1 Estación Experimental La Mayora, Consejo Superior de Investigaciones Científicas, 29750 Algarrobo-Costa, Málaga, Spain
2 Centro Nacional de Biotecnología (CNB), Consejo Superior de Investigaciones Científicas, Campus Universidad Autónoma, 28049 Cantoblanco, Madrid, Spain
3 Centro de Edafología y Biología Aplicada del Segura (CEBAS), Consejo Superior de Investigaciones Científicas, Campus Universitario de Espinardo, Apdo Correos 164, 30100 Espinardo, Murcia, Spain
Correspondence
Miguel Aranda
m.aranda{at}cebas.csic.es
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ABSTRACT |
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The GenBank accession numbers of the sequences reported in this paper are AY242077 (CYSDV RNA 1) and AY242078 (CYSDV RNA2).
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INTRODUCTION |
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In recent years, the number of newly identified whitefly-transmitted criniviruses has increased rapidly (Wisler et al., 1998). Despite this and the very significant negative impact that criniviruses have on crop production, the only reported crinivirus complete genome sequences available to date are those of Lettuce infectious yellows virus (LIYV; Klaasen et al., 1995
), the type member of the genus Crinivirus, Sweet potato chlorotic stunt virus (SPCSV; Kreuze et al., 2002
) and Cucumber yellows virus (CuYV; Hartono et al., 2003
). Analyses of the genome sequences of these three viruses revealed several new features compared to other closteroviruses, for example the identification of a particular gene arrangement of the five-gene module typical of the family Closteroviridae (Karasev, 2000
; Klaasen et al., 1995
; Martelli et al., 2000
, 2002
) and the presence of a unique open reading frame (ORF) in the SPCSV genome that putatively encodes a protein belonging to the RNase III family (Kreuze et al., 2002
). Livieratos & Coutts (2002)
cloned and sequenced cDNAs of the RNA 2 of a CYSDV isolate. Their work showed that the RNA 2 of this isolate is 7281 nt long and contains seven ORFs flanked by 5'- and 3'-untranslated regions of 486 nt and 223 nt, respectively. Going from 5' to 3', proteins potentially encoded by CYSDV RNA 2 are a 5 kDa hydrophobic protein, a 62 kDa heat shock protein 70 homologue (Hsp70h), 59 kDa and 9 kDa proteins of unknown function, a 28·5 kDa coat protein (CP), a 53 kDa coat protein minor (CPm) and a 26·5 kDa protein of unknown function (Livieratos & Coutts, 2002
). Thus, the CYSDV RNA 2 contains the closterovirus hallmark gene array, with a similar arrangement to those of LIYV, SPCSV and CuYV (Hartono et al., 2003
; Klaassen et al., 1995
; Kreuze et al., 2002
; Martelli et al., 2000
, 2002
). Although information is available on CYSDV RNA 2 genomic sequences, including isolate variability of the CP and Hsp70h genes (Rubio et al., 1999
, 2001
), the CYSDV RNA 1, with an estimated size of 9 kb (Célix et al., 1996
), has not been sequenced. In this paper, we report the complete nucleotide (nt) sequence of RNAs 1 and 2 of a Spanish isolate of CYSDV. Analyses of the two genomic RNAs showed further new features compared to the other sequenced criniviruses, such as the identification of a new ORF in the 3' region of RNA 1 and a unusually long 5' region in RNA 2 in which no significant ORF could be predicted. In addition, we present data on the possible mechanisms of expression of CYSDV RNAs 1 and 2.
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METHODS |
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Double-stranded RNA (dsRNA) extracts were prepared according to the method of Valverde et al. (1990) with the modifications introduced by Célix et al. (1996)
. Total RNA was extracted from healthy and CYSDV-infected plants using TRIzol reagent (Sigma) following the manufacturer's instructions.
cDNA synthesis, cloning and sequencing.
