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Journal of Virology, October 1998, p. 8316-8320, Vol. 72, No. 10
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
In Vivo Translation of the Triple Gene Block of
Potato Virus X Requires Two Subgenomic mRNAs
Jeanmarie
Verchot,
Susan M.
Angell, and
David C.
Baulcombe*
Sainsbury Laboratory, Norwich Research Park,
Colney, United Kingdom NR4 7UH
Received 9 March 1998/Accepted 2 June 1998
 |
ABSTRACT |
The 25-kilodalton (25K), 12K, and 8K movement proteins of potato
virus X are derived from overlapping open reading frames (ORFs). Using
an in vivo complementation assay, we have shown that the 25K protein is
expressed from a functionally monocistronic mRNA, whereas the 12K and
8K proteins are from a bicistronic mRNA. Translation of the 8K ORF is
by leaky ribosome scanning through the 12K ORF.
 |
TEXT |
The triple gene block (TGB) of
potato virus X (PVX) encodes three proteins (25-kilodalton [25K],
12K, and 8K proteins) required for virus transport (5) and
is a genetic module that is conserved among members of the
Potexvirus, Hordeivirus, Furovirus,
and Carlavirus genera (18, 21, 28). Several
properties of the TGB proteins have been identified, but it is not
known how all of these activities contribute to virus movement. For
example, consistent with the predicted properties of a viral movement
protein, the 25K protein of PVX influences plasmodesma gating
(2) and the homologous 25K protein of foxtail mosaic
potexvirus has ATPase and RNA binding activity (29).
However, unlike some viral movement proteins affecting plasmodesma
gating, the 25K protein of PVX accumulates in laminate inclusions and
has not been localized to plasmodesmata (11). The 12K and 8K
proteins of PVX were studied by using in vitro translation systems and
may be membrane-associated proteins (27).
In vivo studies indicated that two separate subgenomic RNAs (sgRNAs)
are required for expression of the entire TGB of barley stripe mosaic
hordeivirus and beet necrotic yellow vein furovirus (16,
31). In both cases, expression of the first open reading frame
(ORF) was from a functionally monocistronic mRNA, while expression of
the two downstream ORFs was from a bicistronic mRNA (7, 32).
In the case of PVX, the results of in vitro translation studies
(12, 26) using synthesized RNAs indicated that two 3'
coterminal sgRNAs (sgRNA1 and sgRNA2) of 2.1 and 1.4 kb were necessary
for translation of the entire TGB, while a third sgRNA of 0.9 kb
(sgRNA3) was required for expression of the viral coat protein (CP;
Fig. 1A).

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FIG. 1.
(A) Diagrammatic representation of the PVX genome. Open
boxes indicate the five viral ORFs, and the shaded box indicates an
introduced marker gene (in this study, GUS or GFP). The proteins
encoded by each ORF include the RNA-dependent RNA polymerase (RdRp),
the TGB proteins (25K, 12K, and 8K), and CP. The sgRNAs of the TGB and
CP genes are labelled sgRNA1, -2, and -3. The sgRNA-M is synthesized
from a duplicated sgRNA promoter and is the mRNA for the marker
protein. The Northern blot of RNA from infected tobacco protoplasts
shows each of the predicted genomic and sgRNAs. (B) Diagrammatic
representation of the 3' region of the genomes of PVX.GUS (WT) and
three mutant derivatives, 25K, 12K, and 8K+6. Black
boxes within the TGB ORFs of the 25K and 12K viruses depict
deletions of 357 and 11 nt, respectively. The 8K+6 mutant
has an insertion of 6 nt.
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|
However, in several studies of PVX (9, 10, 20) and of the
related potexviruses, foxtail mosaic virus, white clover mosaic virus,
and clover yellow mosaic virus (3, 6, 14), only two RNA
species corresponding to sgRNA1 and sgRNA3 were observed in infected
plants. These data suggest that the PVX sgRNA2 either accumulates to a
very low level or is not required for complete translation of the TGB.
Here we describe an in vivo analysis of the PVX TGB translation
strategy. We produced plants carrying TGB transgene constructs corresponding to possible mono-, bi-, and tricistronic mRNAs. The
results of this study confirm the results of in vitro studies indicating that the 25K ORF is translated from a functionally monocistronic mRNA and that the 12K and 8K proteins are derived from a
functionally bicistronic mRNA. Additionally, we present evidence that
the 8K ORF is translated by leaky ribosome scanning through the 12K
ORF.
Separate RNAs are required for expression of the PVX TGB
proteins.
To determine the expression strategy of the PVX TGB, we
produced transgenic tobacco expressing mono-, bi-, and tricistronic mRNAs mimicking TGB mRNAs that may be present in infected plants. Six
binary plasmids containing PVX sequences were prepared for Agrobacterium-mediated transformation of tobacco leaf discs
and are described in Table 1. To produce
these constructs, the entire TGB and fragments of the TGB were
amplified by PCR and inserted between the 35S promoter and octapine
synthase terminator in the plasmid SLJ4D4 (19). Fragments
containing the promoter, PVX ORF(s), and terminator were recovered from
the SLJ4D4 plasmid by digestion with EcoRI and
HindIII and were ligated to the
EcoRI-HindIII-linearized binary plasmid
SLJ7292 as described previously (1).
Expression of the TGB100, TGB300, TGB400, and TGB500 transgenes was
analyzed by Western blotting using anti-25K and anti-8K
sera (data not
shown). Expression of the transgenes was also analyzed
by Western or
Northern blotting with selected T1 transgenic plants
(data not shown).
Accumulation of the 12K protein in the TGB200
and TGB600 lines was not
analyzed by Western blotting because
we did not have suitable antisera.
Two versions of PVX were used in these studies and were derived from
PVX.GUS and PVX.GFP plasmids. These plasmids contain
cDNA of PVX with
either the

