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Journal of Virology, August 2000, p. 7656-7665, Vol. 74, No. 16
Department of Medical Microbiology,
Cardiovascular Research Institute Maastricht, University of
Maastricht, 6202 AZ Maastricht, The Netherlands
Received 18 February 2000/Accepted 16 May 2000
We have determined the complete genome sequence of the Maastricht
strain of rat cytomegalovirus (RCMV). The RCMV genome has a length of
229,896 bp and is arranged as a single unique sequence flanked by
504-bp terminal direct repeats. RCMV was found to have counterparts of
all but one of the open reading frames (ORFs) that are conserved
between murine CMV (MCMV) and human CMV (HCMV). Like HCMV, RCMV lacks
homologs of the genes belonging to the MCMV m02
glycoprotein gene family. However, RCMV contains 15 ORFs
with homology to members of the MCMV m145 glycoprotein gene
family. Four ORFs are predicted to encode homologs of host proteins;
R33 and R78 both putatively encode G protein-coupled receptors, whereas r144 and r131 encode homologs of major histocompatibility class I heavy
chains and CC chemokines, respectively. An intriguing feature of the
RCMV genome is the presence of an ORF, r127, with similarity to the
rep gene of parvoviruses as well as ORF U94 of human
herpesvirus 6A (HHV-6A) and HHV-6B. Counterparts of these ORFs have not
been found in the other sequenced herpesviruses.
As a model for cytomegalovirus (CMV)
infection and disease, we study the interaction between rat CMV (RCMV)
and its host. The RCMV-rat model is attractive, since the pathogenesis
of infection in RCMV-infected rats is similar to that in human CMV
(HCMV)-infected humans. To fully exploit the RCMV-rat model, it is
important to have a detailed picture of the genomic
organization of RCMV. In this report, we present the complete DNA
sequence of the RCMV (Maastricht) genome. Following HCMV
(11) and murine CMV (MCMV) (37), RCMV is
the third CMV for which the complete genome sequence has been
determined. In addition, the RCMV genome represents the first complete
sequence of a rat-specific herpesvirus.
General features of the RCMV genome sequence.
The RCMV genome
was sequenced in a directed fashion, by using the previously cloned
EcoRI and XbaI genomic subclones
(32) as starting points. Overlapping plasmid clones of the
genome were generated by using various restriction endonucleases.
Both strands of each plasmid insert were sequenced by the
dideoxynucleotide chain termination method. The final sequence was
determined on both strands over 100% of the RCMV genome. Sequence
assembly and analysis was done with the program PC/Gene (version 2.11;
IntelliGenetics, Mountain View, Calif.) and with software from the
United Kingdom human genome mapping project resource center
(Hinxton, Cambridge, United Kingdom
[http://www.hgmp.mrc.ac.uk]).
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Complete DNA Sequence of the Rat Cytomegalovirus Genome
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Identification of RCMV protein-coding ORFs.
The strategy used
to identify RCMV open reading frames (ORFs) likely to be coding was
essentially based on that used in the sequence analysis of the MCMV
genome (37). The major criteria for identifying a coding
sequence were the presence of an ORF with a minimum length of 300 bp
and a less than 60% overlap with adjacent ORFs. Obviously, the
demonstration of similarity between predicted amino acid sequences
encoded by RCMV ORFs and those encoded by well-characterized genes of
other origin was also used as an indication of an ORF being protein
coding. ORFs which were found to overlap more than 60% and not show
any similarity to known sequences were included in the list of ORFs
(Table
1), irrespective of their length and positional base preference. Database searches for homologous amino acid sequences were carried out with the
TBLASTN program (version 2.0; National Center for Biotechnology Information, National Institutes of Health,
Bethesda, Md. [http://www.ncbi.nlm.nih.gov/blast/blast.cgi?Jform=1]) against
nonredundant combined nucleotide sequence databases. The TBLASTN
program compares a protein query sequence against a nucleotide sequence
database dynamically translated in all reading frames. The use of this
program allows finding homologous amino acid sequences, even when these
sequences are not available from protein sequence databases. The naming
system used for RCMV ORFs numbers them from the left to the right end
of the genome in a similar fashion as has been described for MCMV
(37). The left-to-right orientation of the RCMV genome has
previously been established (8, 50). As the RCMV and MCMV
genomes were found to be largely colinear, the numbering system for the
RCMV genes is congruent with the MCMV numbering system. RCMV ORFs with
homologs in HCMV are indicated by uppercase prefixes (e.g., R23),
whereas ORFs without significant sequence similarity with HCMV genes
are indicated by lowercase prefixes. In order to maintain the
correlation between the numbering system of the CMV genes, suffixes (as
in r25.1) were introduced when additional unique RCMV ORFs were
identified between homologs of MCMV and HCMV genes. These
suffixes do not necessarily indicate any similarity between these RCMV
ORFs. Also, these suffixes were used when RCMV ORFs showed
similarity with MCMV ORFs with similar suffixes.
