Engelberg-Kulka H, Liu Z, Li C, Reches M
Department of Molecular Biology, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.
Biofactors. 2001;14(1-4):61-8. doi: 10.1002/biof.5520140109.
The genetic code, once thought to be rigid, has been found to permit several alternatives in its reading. Interesting alternative relates to the function of the UGA codon. Usually, it acts as a stop codon, but it can also direct the incorporation of the amino acid selenocysteine into a polypeptide. UGA-directed selenocysteine incorporation requires a cis-acting mRNA element called the "selenocysteine insertion sequence" (SECIS) that can form a stem-loop RNA structure. Here we discuss our investigation on the E. coli SECIS. This includes the follows: 1) The nature of the minimal E. coli SECIS. We found that in E. coli only the upper-stem and loop of 17 nucleotides of the SECIS is necessary for selenocysteine incorporation on the condition that it is located in the proper distance from the UGA [34]; 2) The upper stem and loop structure carries a bulged U residue that is required for selenocysteine incorporation [34] because of its interaction with SelB; and 3) We described an extended fdhF SECIS that includes the information for an additional function: The prevention of UGA readthrough under conditions of selenium deficiency [35]. This information is contained in a short mRNA region consisting of a single C residue adjacent to the UGA on its downstream side, and an additional segment consisting of the six nucleotides immediately upstream from it. These two regions act independently and additively and probably through different mechanisms. The single C residue acts as itself; the upstream region acts at the level of the two amino acids, arginine and valine, for which it codes. These two codons at the 5' side of the UGA correspond to the ribosomal E and P sites. Finally, we present a model for the E. coli fdhF SECIS as a multifunctional RNA structure containing three functional elements. Depending on the availability of selenium the SECIS enables one of two alternatives for the translational machinery: Either selenocysteine incorporation into a polypeptide or termination of the polypeptide chain.
遗传密码曾被认为是固定不变的，但现在发现它在解读时有几种变通方式。一个有趣的变通方式与UGA密码子的功能有关。通常，它作为一个终止密码子起作用，但它也能指导将氨基酸硒代半胱氨酸掺入多肽中。UGA指导的硒代半胱氨酸掺入需要一个称为“硒代半胱氨酸插入序列”（SECIS）的顺式作用mRNA元件，该元件能形成茎环RNA结构。在这里，我们讨论我们对大肠杆菌SECIS的研究。这包括以下内容：1）最小的大肠杆菌SECIS的性质。我们发现，在大肠杆菌中，只有SECIS的17个核苷酸的上部茎和环对于硒代半胱氨酸掺入是必要的，条件是它位于距UGA适当的距离处[34]；2）上部茎和环结构带有一个凸出的U残基，由于它与SelB相互作用，所以是硒代半胱氨酸掺入所必需的[34]；3）我们描述了一个扩展的fdhF SECIS，它包含了另一个功能的信息：在硒缺乏的条件下防止UGA通读[35]。该信息包含在一个短的mRNA区域中，该区域由紧邻UGA下游的单个C残基以及紧接其上游的六个核苷酸组成的额外片段组成。这两个区域独立且相加地起作用，可能通过不同的机制。单个C残基以其自身起作用；上游区域在它编码的两种氨基酸精氨酸和缬氨酸的水平上起作用。UGA 5'侧的这两个密码子对应于核糖体的E位和P位。最后，我们提出了一个大肠杆菌fdhF SECIS的模型，它是一个包含三个功能元件的多功能RNA结构。根据硒的可用性，SECIS为翻译机制提供两种选择之一：要么将硒代半胱氨酸掺入多肽中，要么终止多肽链。