-glucuronidase (GUS) or green fluorescence
protein (GFP)
genes inserted adjacent to a duplicated sgRNA3 promoter
(
4,
9). Three PVX.GUS derivatives were prepared by PCR
mutagenesis
(
17) and used initially to assess transgenic
complementation
of viral movement defects (Fig.
1B). The

25K mutant
contains
a deletion mutation extending between genomic nucleotides (nt)
4587 to 4945 as described previously (
2), the

12K mutant
contains
a frameshift mutation resulting from the deletion of 11 nt
between
genomic nt 5240 to 5250, and the 8K
+6 mutant has 6 nt (CGCGTA) inserted after nt 5542 in the PVX genome.
The
transgenic tobacco plants were inoculated with infectious
transcripts
(of the 8K
+6 mutant) or with virions of

25K and

12K.
The virions obtained
from transgenic tobacco expressing the

25K and

12K viral genomes
(
1) were used in this study as a source
of inoculum for approximately
1,000 plants that was produced more
economically than by transcription
of cDNA clones.
Leaves of T1 transgenic plants and nontransgenic plants inoculated with

25K,

12K, and 8K
+6 were detached at 5 days
postinoculation (dpi) and processed for
histochemical assay of GUS, as
described previously (
13). By
5 dpi on nontransgenic plants,
PVX.GUS foci were 10 to 15 cells
in diameter (Fig.
2). However, the three mutant viruses
were defective
for cell-to-cell movement (Fig.
2), and in the
inoculated leaves
the GUS activity was restricted to initially infected
cells.

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FIG. 2.
Results of GUS histochemical analysis of tobacco leaves
at 5 dpi with PVX.GUS, 25K, 12K, and 8K+6 mutant
viruses. Expanding infection foci on leaves inoculated with PVX.GUS
were photographed with a 1× objective and a Leica MZ12 microscope. The
GUS activity in single infected cells on leaves inoculated with mutant
viruses was photographed with a 20× objective and a Zeiss Axiophot
microscope.
|
|
The cell-to-cell movement defects were complemented in
trans
when the mutant viruses were inoculated to plants expressing
the
individual wild-type TGB ORFs (Table
1). Thus, on the TGB100
lines
expressing the 25K gene, there was complementation of the

25K mutant
(16 of 22 lines tested positive); in the TGB200 lines
expressing the
12K gene there was complementation of the

12K
mutant (3 of 9 lines
tested positive); and in the TGB300 lines
expressing the 8K gene there
was complementation of the 8K
+6 mutant (7 of 11 lines
tested positive; Table
1). These data
confirm findings that the 25K
protein is required for PVX cell-to-cell
movement (
2,
25)
and demonstrate directly that the 12K and
8K proteins are also required
for virus movement out of the initially
infected cells.
There was also complementation of the TGB mutants inoculated to plants
carrying bi- or tricistronic transgenes. However, on
these lines, with
one exception, complementation occurred only
if the PVX mutation
corresponded to the 5'-most ORF in the transgene
(Table
1). Thus, the