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Spliced transcripts. Splicing has previously been reported for RCMV R89 (Y. K. Gruijthuijsen, C. A. Bruggeman, and C. Vink, unpublished data) and R122/R123 (MIE) (3). Splicing of two other RCMV transcripts, R112 and r133, could be predicted on the basis of amino acid sequence alignments and the presence of consensus splice donor and acceptor sites (data not shown). In contrast to what was predicted for the MCMV M36 mRNA, transcripts from its RCMV homolog (R36) are probably not spliced. This notion was deduced from an alignment of the amino acid sequence derived from the unspliced R36 ORF with that derived from the spliced M36 ORF (data not shown). Similarly, transcripts from both the MCMV M33 and HCMV UL33 genes were reported to be spliced (16), whereas transcripts from their RCMV counterpart (R33) were demonstrated to be unspliced (5).
DNA sequences and proteins involved in nucleotide and DNA metabolism and DNA replication. The origin of lytic-phase DNA replication (oriLyt) of RCMV has previously been mapped to a 3.3-kb region between ORFs R57 and R69 and was shown to be highly complex, containing 23 DRs and 16 inverted repeats of lengths greater than 10 bp (50). Like MCMV and HCMV, RCMV contains six ORFs that may be essential for viral DNA replication. These ORFs, R44, R54, R57, R70, R102, and R105, have the potential to code for DNA polymerase accessory protein (DPAP), DNA polymerase (8), major DNA binding protein (8), and three components of the helicase-primase complex, respectively. Homologs of genes with a role in nucleotide metabolism are also found in the RCMV genome. These genes include ORFs R45, R72, and R114, which putatively encode the ribonucleotide reductase large subunit (RRL), dUTPase, and uracil-DNA glycosylase, respectively. ORF R97 is the homolog of HCMV UL97, which encodes a phosphotransferase (28).
ORFs encoding IE/regulatory proteins. Previously, several HCMV immediate early (IE) proteins were reported to play a role in the regulation of viral gene expression. These proteins are encoded by the UL122 to -123 (the MIE locus) (43), UL36 to -38 (27), UL69 (51), TRS1-IRS1 (42), and US3 (13) genes. RCMV possesses sequence and positional homologs of the first three loci (R122-123, R36-38, and R69). The organization of the R122-123 MIE locus (3) was previously shown to be similar to that of HCMV (43), MCMV (25), simian CMV (10), and the England strain of RCMV (39, 40). Various spliced transcripts are derived from each of these loci. Similarly, the HCMV UL36 to -38 IE locus was found to be transcribed in multiply spliced mRNAs. Some of the proteins that are encoded by these mRNAs have been reported to function in the activation of gene transcription (13, 24). One of the spliced transcripts from the UL36 to -38 gene cluster is the UL37 mRNA, which is composed of three exons. Only exon 3 of UL37 has homologs in the other sequenced betaherpesviruses.
Another potential IE gene of RCMV is r128, which is positioned several kilobases upstream of the MIE locus. ORF r128 is the homolog of the MCMV m128 (or ie2) IE gene (33) and has sequence and positional homology with the U95 ORFs of HHV-6A (20), HHV-6B (17, 22), and HHV-7 (36). ORF m128 was previously shown to have sequence similarity with members of the US22 gene family (33).Structural proteins. Homologs of all MCMV genes that have the capacity to encode the well-known structural proteins were also detected in the RCMV genome. These genes are likely to encode the major and minor capsid proteins (R86 and R46, respectively), the large tegument protein (R48), the upper (R82) and lower (R83) matrix proteins, and the major and small tegument phosphoproteins (R32 and R99, respectively) (10). In addition, the RCMV genome carries a homolog (R25) of MCMV M25, which was recently reported to code for a component of the viral tegument (52). Like their MCMV counterparts (14), RCMV ORFs R82, R83, and R84 were found to share sequences. The overall identities among the amino acid sequences that are deduced from these ORFs are 21.2% between R82 and R83, 22.4% between R82 and R84, and 19.1% between R83 and R84. When the amino acid sequences encoded by R82, R83, and R84 were compared to their MCMV counterparts, the highest similarities were seen between sequences derived from congruent positions within their respective genomes. Interestingly, when the amino acid sequences deduced from R82, R83, and R84 were compared to those from their HCMV counterparts (UL82, UL83, and UL84, respectively), the three RCMV sequences each showed a higher similarity with sequences derived from UL82 than with those from either UL83 or UL84. Conversely, the three HCMV sequences each displayed higher similarities with the amino acid sequence encoded by R82 than with the sequences deduced from the other two RCMV genes (data not shown).
