA Granule Formation after Heat Shock of Procyclic Form Trypanosoma brucei: Many Heat-Induced mRNAs Are also Increased during Differentiation to Mammalian-Infective Forms

African trypanosome procyclic forms multiply in the midgut of tsetse flies, and are routinely cultured at 27°C. Heat shocks of 37°C and above result in general inhibition of translation, and severe heat shock (41°C) results in sequestration of mRNA in granules. The mRNAs that are bound by the zinc-finger protein ZC3H11, including those encoding refolding chaperones, escape heat-induced translation inhibition. At 27°C, ZC3H11 mRNA is predominantly present as an untranslated cytosolic messenger ribonucleoprotein particle, but after heat shocks of 37°C—41°C, the ZC3H11 mRNA moves into the polysomal fraction. To investigate the scope and specificities of heat-shock translational regulation and granule formation, we analysed the distributions of mRNAs on polysomes at 27°C and after 1 hour at 39°C, and the mRNA content of 41°C heat shock granules. We found that mRNAs that bind to ZC3H11 remained in polysomes at 39°C and were protected from sequestration in granules at 41°C. As previously seen for starvation stress granules, the mRNAs that encode ribosomal proteins were excluded from heat-shock granules. 70 mRNAs moved towards the polysomal fraction after the 39°C heat shock, and 260 increased in relative abundance. Surprisingly, many of these mRNAs are also increased when trypanosomes migrate to the tsetse salivary glands. It therefore seems possible that in the wild, temperature changes due to diurnal variations and periodic intake of warm blood might influence the efficiency with which procyclic forms develop into mammalian-infective forms.非洲锥虫前循环体（African trypanosome procyclic forms）可在采采蝇（tsetse flies）的中肠内增殖，常规培养温度为27℃。37℃及以上的热激（heat shocks）会导致翻译过程的普遍抑制，而剧烈热激（severe heat shock，41℃）则会使mRNA被隔离至颗粒中。被锌指蛋白ZC3H11（zinc-finger protein ZC3H11）结合的mRNA，包括编码重折叠分子伴侣的mRNA，可逃脱热激诱导的翻译抑制（translation inhibition）。在27℃时，ZC3H11 mRNA主要以未翻译的胞质信使核糖核蛋白颗粒形式存在，但经37℃~41℃热激后，ZC3H11 mRNA会转移至多聚体组分（polysomal fraction）中。为探究热激翻译调控（translational regulation）与颗粒形成（granule formation）的范围及特异性，我们分析了27℃及39℃热激1小时后mRNA在多聚体上的分布，以及41℃热激颗粒中的mRNA含量。研究发现，结合ZC3H11的mRNA在39℃时仍留存于多聚体中，并在41℃时免受颗粒隔离。正如此前在饥饿应激颗粒（starvation stress granules）中观察到的结果，编码核糖体蛋白（ribosomal proteins）的mRNA会被排除在热激颗粒之外。经39℃热激后，共有70种mRNA向多聚体组分转移，260种mRNA的相对丰度上升。令人意外的是，其中许多mRNA在锥虫迁移至采采蝇唾液腺（tsetse salivary glands）时也会出现丰度升高。因此在自然环境中，昼夜变化（diurnal variations）及周期性摄入温热血液所引发的温度变化，可能会影响前循环体发育为哺乳动物感染型（mammalian-infective forms）的效率。