25K mutant was complemented on TGB400 (21
of 21 lines tested
positive) and TGB500 lines (5 of 5 lines tested
positive). The 25K ORF
was the 5'-most ORF in both of these transgenes.
The

12K mutant was
complemented on TGB600 lines (four of six
lines tested positive), in
which the 5'-most ORF encoded the 12K
protein, but not on lines TGB400
or TGB500, in which the 12K ORF
was internal. The one exception in
which there was complementation
of an internal ORF was with the
8K
+6 mutant inoculated to TGB600 lines. In these lines, the
8K ORF
was expressed as an internal ORF in the transgene RNA. Thus,
there
was expression of the 12K and 8K proteins from a bicistronic
12K/8K
mRNA in the TGB600 lines but not from a tricistronic mRNA
encoding
the entire TGB in the TGB400 and TGB500 lines. These
complementation
data are therefore consistent with the previous in
vitro studies
(
26), suggesting that the 2.1-kb sgRNA1 is
functionally monocistronic
for the 25K protein and that the 12K and 8K
proteins are encoded
in sgRNA2.
The translation strategy of the PVX 12K and 8K proteins from a
bicistronic RNA.
The complementation data (Table 1) show that the
TGB600 mRNA served as a bicistronic mRNA for expression of the 12K and
8K proteins. If there is a corresponding bicistronic viral mRNA in PVX-infected cells, we predicted that certain types of mutations in the
12K ORF of inoculated PVX would act in cis to prevent
translation of the 8K ORF. The type of 12K mutation that would prevent
8K protein expression depends on the mechanism by which the 8K ORF is
translated from a bicistronic mRNA. Thus, six PVX.GFP mutants (Fig.
3) containing mutations in the 12K ORF
were prepared by PCR mutagenesis (17). The 12FS mutant has a
deletion mutation identical to that of
12K. In the 12I mutant, the
AUG translation initiation codon was converted to an AUA codon by
replacing the methionine codon with one for isoleucine. The 12STOP
mutant contains the sequence C UAG UGA UAA in which the C replaces a U
in the PVX and has the UGA UAA motif inserted after nt 5167. This motif introduced two stop codons 20 nt downstream of the 12K ORF initiation codon. The mutant 12KOZ contains the sequence
GGGGACCAUGAUGAUGGCAG GG inserted at this same
position. This sequence contains three translation initiation codons
within a Kozak consensus sequence (23, 30). The 12D mutant
lacks the coding sequence for the entire 12K ORF between nt 5170 to
5423, leaving intact only the first 20 nt, overlapping the 25K ORF, and
the last 70 nt, of which 67 nt overlap the 8K ORF. The 12ID mutant
contains a replacement of the translation initiation codon with a codon
for isoleucine and a deletion of 20 nt within the 12K ORF.

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FIG. 3.
Diagrammatic representation of PVX.GFP and six mutant
derivatives. The 12FS mutation interferes with expression of a region
of the 12K ORF, as indicated by the shaded box. Mutations converting a
methionine start codon to an isoleucine codon are indicated above the
12K ORF of mutants 12I and 12ID. Nucleotide sequence insertions in the
12STOP and 12KOZ mutants are also indicated above the 12K ORF.
Deletions in mutants 12D and 12ID are indicated by gaps in the 12K
ORF.
|
|
The mutations in 12FS, 12I, 12ID, and 12STOP (Fig.
3) would each
prevent translation of the 8K ORF if it was translated by
ribosome
frameshifting or by readthrough of the 12K ORF stop codon.
The
ribosomes either would not initiate translation of the 12K
ORF or would
disengage before reaching the 8K ORF. The 12D mutant
would prevent
internal entry of ribosomes translating the 8K ORF
by removal of any
sequences that could potentially function as
an internal ribosome entry
site. Finally, the 12KOZ mutant would
prevent translation of the 8K ORF
by leaky ribosome scanning because
the mutation introduces several
initiation codons at the start
of the 12K ORF. All three of these
initiation codons lie within
a good translation context (
22,
30) so that leaky ribosome
scanning would be prevented.
Each of these 12K mutations was introduced into the PVX.GFP background
so that cell-to-cell movement could be monitored directly
under UV
illumination over a 7-day period (Fig.
4). On nontransgenic
plants, each 12K
mutant virus was restricted to the initially
infected cells, as
evidenced by GFP expression. However, all 12K
mutant viruses, except
12KOZ, moved from cell to cell on TGB200
plants expressing the 12K
protein. The phenotype of 12KOZ is likely
due to an effect of the
mutations on expression of the 8K ORF,
because the cell-to-cell
movement defect was complemented on TGB600
plants expressing both the
12K and 8K proteins. Therefore, we
conclude that expression of the 8K
protein in PVX is achieved
by leaky ribosome scanning through the 12K
ORF.

