7.2.3 Editing the Sequence of RNA
In addition to RNA splicing, there is a second pathway by which RNA is transformed into a different
sequence than originally encoded by the parent DNA. Many organisms employ diverse mechanisms for
RNA editing, during which the identity of individual bases is altered. This can change amino acid identity
at a specific position or introduce a new stop codon, thereby resulting in major changes in gene expression.
7.2.3.1 Transversional Editing. The mRNA from humans and other higher eukaryotes commonly
undergoes transversional editing, which changes the identity of individual bases. The most common base
changes are C→→U andA→→I.42,43In the latter case, the inosine residue is read as a guanosine by the trans-
lational apparatus and by polymerases that are used to amplify RNA gene products. Transversional editing
RNA Structure and Function 269
Figure 7.22 The mechanism of group II intron insertion and mobility into duplex DNA. Mobility is catalyzed by a
ribonucleoprotein particle that contains a lariat group II intron RNA (black line), which is bound to a pro-
tein cofactor (grey) that is encoded by the intron itself. Both protein and RNA contain active sites for cata-
lysis of the various steps of intron insertion. (a) After recognition of its target site, the 3-OH group of the
lariat RNA attacks the sense strand of DNA in a reverse-splicing reaction that is catalyzed by the intron.
(b) An endonuclease motif within the protein (grey) then attacks the antisense strand. (c) The second step
of reverse splicing. (d) Concomitant with or after the second splicing step, a reverse-transcriptase motif
makes a DNA copy of the inserted RNA, by use of the cut antisense strand as a primer