To generate cDNAs of the 5'- and 3'-ends of CYSDV dsRNAs we proceeded as follows. First, poly(A) was added to the 3'-ends of the dsRNA minus and plus strands. One microlitre of 0·1 M methylmercuric hydroxide (Sigma) was added to 9 µl of dsRNA extract (10 ng µl-1) and incubated for 10 min at room temperature. One microlitre of 1·4 M -mercaptoethanol was then added, mixed and incubated on ice for 10 min. To that mixture, 4 µl of 5x yeast poly(A) polymerase buffer (USB) was added together with 1 µl of 2 mM ATP, 1 µl of RNase inhibitor (10 U µl-1; Amersham Pharmacia Biotech), 1 µl of yeast poly(A) polymerase (USB) and 2 µl of sterile distilled water. The mixture was incubated for 30 min at 37 °C, phenol/chloroform extracted and ethanol-precipitated. After drying, the resulting concentrate was dissolved in 11 µl of water and 1 µl of 0·1 M methyl mercuric hydroxide was added. The sample was incubated for 10 min at room temperature and then 1 µl of 1·4 M
-mercaptoethanol added. cDNAs were synthesized by adding 5 µl of 5x reverse transcriptase buffer (Roche Diagnostics), 2 µl of 5 mM dNTPs, 1 µl of RNase inhibitor (10 U µl-1; Amersham Pharmacia Biotech), 1 µl of 100 mM DTT, 1 µl of reverse transcriptase (Expand RT, Roche Diagnostics) and 2 µl of oligo(dT) (100 ng µl-1) and the mixture was then incubated for 90 min at 37 °C. The cDNA was PCR-amplified using specific oligonucleotides designed from known internal sequences (Célix et al., 1996
; Livieratos & Coutts, 2002
; E. Rodríguez-Cerezo & M. A. Aranda, unpublished results) and the Expand High Fidelity kit (Roche Diagnostics) following the manufacturer's instructions.
To generate cDNAs internal to the 5'- and 3'-ends of CYSDV RNAs, specific oligonucleotides designed from internal sequences (Célix et al., 1996; Livieratos & Coutts, 2002
; E. Rodríguez-Cerezo & M. A. Aranda, unpublished results) were used as primers in RT-PCR reactions in which total RNA from CYSDV-infected plants was used as a template. Reverse transcription and PCR were carried out essentially as described for the generation of cDNAs to the 5'- and 3'-ends.
DNA fragments obtained after PCR were fractionated by electrophoresis in 1 % agarose gels and purified. Some DNA fragments were ligated to the plasmid pCR-BluntII-TOPO (Invitrogen) or pGEM-T Easy vector System II (Promega) and cloned in E. coli. For each cDNA fragment two clones (Fig. 1) were sequenced and each nucleotide position of these clones was read at least twice. Other DNA fragments were directly sequenced. In this case, sequences from at least two independent RT-PCR reactions were obtained for each genomic region (Fig. 1
).
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Northern blot analyses.
For Northern blot analysis, 10 µg of total RNA was incubated at 55 °C for 75 min in a solution containing 1 M glyoxal and 50 % DMSO in 0·5 M BES pH 6·7. Denatured samples were electrophoresed in a 0·8 % agarose gel in 0·5 M BES pH 6·7 running buffer. RNAs were transferred to positively charged nylon membranes (Roche Diagnostics) by blotting. After UV-cross-linking, membranes were hybridized in a solution containing 50 % formamide at 65 °C for at least 8 h. For hybridizations, digoxigenin-11-UTP-labelled cRNA probes were used. Probes were synthesized by in vitro transcription from plasmids containing inserts corresponding to different genomic regions. Thus, probes I to III correspond to nt 1 to 950, 7539 to 8195 and 8342 to 9085 of CYSDV RNA 1, respectively. Probes IV to IX correspond to nt 11 to 430, 1844 to 2327, 3350 to 4161, 4842 to 5523, 5634 to 6190 and 7099 to 7957 of CYSDV RNA 2, respectively (see below). Plasmids used to prepare the probes were either those obtained for sequencing (Fig. 1) or by subcloning the specific regions following standard protocols (Sambrook & Russell, 2001
). Probes were transcribed from plasmids following standard protocols (Sambrook & Russell, 2001
). After hybridization, membranes were washed for 15 min, once at room temperature in 2x SSC and twice at 65 °C in 0·1x SSC. Chemiluminescent detection was carried out using the reagents and protocols supplied with a DIG-labelling and detection kit (Roche Diagnostics).
