Paz I, Abramovitz L, Choder M
Department of Molecular Microbiology and Biotechnology, Tel-Aviv University, Ramat Aviv 69978, Israel.
J Biol Chem. 1999 Jul 30;274(31):21741-5. doi: 10.1074/jbc.274.31.21741.
Internal initiation of translation, whereby ribosomes are directed to internal AUG codon independently of the 5' end of the mRNA, has been observed rarely in higher eucaryotes and has not been demonstrated in living yeast. We report here that starved yeast cells are capable of initiating translation of a dicistronic message internally. The studied element that functions as an internal ribosome entry site (IRES) is hardly functional or not functional at all in logarithmically growing cells. Moreover, during the logarithmic growth phase, this element seems to inhibit translation reinitiation when placed as an intercistronic spacer or to inhibit translation when placed in the 5'-untranslated region of a monocistronic message. Inhibition of translation is likely due to the putative strong secondary structure of the IRES that interferes with the cap-dependent scanning process. When cells exit the logarithmic growth phase, or when artificially starved for carbon source, translation of the IRES-containing messages is substantially induced. Our findings imply that the capacity to translate internally is a characteristic of starved rather than vegetatively growing yeast cells.
翻译起始的内部机制，即核糖体独立于mRNA的5'端被引导至内部AUG密码子，在高等真核生物中很少被观察到，并且在活酵母中尚未得到证实。我们在此报告，饥饿的酵母细胞能够在内部起始双顺反子信息的翻译。在对数生长期细胞中，作为内部核糖体进入位点（IRES）起作用的研究元件几乎没有功能或完全没有功能。此外，在对数生长期，当该元件作为顺反子间间隔区放置时，似乎会抑制翻译重新起始；当置于单顺反子信息的5'非翻译区时，似乎会抑制翻译。翻译抑制可能是由于IRES假定的强二级结构干扰了帽依赖性扫描过程。当细胞退出对数生长期，或人为地缺乏碳源饥饿时，含IRES信息的翻译会被显著诱导。我们的发现表明，内部翻译能力是饥饿酵母细胞而非营养生长酵母细胞的一个特征。