Glycoproteins. Among the ORFs potentially encoding glycoproteins are R55 (8), R75, R100, and R115, which code for homologs of the conserved herpesvirus glycoproteins gB, gH, gM, and gL, respectively. In particular, the sequence of the putative RCMV gM protein is very similar to the sequence of the corresponding MCMV protein (68.9% identity). A glycoprotein may also be encoded by ORF r138, which is a homolog of the MCMV m138 gene (or fcr-1) (44). Homologs of this gene have not been identified in other betaherpesviruses. The m138 gene-encoded protein has been reported to be a receptor for the Fc domain of murine immunoglobulin G molecules (44). Recombinant MCMV strains that lack a functional m138 gene displayed severely restricted replication in comparison with wild-type MCMV in vivo (15).
Families of related RCMV ORFs. Five families of related genes were identified in the RCMV genome: (i) the UL25 family, including ORF R25 and R35; (ii) the UL82 family, including R82, R83, and R84; (iii) the US22 family; (iv) the m145 family; and (v) the G protein-coupled receptor (GPCR) homolog gene family, including R33 and R78 (Fig. 1). These families are also represented in the MCMV genome (37), whereas all but one of them (the m145 family) are represented in the HCMV sequence (11).
Members of the US22 gene family are present in all sequenced betaherpesviruses. The RCMV members of this family include R23, R24, r25.1, R36, R43, r128, r139, r140, r141, r142, and r143. The sequence as well as the position of these genes are conserved between RCMV and MCMV. Within the RCMV US22 family, the highest level of similarity is seen between the amino acid sequences derived from R24 and r25.1 (25.8% identity). A relatively high amino acid sequence similarity is also observed between r140 and r141 (25.6% identity) and between r140 and r143 (22.9% identity). An RCMV homolog of one MCMV member of the US22 family, m25.2, was not found. As described above, RCMV contains 15 members of the m145 glycoprotein gene family (Fig. 1). One of the members of this family, m152, has been shown to interfere with the major histocompatibility complex (MHC) class I pathway of antigen presentation (54). Six of the RCMV m145-like ORFs (r145, r150, r151, r152, r155, and r157) have both positional and sequence homology to the corresponding MCMV ORFs (37). Five others (r149, r151.3, r152.2, r152.3, and r152.4) show similarity in sequence, but not position, with MCMV ORFs. ORF r149 displays highest similarity with MCMV m17, whereas r151.3 scores highest with m145. ORFs r152.2, r152.3, and r152.4 are more similar to m152 than to other MCMV m145-like genes (data not shown). Four m145 family members, ORFs r70.2 through r70.5, are located at a unique position within the RCMV sequence, between conserved ORFs R70 and R72. Unlike their counterparts at the right side of the prototype genome, these ORFs are orientated from left to right. ORFs r70.2 to r70.5 were found to have the highest level of sequence similarity among each other. In particular, the deduced amino acid sequences of r70.2 and r70.4 are highly related (44.1% identity). ORFs r70.2 to r70.5 were also found to show a relatively high degree of similarity with ORF r152.2 (data not shown).ORFs encoding homologs of cellular proteins. Similar to MCMV, RCMV contains four ORFs that encode homologs of cellular proteins. R33 and R78 both encode homologs of GPCRs (2, 5), whereas r131 and r144 (4) encode homologs of chemokines and MHC class I molecules, respectively.
(i) ORFs encoding GPCR homologs.
RCMV ORF R33 belongs to the
HCMV UL33 gene family (5, 48). Currently, this family
consists of six members: UL33 (12), R33 (5), MCMV
M33 (16), and the U12 ORFs of HHV-6A (20), HHV-6B
(17, 22), and HHV-7 (36). Sequence and genome
location of these genes are conserved among the betaherpesviruses. The predicted amino acid sequences of the proteins encoded by members of
the UL33-like gene family were found to comprise several features characteristic of chemokine receptors (5, 16). In accordance with this, the HHV-6 U12-encoded protein was reported to be a functional receptor for
-chemokines in vitro (23). It has
been shown that the UL33, M33, and R33 genes are dispensable for in vitro replication of HCMV (31), MCMV (16),
and RCMV (5), respectively. However, both M33 and R33 were
shown to be essential for in vivo replication of MCMV
(16) and RCMV (5), respectively.
(ii) An ORF encoding an MHC class I homolog. RCMV contains an ORF putatively encoding a homolog of MHC class I heavy chains. This ORF, r144 (4), possesses positional as well as sequence similarity to the MCMV m144 gene (37). We recently reported that an r144-deleted RCMV strain shows similar replication characteristics as wild-type RCMV both in vitro and in immunocompromised rats (4). In contrast, an m144-deleted MCMV strain was shown to be attenuated during the primary phase of infection in mice (18).