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RESULTS AND DISCUSSION |
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CYSDV RNA1 identification of a new ORF
An analysis of the coding capacity of RNA1 showed that it contains at least five ORFs, which we have designated ORF1a, ORF1b, ORF2, ORF3 and ORF4. These ORFs are flanked by two putatively untranslated regions (UTRs) (Fig. 1). The 5'-UTR of CYSDV RNA1 is 94 nt and the 3'-UTR is 221 nt.
The first AUG translation initiation codon is at position 95 downstream of the 5'-end and, consequently, ORF1a extends from nt 95 to 6028 and potentially encodes a 1977 amino acid protein with a calculated molecular mass of 226·7 kDa. In this putative protein two motifs conserved among proteins associated with virus replication could be identified. A methyltransferase domain (MTR) (Rozanov et al., 1992) is located between amino acids 514 and 871 and this sequence has high similarity to the corresponding domain of other criniviruses (Table 1
). In the carboxy-terminal portion (between amino acids 1678 to 1941) there is a helicase domain (HEL) (Gorbalenya & Koonin, 1993
); again, the homologous domains of other criniviruses have high similarity to this sequence (Table 1
). The region between the MTR and HEL motifs does not show significant similarity to any protein sequence available in databases. Interestingly, in this region, the TMHMM program (Krogh et al., 2001
; Möller et al., 2001
) predicts two transmembrane domains between amino acid positions 1238 to 1260 and 1352 to 1374, suggesting that this protein may be localized to membranes, analogous to the replicase of the closterovirus Beet yellows virus (Erokhina et al., 2000
, 2001
). An alignment of the amino acid sequences upstream of MTR in SPCSV, CuYV and LIYV with that of the similar region of CYSDV showed the existence of a putative papain-like cysteine proteinase domain (L-Pro), similar to that which has been reported for other viruses of the family Closteroviridae (Peng et al., 2001
). The conserved cysteine and histidine residues that are predicted to participate directly in catalysis (Peng et al., 2001
) are Cys-393 and His-442, and the presumed cleavage site is between Gly-461 and Val-462. Thus, autoproteolysis would yield a putative leader proteinase of 52·3 kDa.
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The organization of the ORFs downstream, ORFs1a/1b, seems to be unique to CYSDV among criniviruses. ORF2 (nt 7554 to 7700) encodes a putative small protein (p5) of 48 amino acids with a predicted molecular mass of 5·2 kDa. In this protein, the TARGETP program (Emanuelsson et al., 2000) predicts the existence of an amino-terminal signal peptide which might be responsible for targeting the protein to the ER. Interestingly, there is a putative N-myristoylation site near the signal peptide cleavage site predicted by TARGETP. Thus, after processing, p5 might remain attached to the plasma membrane anchored by a myristoyl residue (Cross, 1990
). ORF 3 (nt 7704 to 8342) encodes a putative protein (p25) of 212 amino acids, with a predicted molecular mass of 25·1 kDa. No sequence homology has been found between p25 and any other protein in the databases nor has any conserved motif been identified in its sequence. The putative protein encoded by ORF 4 (p22; nt 8324 to 8902) is 192 amino acids and has a predicted molecular mass of 22·4 kDa. p22 has a similar size to the proteins encoded from the most 3'-terminal genes of the RNA1 of SPCSV and LIYV, and the BYV genome. All these proteins share some sequence similarity (Table 1
; results not shown) which suggests that they could have some functional equivalence.
