Biology Reference
In-Depth Information
Preferential target sites for LINE elements are also apparent in mammalian
genes during evolution. For example, the interleukin-6 genes of rodents represent
hotspots for LINE element retrotransposition (Qin
et al
., 1991). During mam-
malian evolution, the introduction of LINE elements in the vicinity of genes has
sometimes altered gene expression as a consequence of their being recruited to
perform a regulatory function (examples of this phenomenon are given in Chapter
5, section 5.1.12,
Alu sequences
). LINE elements have also served to promote
genetic rearrangements and indeed they may well have mediated both gene inver-
sion (Chapter 9, section 9.1) and duplication (Section 8.5) events during evolu-
tion. Finally, and perhaps most importantly, LINE elements may have repeatedly
transduced exons from one genomic location to another, thereby potentiating
exon shuffling (Chapter 3, section 3.6.1), the transfer of exons encoding specific
protein modules between genes.
8.4.3
Alu
sequences
Evolution of
Alu
sequences.
The fossil
Alu
monomer is thought to have arisen
by the deletion of the central S domain of 7SL RNA (
RN7SL
) followed by the
addition of a 3
poly(A) tract which may have facilitated reverse transcription of
these RNA polymerase III transcripts (Mighell
et al.
, 1997;
Figure 8.5
). The free
left arm monomer then arose by deletion of 42 bp from the fossil
Alu
monomer
whilst the free right arm monomer arose by deletion of 11 bp from the fossil
Alu
monomer (Mighell
et al.
, 1997;
Figure 8.5
). The first
Alu
sequence may have been
formed
7SL RNA
Sdomain
1
83
97
239
267
300
247 259
Fossil
Alu
monomer (FAM)
A
n
Free left arm monomer (FLAM)
Freerightarmmonomer(FRAM)
A
n
A
n
First dimeric Alu (~280bp)
5'
A
n
3'
A
n
Box A
TRGCNNAGY
Box B
GGTTCGANTCC
Figure 8.5.
Proposed model of dimeric
Alu
formation via intermediate monomeric units
derived from 7SL RNA which is neither capped nor polyadenylated. Important
nucleotide positions are marked on the schematic 7SL RNA moiety. See text for
explanation of the progression from 7SL RNA to the first dimeric
Alu
repeat. The
approximate positions and consensus sequences of the RNA pol III promoter boxes A and
B in the
Alu
sequence are marked. Redrawn from Mighell
et al
. (1997).
