Insertion of selenocysteine into a growing peptide
requires the unusual tRNASec (Zinoni et al.,
1987; Stadtman, 1990; Böck et al., 1991). This tRNA
has an extended D-stem containing six base pairs, which,
in the case of eukaryotic tRNASec (euk-tRNASec),
is the key identity element for selenylation and phosphorylation
(Wu & Gross, 1994; Amberg et al., 1996). Two secondary
structures have been proposed for the euk-tRNASec,
which differ in the base pairing of the acceptor/T helical
domain (Diamond et al., 1981; Böck et al., 1991; Sturchler
et al., 1993). One structure has the normal seven base
pairs in the acceptor stem and five base pairs in the T-stem
(7/5 structure, Fig. 1, left), and is characterized by
an unusually long four-nucleotide unpaired region between
the acceptor and D-stems (Connector 1) and an unpaired
nucleotide, C64a, in the T-stem. The alternate structure
features the normal two nucleotides in Connector 1 and
a 13-base pair acceptor/T domain comprised of nine base
pairs in the acceptor stem and four in the T-stem (9/4
structure, Fig. 1, right). This 9/4 structure was initially
proposed by analogy with the prokaryotic tRNASec
(prok-tRNASec), which also contains 13 base
pairs in the acceptor/T helical domain. However, in this
case, there are eight and five base pairs in the acceptor
and T-stems, respectively. The acceptor/T helical domain
having 13 base pairs is thought to be a key structural
element determining the functionalities pattern of tRNASec
in both prokaryotes and eukaryotes (Böck et al., 1991).