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Journal of Virology, April 2002, p. 4119-4124, Vol. 76, No. 8
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.8.4119-4124.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Inter- and Intramolecular Recombinations in the Cucumber Mosaic Virus Genome Related to Adaptation to Alstroemeria
Yuh-Kun Chen,1,2 Rob Goldbach,1 and Marcel Prins1*
Laboratory of Virology, Wageningen University, Wageningen, The Netherlands,1
Department of Plant Pathology, National Chung Hsing University, Taichung, Taiwan2
Received 2 November 2001/
Accepted 23 January 2002

ABSTRACT
In four distinct alstroemeria-infecting cucumber mosaic virus
(CMV) isolates, additional sequences of various lengths were
present in the 3' nontranslated regions of their RNAs 2 and
3, apparently the result of intra- and intermolecular recombination
events. Competition experiments revealed that these recombined
RNA 2 and 3 segments increased the biological fitness of CMV
in alstroemeria.

TEXT
The genome of
Cucumber mosaic virus (CMV), the type species
of the genus
Cucumovirus in the family
Bromoviridae (
38), consists
of three single-stranded RNA molecules: RNAs 1 and 2 encode
components of the viral RNA-dependent RNA polymerase (RdRp)
(
31,
32), while the bicistronic RNA 3 encodes the movement protein
(MP) (
43) and coat protein (CP) (
39). The CP is translated from
a subgenomic RNA, RNA 4, which is encoded by the 3' half of
the RNA 3 (
19). A small overlapping gene (2b), residing in RNA
2 and implicated in suppression of gene silencing (
4), was discovered
more recently and is most likely expressed through a second
subgenomic RNA (
13). Based on their genomic sequences, CMV isolates
have been classified into two major subgroups, I and II (
2,
32,
35). A further division of subgroup I, into IA and IB, has
been proposed based on the nucleotide sequences of 5' nontranslated
regions (NTRs) of RNA 3 (
34,
37). With a host range of over
1,000 plant species, CMV is one of the most widespread plant
viruses in the world (
32). Also, the ornamental plant alstroemeria
has been reported as a host of CMV (
21), although few data on
the interaction between CMV and this host are available.
RNA recombination is generally considered one of the major forces that favor the evolution and adaptation of RNA viruses (14, 24, 36, 41, 42). Most of the data collected from different experimental systems suggest that RNA recombination prefers replicase (RdRp)-driven template switching models or copy choice mechanisms (6, 30, 41). Alternatively, the breakage-and-ligation mechanism has been suggested and proven as a possible means of in vitro RNA recombination as well (12). RNA recombination has been classified according to the extent of involvement of homologous sequences (23, 24, 30).
Evidence from nucleotide sequence data suggests that recombinational events have occurred in satellite RNA associated with CMV strain Y (27) and defective RNAs of the Fny strain of CMV (20). Recombinational events have also been observed in artificial pseudorecombinant viruses composed of CMV and tomato aspermy cucumovirus (TAV) RNAs (16, 28). However, a systematic search of available CMV nucleotide sequences did not indicate that potential recombinational events among numerous analyzed CMV strains occur (7). No natural mixed infection and no genetic exchange between CMV subgroup I and II strains were found (17), indicating that most heterologous genetic recombinations seem to be at a competitive disadvantage (18), although a case of natural interspecies recombination between CMV and TAV has been reported (1). More recently, a homologous recombinational event between RNA 1 and RNA 2 or RNA 3 was demonstrated in vivo in subgroup I CMV when transgene sequences were used as donors (8).
In our previous work we described an alstroemeria-infecting subgroup II CMV isolate (ALS-CMV) that contained an additional sequence of 218 nucleotides (nt) in the 3' NTR of RNAs 3 and 4 (10). This additional sequence appeared to be identical to parts of the 3' NTRs of RNAs 1 and 2 of the homologous virus, suggesting that an RNA recombination event occurred between RNA 3 and RNA 1 or RNA 2. To investigate the commonness of additional sequences in the 3' termini of alstroemeria-infecting CMV isolates, the genomic RNAs of five distinct ALS-CMV isolates were analyzed. Furthermore the biological relevance of the recombinational events encountered are tested by competition experiments.
