KEY CONCEPTS:
- Changes in the reading of specific codons can occur in individual genes.
- The insertion of seleno-Cys-tRNA at certain UGA codons requires several proteins to modify the Cys-tRNA and insert it into the ribosome.
- Pyrrolysine can be inserted at certain UAG codons.
Specific changes in reading the code occur in individual
genes. The specificity of such changes implies that the reading of the
particular codon must be influenced by the surrounding bases.
A striking example is the incorporation of the modified
amino acid seleno-cysteine at certain UGA codons within the genes that code for
selenoproteins in both prokaryotes and eukaryotes. Usually these proteins
catalyze oxidation-reduction reactions, and contain a single seleno-cysteine
residue, which forms part of the active site. The most is known about the use of
the UGA codons in three E. coli genes coding for formate dehydrogenase
isozymes. The internal UGA codon is read by a seleno-Cys-tRNA. This unusual
reaction is determined by the local secondary structure of mRNA, in particular
by the presence of a hairpin loop downstream of the UGA.
Mutations in 4 sel genes create a deficiency in
selenoprotein synthesis. selC codes for tRNA (with the anticodon
ACU![]()
) that is
charged with serine. selA and selD are required to modify the
serine to seleno-cysteine. SelB is an alternative elongation factor. It is a
guanine nucleotide-binding protein that acts as a specific translation factor
for entry of seleno-Cys-tRNA into the A site; it thus provides (for this single
tRNA) a replacement for factor EF-Tu. The sequence of SelB is related to both
EF-Tu and IF-2 (for review see Bock, 1991).
Why is seleno-Cys-tRNA inserted only at certain UGA codons?
These codons are followed by a stem-loop structure in the mRNA. Figure 7.12 shows that the stem of this structure is
recognized by an additional domain in SelB (one that is not present in EF-Tu or
IF-2). A similar mechanism interprets some UGA codons in mammalian cells, except
that two proteins are required to identify the appropriate UGA codons. One
protein (SBP2) binds a stem-loop structure far downstream from the UGA codon,
while the counterpart of SelB (called SECIS) binds to SBP2 and simultaneously
binds the tRNA to the UGA codon (Fagegaltier et al., 2000).
Another example of the insertion of a special amino acid is
the placement of pyrrolysine at a UAG codon. This happens in both an archaea and
a bacterium (Srinivasan, James, and Krzycki, 2002; Hao et al., 2002). The mechanism is probably similar
to the insertion of seleno-cysteine. An unusual tRNA is charged with lysine,
which is presumably then modified. The tRNA has a CUA anticodon, which responds
to UAG. There must be other components of the system that restricts its response
to the appropriate UAG codons.