The unique tRNA(Sec) and its role in selenocysteine biosynthesis

Selenium (Se) is an essential trace element for several organisms and is mostly present in proteins as l-selenocysteine (Sec or U). Sec is synthesized on its l-seryl-tRNA(Sec) to produce Sec-tRNA(Sec) molecules by a dedicated selenocysteine synthesis machinery and incorporated into selenoproteins at specified in-frame UGA codons. UGA-Sec insertion is signaled by an mRNA stem-loop structure called the SElenoCysteine Insertion Sequence (SECIS). tRNA(Sec) transcription regulation and folding have been described showing its importance to Sec biosynthesis. Here, we discuss structural aspects of Sec-tRNA(Sec) and its role in Sec biosynthesis as well as Sec incorporation into selenoproteins. Defects in the Sec biosynthesis or incorporation pathway have been correlated with pathological conditions.