The CMV strains used in this study, designated ALS-0 through -4 (Table 1), were all independently isolated from alstroemeria plants (ALS-CMV) and were stock material of the Bulb Research Center, Lisse, The Netherlands, the Inspection Service for Floriculture and Arboriculture (NAKtuinbouw), Roelofarendsveen, The Netherlands, and Plant Research International, Wageningen, The Netherlands. ALS-0, although originally isolated from alstroemeria, had been maintained on tobacco for an extended period (at least 5 years); the others were exclusively passaged on alstroemeria. For short-term propagation, all five ALS-CMV isolates were mechanically inoculated onto Nicotiana benthamiana, while alstroemeria plants (clone VV024-6) (25) were used for pathogenicity assays. Purifications of virions, viral RNA, and total RNA were carried out by using published protocols (26, 33, 40). Reverse transcriptase (RT)-PCR amplification of CMV genomic RNAs was performed as previously described (9) with degenerate primer sets for RNA 1 (corresponding to nt 1 to 23 and 1098 to 1108, nt 1074 to 1087 and 2238 to 2255, and nt 2238 to 2248 and 3307 to 3391), RNA 2 (corresponding to nt 1 to 22 and 1105 to 1118, nt 1105 to 1118 and 2062 to 2075, and nt 1937 to 1951 and 3025 to 3039), or RNA 3 (corresponding to nt 1 to 34 and 1120 to 1131 and nt 1087 to 1111 and 2195 to 2204). RT-PCR products were cloned into pGEM-T Easy (Promega), and inserts were sequenced in both directions. Programs of the University of Wisconsin Genetics Computer Group package were used for sequence analysis (Table 1).
The sequences obtained from all ALS-CMV genomic RNAs appeared
to be highly homologous (>97%) to those of reported subgroup
II CMV strains. Whereas no large differences were found in the
RNAs 1 of the three ALS-CMV isolates tested (ALS-0, ALS-1, and
ALS-2), the 3' NTRs of both RNA 2 and RNA 3 were found to vary
significantly in length (Table
1). The RNA 2 molecules did not
differ in their 5' NTR (92 nt), 2a open reading frame (ORF)
(2,523 nt), or 2b ORF (303 nt) but varied considerably in their
3'-NTR length (Table
1). Also, the RNA 3 molecules had similar
lengths in their 5' NTRs (96 nt), 3a ORF (840 nt), intercistronic
regions (287 to 290 nt), and CP ORF (657 nt) but were notably
different in 3'-NTR length (Table
1). The additional sequences
were not PCR artifacts, as upon electrophoresis RNAs 3 and 4
of ALS-CMV strains 1 to 4 were visibly larger (Fig.
1) than
those of a wild-type subgroup II CMV isolate. Furthermore, the
size addition to RNA 2 made this RNA indistinguishable in size
from RNA 1 in Northern blots (Fig.
1). ALS-0 had RNAs of wild-type
size and sequence, and strikingly, this was the only alstroemeria
strain maintained (for several years) on
N. benthamiana, instead
of its original host. Hence, it cannot be excluded that it originally
also contained additional sequences.
The wild-type (and ALS-0) 3' NTR of RNA 2 can be arbitrarily
divided into five regions (A through E) (Fig.
2A), and that
of RNA 3 can be divided into four regions (A, B, C, and F) (Fig.
2B). Regions A through F are 13, 109, 16, 170, 116, and 183
nt long, respectively. Regions A, B, and C are common to all
reported CMV segments and can be folded into a tRNA-like structure
that is involved in viral minus-strand synthesis (
3). In ALS-0,
secondary structures consisting of stem-loop configurations
are predicted between regions C and D (positions 2868 to 2906)
and between regions D and E (positions 2698 to 2731) (Fig.