CYSDV RNA 2 prediction of an unusually long 5'-UTR
RNA2 is 7976 nt. This is 695 nt longer than the sequence determined by Livieratos & Coutts (2002) for the same RNA of another isolate of CYSDV. An alignment of both sequences showed that of these 695 nt, 691 nt are in the 5'-terminal region (Fig. 2
), and that the overall nucleotide similarity between both sequences in the shared region is very high (99 % nt identity). In order to confirm the presence of these extra nucleotides in the 5'-end of the genomic RNA2 of CYSDV-AlLM (the CYSDV isolate characterized in this study; see Methods), total nucleic acid extracts of CYSDV-AlLM-infected plants were analysed in Northern blots using a probe complementary to nt 11 to 430 of CYSDV RNA2 sequence described here. Consistently, this probe detected an RNA species of around 8 kb which likely corresponds to the genomic RNA2 (Fig. 3
c). In addition, the six 5'-terminal nucleotides (5'-GAAATA-3') of CYSDV-AlLM RNA2 are identical to the six 5'-terminal nucleotides of RNA1, similar to what has been described for SPCSV (Kreuze et al., 2002
) and LIYV (Klaasen et al., 1995
). Based on these results, we concluded that these extra 691 nucleotides are really a part of the RNA2 5'-terminal region of CYSDV-AlLM. Differences between this and the sequence determined by Livieratos & Coutts (2002)
could be the result of the natural variability of CYSDV isolates. Thus, we decided to analyse the RNA2 5'-terminal regions of four CYSDV field isolates from Almería (Spain), Málaga (Spain) and Texas (USA) by preparing and sequencing RT-PCR products for this genomic region. The results indicated that all four CYSDV isolates have very similar nucleotide sequences to the sequence of CYSDV-AlLM (Fig. 2
).
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Further analyses of the CYSDV RNA2 sequence showed that it contained at least eight ORFs, potentially encoding proteins p5 (ORF5; nt 1178 to 1303), Hsp70h (ORF6; nt 1234 to 2895), p6 (ORF7; nt 2902 to 3066), p59 (ORF8; nt 3060 to 4616), p9 (ORF9; nt 4595 to 4834), CP (ORF10; nt 4927 to 5682), CPm (ORF11; nt 5682 to 7061) and p26 (ORF12; nt 7066 to 7752). Therefore, the predicted genomic structure of CYSDV RNA2 is essentially similar to that proposed by Livieratos & Coutts (2002). However, an additional ORF, ORF7, was predicted from our data (Fig. 1
, Table 2
). This ORF was in-frame with ORF6 and extended from nt 2902 to 3066. Its putative AUG initiation codon, which is not in an optimal context for translation initiation (Kozak, 1991
), is only six nucleotides downstream of the ORF6 termination codon. If this AUG codon is in fact functional, then ORF7 would encode the p6 protein, which would have 54 amino acids and a predicted molecular mass of 6·5 kDa. Nevertheless, no sequence homology has been found between p6 and any other protein in the databases, nor has any conserved motif been identified in its sequence. Further studies are needed to analyse the biological significance of ORF7. The remaining proteins potentially encoded by CYSDV RNA2 according to our analyses are very similar (Table 2
) to those proposed by Livieratos & Coutts (2002)
and their characteristics have been discussed by those authors.
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Expression of internal ORFs via subgenomic RNAs
Several mechanisms, such as production of 3' co-terminal sgRNAs, frameshifting and polyprotein processing, have been shown or proposed to be used by closteroviruses to facilitate the expression of internal ORFs (reviewed by Karasev, 2000). To identify putative CYSDV sgRNAs, we carried out Northern blot analyses on total ssRNAs from healthy and infected plants with probes for different regions of CYSDV RNAs 1 and 2 (Fig. 3
).
