ABSTRACT
Universal genetic code provides the ability to encode only twenty "canonical" amino acids, whereas the twenty first amino acid--selenocysteine (Sec), is encoded by one of three well-known stop codon (UGA). In terms of molecular biology, selenocysteine is of exceptional interest, the mechanism of its incorporation into synthesized polypeptide chains is very different from that of the other typical 20 amino acids. This process involves some cis- and trans-active factors, such as the SECIS element (Selenocystein insertion sequence), a structure located in the 3'-untranslated region of eukaryotic mRNA, and in the open reading frame immediately after the UGA-selenocysteine codon in bacteria, which, in turn, leads to differences in the mechanism of selenocysteine incorporation in these domains of life. The trans-factors include Sec-tRNA([Ser]Sec) that has a unique system of biosynthesis, Sec-specific elongation factor EFsec and SBP2--Sec binding protein. Thus, for realization of the selenocysteine incorporation process during translation a large number of additional molecules must be synthesized in the cell, this fact makes the selenocysteine containing proteins rather "expensive" and emphasizes their crucial role in metabolism.
