Affiliations: Department of Molecular Biology, The Hebrew
University-Hadassah Medical School, Jerusalem 91120, Israel
Note: [] Department of Molecular Biology, The Hebrew University-Hadassah
Medical School, PO.BOX 12272, Jerusalem 91120, Israel. Tel.: +972 2 6758250;
Fax: +972 2 6784010; E-mail: [email protected]
Abstract: 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 {\it E. coli}
SECIS. This includes the follows: 1) The nature of the minimal {\it E. coli}
SECIS. We found that in {\it 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 {\it 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 {\it 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.
Keywords: selenoprotein biosynthesis, E. coli, fdhF SECIS, functional elements, UGA translation, genetic code
Journal: BioFactors, vol. 14, no. 1-4, pp. 61-68, 2001
