"Frontmatter". In: Plant Genomics and Proteomics

R
OLE OF
5
¢ S
EQUENCES IN
G
ENE
E
XPRESSION
Certain 5¢ leader sequences have been shown to enhance the expression 
of transgenes. Both viral and endogenous plant 5¢ untranslated leader
sequences have effects. The ability of 5¢ leader sequences to enhance gene
expression has been used to design more efficient transgene expression
systems for plants (Groenewald et al., 2000). However, the 5¢ untranslated
leader sequences from dicots appear to work better in dicots than those from
monocots and vice versa. 
R
OLE OF
3
¢ S
EQUENCES IN
G
ENE
E
XPRESSION
There is evidence that the transcription-enhancing effects of the 3¢ coding
sequence for transgenes may be related to the effectiveness of transcription
termination (Ali and Taylor, 2001; Bieri et al., 2002). As with many of the
R
O L E O F
3
¢ S
E Q U E N C E S I N
G
E N E
E
X P R E S S I O N
9 3

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transcription signals, the generality of these conclusions may not be equally
applicable across the whole plant kingdom. 
R
OLE OF
I
NTRONS IN
G
ENE
E
XPRESSION
Introns have been shown to be important for improving transgene 
expression (Ibrahim et al., 2001) and can also play an important role in 
9 4
5. C
O N T R O L O F
G
E N E
E
X P R E S S I O N
−
343
−
271
−
172
−
90
−
186
−
256
[
−
]
[
−
]
[
−
]
[
+
]
[
+
]
[
+
]
[
+
]
mas 1
′
mas 2
′
masA
masB
masC
ocs
+
2
ocs
+
2 - element
ocs
+
1 - element
MJ Response
Wound Response
RNA
PoIlI
RNA
PoIII
FIGURE 5.2.
Functional model of MAS bidirectional promoter activity. The capac-
ity of MAS subdomains to bind specific nuclear proteins was defined by footprint-
ing analysis. The relative positions of ocs-like elements and MAS promoter fragments
with enhancer or silencer activity are indicated. The ocs-binding factors at the ocs-
like elements and the putative locations of the plant nuclear proteins that bind to the
MAS promoter are depicted according to Ni et al. (1996). The relative strength and
transcriptional orientation of the mas2 and mas1 promoters are indicated by the hor-
izontal arrows. The orientation-dependent function of the each fragment is indicated
by the curved arrows. Positive regulatory effects are indicated by [+] signs and heavy
curved arrows, whereas negative regulatory effects are indicated by [–] signs and
stippled curved arrows. The suppressor effect that wounding and methyl jasmonate
(MJ) have on the silencer properties of masB and masCocs+2 regions is represented
by the large X. (Reprinted with permission from Guevara-Garcia et al., 1999. Copy-
right Springer-Verlag GmbH & Co. 1999).

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determining the tissue specificity of gene expression. The enhancement can
be more than 100-fold (Maas et al., 1991; Zhang and Singh, 1994), but is
usually in the range of 2- to 10-fold. The effect is generally larger in mono-
cots than dicots. The intron must be within the transcribed portion of the
gene for the intron-mediated enhancement (IME) to occur, because there is
no effect if the intron is placed upstream of the promoter. Therefore, the
expression effect is not due to any enhancer-like activity. A number of mech-
anisms have been proposed to explain this so-called intron-dependent gene
expression:
∑ Spliceosome formation plays a role in stabilizing transcripts in the
nucleus (Luehrsen et al., 1994) 
∑ The presence of negative or positive cis-acting sequences in genes
determines whether or not introns are necessary for proper expres-
sion. Any cis-acting elements in a particular gene, in the absence of
an intron, act to impair expression of genes (Rethmeir et al., 1998).
Therefore, introns do not necessarily enhance gene expression but act
to overcome repression of the normal expression levels of the genes
in question. 
Results from use of either of the first two introns of the Arabidopsis tryp-
tophan pathway gene PAT1 showed a roughly fivefold elevation of mRNA
accumulation from a PAT1:b-glucuronidase (GUS) fusion without affecting
the rate of PAT1:GUS transcription. An analysis of the modification of such
constructs eliminated the possibility that the intron stimulated mRNA accu-
mulation via a unique RNA-stabilizing sequence or through the completed
act of splicing. However, the results were consistent with a possible role for
redundant intron sequence elements or an association of the pre-mRNA with
the spliceosome.
As has been described with other control sequences, any particular
intron does not work equally well when introduced into monocots and
dicots. Therefore, the requirements for correct splicing may not be the same
in these two groups. Introns that stimulate expression in monocots include
those from the maize Adh1, Sh1, Bz1, Hsp82, actin, and GapA1 genes and the
rice salT, Act1, and tpi genes (cited in Rose and Beliakoff, 2000). Dicot introns
that elevate expression include those from the petunia rbcS gene SSU301, the
potato ST-LS1 gene, and the Arabidopsis UBQ3, UBQ10, PAT1, atpk1, A1 EF-
1a, and At eEF-1b genes (cited in Rose and Beliakoff, 2000).