2C).
Interestingly, in all four other ALS-CMV isolates the additional
sequences were located in the central region of the 3' NTRs
of both RNA 2 and RNA 3. In the RNA 2 of both ALS-1 and ALS-3,
the extra sequences were 295 nt long and represented an exact
duplicate of regions D, C, and B (Fig.
2A). Shorter additional
sequences of 250 and 158 nt were found in the RNAs 2 of ALS-2
and ALS-4, respectively, and seemed to have arisen from deletions
following the original duplication that resulted in the RNAs
2 of ALS-1 and ALS-3.
In the 3' NTR of RNA 3, the ALS-CMV isolates harbored an additional
sequence of 218 nt (ALS-1 and -2) or 295 nt (ALS-3 and -4) (Fig.
2B). Strikingly, the additional 295 nt inserted in the RNA 3
of ALS-3 were completely identical to those of the homologous
RNA 2, both in length and in sequence (Fig.
2).
Their presence in the genome of four ALS strains maintained in their original host, and their absence in the only ALS strain not maintained in this host, suggests that the additional sequences may play a role in host adaptation. Indeed, based on visual observations, all isolates containing recombinant RNAs 2 and 3 (ALS-1 through -4) were more virulent than isolates lacking these repetitions, when inoculated into alstroemeria (data not shown), suggesting a greater fitness on this host. To verify this possibility, the ALS-CMV strains were tested in competitive inoculation assays. Tobacco and alstroemeria plants were inoculated with mixtures of various ratios of purified virions of wild-type ALS-0 and one of the recombinant isolates (ALS-2). In systemically infected leaves of tobacco plants, only nonrecombinant sequences could be amplified in subsequent RT-PCR analysis, except when ALS-2 was inoculated alone. In alstroemeria plants, however, the outcome was reversed: only RNAs of recombinant ALS-2 were detected in mixed-inoculation plants (Fig. 3). These results demonstrated a higher replicative ability of the nonrecombinant ALS-0 than of ALS-2 in tobacco plants, while the recombinant ALS-2 isolate was more competitive in alstroemeria.
To further delineate the difference in replication speed, tobacco
and alstroemeria plants were inoculated with mixtures of virus
isolates in which the least fit virus was added in 10-fold excess.
Viral RNA replication was monitored in the inoculated leaves
by Northern blot analyses and RT-PCR assays (Fig.
4). In tobacco,
the enlarged RNAs 3 and 4 of ALS-2 could be detected only until
the fourth day postinoculation, while those of ALS-0 could be
detected from the first day postinoculation, with the signals
further increasing over time. However, on alstroemeria, ALS-0,
even though inoculated in 10-fold excess, remained undetectable
on inoculated leaves. This experiment indicated that ALS-0 and
ALS-2 are outcompeted during the early stages of infection on
alstroemeria and tobacco, respectively, in spite of the initial
molecular excess over the competitor. Therefore, the additional
sequences in the 3' NTRs of RNAs 2 and 3, which were the only
differences observed between these isolates, were responsible
for the enhanced fitness of the virus in alstroemeria but produced
an adverse effect in tobacco plants. The significance of this
finding is further underscored by the fact that four different
alstroemeria isolates have these extra sequences, indicating
that the occurrence of such recombination is not a single incidence.
Though many cases of recombination events between RNA segments
of viruses or between viral RNAs and their satellite RNAs have
been described (e.g., see references
5,
28, and
29), naturally
occurring stable recombinations are rare (
17). An increase in
the biological fitness of a virus by recombination within or
between its segments has not been reported previously without
applying selection pressure in favor of the recombinant virus.