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FIG. 4.
Time course analysis of expanding PVX infection foci,
measured by using GFP as a visual marker to count the number of
infected epidermal cells across an infection focus. The average
diameters of 10 infection foci on an inoculated leaf of nontransgenic
( ), TGB200 ( ), and TGB600 ( ) tobacco plants were measured at
3, 5, and 7 dpi. Dotted lines in two panels indicate the cell-to-cell
movement of wild-type PVX.GFP, while solid lines indicate the
cell-to-cell movement of the mutant viruses named in each panel.
|
|
Leaky ribosome scanning plays a role in translation from downstream
start codons in some positive-stranded RNA viruses and
retroviruses
(
15,
24) and requires that the translation initiation
codon
of the first ORF is in a suboptimal sequence context (
22).
Efficient translation initiation from the 5' end of the mRNA requires
a
purine at position

3 from the start codon and a guanine at
position
+4 from the start codon (
8,
15). Conversely, a suboptimal
context has a pyrimidine at position

3 from the start codon and
position +4 does not contain a guanine. The translation initiation
codon of the PVX 12K ORF is in an unfavorable context for translation
initiation, with a pyrimidine (cytosine) at position

3 and a
uridine
at position +4. The initiation codons of the second ORF
in the TGB of
beet necrotic yellow vein furovirus and barley stripe
mosaic
hordeivirus are also in a suboptimal context, indicating
that, like
PVX, these viruses use leaky scanning as a control
mechanism for
translation of the third TGB ORF (
7,
32).
The conserved linear arrangement of the genes within the TGBs of the
hordeivirus, furovirus, and potexvirus families and the
similar TGB
expression strategies suggest that the mechanisms
used for expression
of the TGB proteins have functional significance.
The most obvious
functional reason for conservation of the TGB
would concern the
stoichiometry of TGB protein production. For
example, by coupling
expression of the 12K and 8K proteins from
the same RNA it could be
ensured that they are produced in relative
amounts that result in the
most efficient function. An indication
that TGB protein function is
dependent on relative levels is from
the complementation of TGB
mutations in beet necrotic yellow vein
furovirus: the complementation
of defective cell-to-cell movement
occurred only if the 13K and 15K TGB
proteins were expressed from
the same bicistronic mRNA (
7).
The situation in PVX is not
precisely the same because, as described
here, cell-to-cell movement
was complemented by expression of any of
the TGB proteins from
monocistronic transgenes. However, in none of our
TGB transgenic
lines was there complementation of long-distance
movement defects
in the inoculated virus (data not shown). Perhaps
precise stoichiometry
of the TGB proteins is required for PVX to enter
or leave the
vascular system.
 |
ACKNOWLEDGMENTS |
The Sainsbury Laboratory is funded by the Gatsby Charitable
Foundation.
We thank Tamas Dalmay, Kostya Kanyuka, Isabelle Malcuit, and Steven
Rudd for comments on the draft script.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: The Sainsbury
Laboratory, John Innes Centre, Norwich Research Park, Colney, United Kingdom NR4 7UH. Phone: 44-1603-452571. Fax: 44-1603-250024. E-mail: baulcombe{at}bbsrc.ac.uk.
Present address: Department of Entomology and Plant Pathology,
Oklahoma State University, Stillwater, OK 74078.
 |
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Journal of Virology, October 1998, p. 8316-8320, Vol. 72, No. 10
0022-538X/98/$04.00+0
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