(iii) An ORF encoding a CC chemokine homolog.
Previously,
genes encoding homologs of chemokines have been identified in both HCMV
(UL146, UL147, and UL152) (9) and MCMV (m131/129) (19,
29, 30). The HCMV-encoded chemokine homologs show similarity to
CXC (or
-) chemokines, whereas the MCMV m131/129-encoded protein is
more related to CC (or
-) chemokines. The MCMV-encoded chemokine
homolog was reported to be produced from a transcript in which the m131
ORF is spliced at its 3' end to the downstream located m129 ORF
(19, 29, 30). Remarkably, RCMV possesses an ORF at a
position congruent to that of MCMV m131 with limited similarity to both
m131 and m129 (Fig. 3). A study of
m131-deleted MCMV strains indicated that the m131/129-encoded
polypeptide may function as a chemokine agonist by recruiting
leukocytes to the sites of infection (19).
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RCMV ORF r127 shows similarity to parvovirus rep
genes.
ORF r127 is unique among the CMVs: a positional and/or
sequence homolog of this ORF was found in neither HCMV nor MCMV.
Surprisingly, a TBLASTN database search revealed that the amino acid
sequence that was deduced from r127 has similarity to the sequences of NS1 (nonstructural protein 1) or Rep proteins that are encoded by the
rep genes of parvoviruses. These viruses have
single-stranded DNA genomes with a length of approximately 5 kb. The
Parvovirinae subfamily of the parvoviruses consists of three
genera, Dependovirus, Parvovirus, and
Erythrovirus. The dependoviruses or adeno-associated viruses
(AAVs) require helper functions which can be supplied either by
genotoxic stimuli or by coinfecting viruses, like adenovirus, HSV-1,
HSV-2, CMV, and pseudorabies virus (for a review, see reference 6). These helper functions are needed for productive
infection and rescue of viruses that are integrated into the host's
genome. Unlike the AAVs, the members of the Parvovirus genus
are all capable of autonomous replication and can be pathogenic. The
RCMV r127-derived amino acid sequence displays highest similarity with
the sequence of the goose parvovirus (GPV) NS1 protein (53).
Lower similarities were observed with the corresponding sequences of
other parvoviruses, like Barbarie duck parvovirus (53) and
AAV-5 (1). Interestingly, the r127-encoded amino acid
sequence also showed similarity to the sequence encoded by the U94 gene
of HHV-6A (46). A homolog of the U94 gene, which displayed
the highest degree of similarity with the rep gene of AAV-2
(41, 46), was also found at a congruent position in the
genome of HHV-6B (17, 22). Remarkably, despite the generally
close genetic conservation between HHV-6A, HHV-6B, and HHV-7, a U94
homolog was not detected in the genome of HHV-7 (36).
ORF 94 is one of only six ORFs (DR3, U6, U9, U22, U83, and U94) that
are conserved between HHV-6A and HHV-6B but not HHV-7 (17).
It is, therefore, surprising that ORF U94 not only conserves sequence
but also genomic position with RCMV r127. In the genomes of
HHV-6A and -6B, ORF U94 is located immediately 5' of the U95 ORF,
running from right to left in the direction opposite to that of U95.
RCMV r127 is similarly situated with regard to the RCMV homolog of U95,
r128. The conserved location as well as orientation of the r127 and U94
genes in their respective genomes indicates that these genes may have
diverged from a common ancestral betaherpesvirus genome. A multiple
sequence alignment of the amino acid sequences encoded by r127, HHV-6A
U94 and GPV rep is shown in Fig.
4. In comparison with the NS1 amino acid sequence, both the r127 and U94 sequences are truncated at their carboxyl termini. The r127-derived sequence is also truncated at its
amino terminus compared to the other two sequences. Strongly conserved regions among the three amino acid sequences include sequences that represent putative nucleoside triphosphate binding helicase motifs termed A, B, B', and C (Fig. 4) (21, 26).
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Nucleotide sequence accession number. The nucleotide and amino acid sequences discussed in this paper have been deposited in the GenBank database under accession number AF232689.
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ACKNOWLEDGMENTS |
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We thank Saskia van der Vlies, Audrey Dasi, Mohamed Siad Farah, and Jasper van Grunsven for technical assistance, Patrick Beisser for helpful discussions, and Suzanne Kaptein and Wil Loenen for critical reading of the manuscript.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Medical Microbiology, University of Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands. Phone: 31 43 3876669. Fax: 31 43 3876643. E-mail: kvi{at}lmib.azm.nl.
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