However, our conclusion that the 3' NTRs of the CMV genome are
important for viral fitness is confirmed by previous observations
that an artificial pseudorecombinant of CMV and TAV, having
RNAs 1 and 2 from CMV and RNA 3 from TAV, gained in fitness
upon a recombinational event in which CMV RNA 2-derived 3'-NTR
sequences were inserted in its TAV-derived RNA 3 (
16).
How the recombinant RNAs in the alstroemeria CMV isolates may have arisen remains subject to further study. Some clues, however, can be obtained from the positions of two stem-loop structures (Fig. 2), in combination with the previously implicated involvement of the CMV RdRp in template switching (22), a mechanism commonly accepted as a major means of generating recombinant RNAs (6, 11, 30, 41, 44). The RNAs 2 of the recombinant isolates show the duplication of a contiguous internal 3'-NTR segment. The same RNA 2-derived sequence was found to be incorporated in the 3' NTR of the RNAs 3 of the recombinant alstroemeria CMV isolates, some of which seem to have undergone further deletions. A complex secondary structure consisting of a stable hairpin (Fig. 2C, panel II) may have caused the RdRp to halt and disengage during minus-strand synthesis of the recombinant RNA 2. Subsequent restarting of replication at the 3' end of a second RNA 2 template resulted in the initial duplication (Fig. 2A). While replicating the 3' end of the recombinant RNA 2, the viral replication complex may have halted and separated at hairpin loop I (Fig. 2C). Subsequently, the RdRp continued replication on the highly homologous 3' NTR of RNA 3, resulting in an RNA 3 molecule with a duplicated 3' NTR of RNA 2.
Recombinational events leading to mutant viruses as observed here may be commonplace for CMV, as suggested by Canto and coworkers (8). These authors were readily able to generate recombinant CMV RNAs in a transgene setting, selecting for such events. Nonetheless, recombinant viruses do not occur frequently in nature, most likely due to reduced fitness (17). Our observations with the mixed infections of tobacco plants are in agreement with that notion. Alstroemeria plants, however, seem to be an exception to that rule, sustaining the presence of specific recombinant viruses. Additional sequences observed in the ALS-CMV isolates are all duplicated 3' NTRs. These 3' ends of positive-strand viral RNAs contain the origin of minus-strand synthesis and are the presumed site of the promoter elements that control this synthesis in cis (15). It can be hypothesized that a duplication within the NTRs enhances the recruiting of host proteins involved in the viral replication complex. If binding of these components is less efficient in alstroemeria than in other host plant species, it may be beneficial to duplicate the regions involved in recruiting these factors. Although the inserted sequences might interrupt the sequence and secondary structure of the 3' termini, the alteration apparently offers biological advantages to alstroemeria-infecting CMV isolates. In addition, the slow infection process of CMV in alstroemeria and low accumulation of viral RNA (results not shown) could indicate that duplications in viral sequences are tolerated in this host, while in other hosts these viruses are increased in their sensitivity to host defense responses, such as gene silencing. The competitive inoculation tests demonstrated the merit of these recombination events, which enhance the fitness of ALS-CMV infecting alstroemeria.
Nucleotide sequence accession numbers.
The sequences of the genomic RNAs of ALS isolates used here have been made available in GenBank (accession numbers AJ304393 through AJ304405).

ACKNOWLEDGMENTS
We thank R. van der Vlugt and I. Bouwen of Plant Research International
and A. van Zaayen of NAKtuinbouw for providing the
Alstroemeria isolates of CMV and M. de Jeu and J. B. Kim of the Laboratory
of Plant Breeding of Wageningen University for providing
Alstroemeria plants.
Y.-K.C. was financially supported by the Ministry of Education of Taiwan.

FOOTNOTES
* Corresponding author. Mailing address: Binnenhaven 11, 6709 PD Wageningen, The Netherlands. Phone: 31 317 483090. Fax: 31 317 484820. E-mail:
marcel.prins{at}viro.dpw.wag-ur.nl.


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Journal of Virology, April 2002, p. 4119-4124, Vol. 76, No. 8
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.8.4119-4124.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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