For CYSDV RNA1, probe I, complementary to the 5'-terminal 950 nt (Fig. 3a), hybridized to an RNA of around 9 kb which most probably corresponds to CYSDV RNA1, but did not hybridize to any RNAs from healthy plants (Fig. 3b
). Probe II, complementary to nt 7539 to 8195 (Fig. 3a
), hybridized to CYSDV RNA1 and also to a single smaller RNA of approximately 1·6 kb (Fig. 3b
). Probe III, complementary to nt 8342 to 9085 of the 3'-terminal region (Fig. 3a
), hybridized to the same RNAs as probe II and, in addition, to a single smaller RNA of around 0·8 kb (Fig. 3b
). We speculate that the 0·8 kb sgRNA detected by probe III may correspond to a subgenomic mRNA for ORF4 and that the sgRNA of around 1·6 kb detected by probes II and III may correspond to a subgenomic mRNA for ORF 2 and/or ORF 3. If the 1·6 kb sgRNA was monocistronic, p5 or p25 might not be expressed. Alternatively, they might be expressed from a different sgRNA that accumulates at undetectable levels, or expressed using a different mechanism. If the 1·6 kb sgRNA was dicistronic, then the expression of p25 might occur by ribosomal leaky scanning or internal ribosome entry, as has been shown for other viruses (reviewed in Maia et al., 1996
), or by a different mechanism. The lack of detection of a sgRNA for ORF1b agrees with the hypothesis proposed previously (see above) that this ORF is expressed by a +1 ribosomal frameshift during translation.
For CYSDV RNA2, probe V, complementary to nt 11 to 430 (Fig. 3a), hybridized to an RNA of around 8 kb (Fig. 3c
), which most likely corresponds to CYSDV RNA2. Probe V, complementary to nt 1844 to 2327 (Fig. 3a
), hybridized to CYSDV RNA 2 and to an RNA of around 6·8 kb (Fig. 3c
). According to this size estimation, this RNA might correspond to a subgenomic mRNA for p5 and/or Hsp70h. However, it would be surprising if p5 needed to be expressed from a subgenomic mRNA, since the corresponding ORF seems to be the first ORF in CYSDV RNA2 and, therefore, it would be expected to be expressed directly from CYSDV RNA2. Probe V also hybridized to several RNAs in healthy samples (Fig. 3c
), perhaps as a consequence of the conservation of Hsp70 genes between the host and the virus. Probes VI to IX (Fig. 3a
) identified three 3'-co-terminal sgRNAs of sizes around 5, 3 and 0·9 kb (Fig. 3c
). According to these size estimations, these RNAs could serve as subgenomic mRNAs for ORFs 8 and/or 9, CP and ORF12. We expected to detect another sgRNA of around 2·3 kb which would serve for the expression of CPm. Such an sgRNA was not seen and therefore CPm could be either expressed from an sgRNA undetectable under our experimental conditions or expressed using a different mechanism. Based on the observations above, a tentative CYSDV sgRNAs map was proposed (Fig. 3a
).
Conclusions
We have determined the complete genomic sequence of CYSDV, the fourth crinivirus genome fully sequenced to date. This analysis has shown new features of crinivirus genome composition. The most striking novel features of CYSDV compared to LIYV, SPCSV and CuYV are a unique gene arrangement in the 3'-terminal region of RNA1, the identification in this region of an ORF potentially encoding a protein with no homologues in the databases and the prediction of an unusually long 5'-non-coding region in RNA2. Additionally, the CYSDV genome resembles those of SPCSV and CuYV in having very similar 3' regions in RNAs 1 and 2 (Hartono et al., 2003; Kreuze et al., 2002
), although in the case of CYSDV similarity in primary structures did not result in predictions of equivalent secondary structures. Overall, these data reinforce the view that the genus Crinivirus contains considerable genetic variation (Karasev, 2000
; Kreuze et al., 2002
). On the other hand, our Northern blot analyses suggested that the generation of subgenomic mRNAs is a strategy used by CYSDV for the expression of internal ORFs. Aspects that deserve more attention are the lack of detection of a candidate sgRNA for the CPm ORF, the precise mapping of the observed sgRNAs and the corresponding analyses of protein expression from these sgRNAs. Further studies on the molecular biology of CYSDV and, in general, on the molecular biology of criniviruses, have the potential to provide interesting information on gene function and regulation of expression of complex positive-strand RNA genomes. In addition, these studies may have considerable practical implications due to the economic importance of diseases caused by viruses in this genus.
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ACKNOWLEDGEMENTS |
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Received 7 March 2003;
accepted 30 April 2003.