Nucleic Acids Research, 2001, Vol. 29, No. 24 5029-5035
© 2001 Oxford University Press
Sequence organization of barley centromeres
Institut für Pflanzengenetik und Kulturpflanzenforschung (IPK), Corrensstrasse 3, D-06466 Gatersleben, Germany
Received September 3, 2001; Revised October 22, 2001; Accepted November 1, 2001.
DDBJ/EMBL/GenBank accession nos+.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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By sequencing, fingerprinting and in situ hybridization of a centromere-specific large insert clone (BAC 7), the sequence organization of centromeric DNA of barley could be elucidated. Within 23 kb, three copies of the Ty3/gypsy-like retroelement cereba were present. Two elements of
7 kb, arranged in tandem, include long terminal repeats (LTRs) (1 kb) similar to the rice centromeric retrotransposon RIRE 7 and to the cereal centromeric sequence family, the primer binding site, the complete polygene flanked by untranslated regions, as well as a polypurine tract 5' of the downstream LTR. The high density (200 elements/centromere) and completeness of cereba elements and the absence of internally deleted elements and solo LTRs from the BAC 7 insert represent unique features of the barley centromeres as compared to those of other cereals. Obviously, the conserved cereba elements together with barley-specific G+C-rich satellite sequences constitute the major components of centromeric DNA in this species. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The centromere of monocentric chromosomes is morphologically recognizable as the primary constriction. Centromeres are essential for correct segregation into daughter cells of sister chromatids during mitosis and meiosis II and of homologous chromosomes during meiosis I (reviewed in 1,2). Although centromere function is highly conserved among eukaryotes, as are the kinetochore proteins including those of higher plants (3,4), centromeric DNA is rather variable.
Centromeric DNA sequences have been described for several eukaryotes. However, except for some yeasts (5,6), their functional importance is at least controversial, the more so since for several species neocentromeric activities at non-centromeric positions have been reported, supporting the idea that the centromere location might be regulated epigenetically (7).
While for some plants no centromere-specific repeats could be isolated (8) such sequences have been found in others. For instance, the 180 bp repeat of Arabidopsis (9), which forms large tandem arrays with the repeat 106B (10) interspersed therein, occupy the central domain of all five Arabidopsis centromeres (11–13).
A few years ago, two centromeric sequences were described for cereals. One is the cereal centromeric sequence (CCS1) family of Brachypodium that also occurs in wheat, rye, barley, maize and rice centromeres (14); the other is the Sau3A9 sequence of sorghum which also hybridized to the primary constriction of the above species (15). Thereafter, using a barley homolog of Sau3A9 as a probe, a 
clone (#9) from a genomic library was detected containing a cereba (centromeric retroelement of barley) element with high similarity to the Ty3/gypsy group of retrotransposons (16). This element hybridized to all barley centromeres. It contained a complete polygene, of which Sau3A9 represents the integrase encoding region, and flanking sequences similar to CCS1, supposed to represent long terminal repeats (LTRs) of cereba. Due to the additional presence of BARE retroelement sequences (dispersed along the chromosome arms of barley; 17) within the 9 clone and a DraI restriction pattern, which differed from that of genomic DNA when probed with the barley homolog of Sau3A9, we assumed that this clone might contain either sequences of a centromere-border or a chimeric insert not really representative of barley centromeres. Meanwhile, further conserved sequences representing parts of gypsy-like retroelements were found within the centromeres of several cereals [CentA in maize (18); pHind22 in sorghum, wheat, maize and rye (19); RCS1 in rice, rye, barley, sorghum and maize (20); RCB11 in rice and crwydryn in oats and rye (21,22); RIRE7 in rice (23–25); R11H in wheat (26)] and even of Beta species [pBv26 and pBp10 (27)]. However, completeness and arrangement of these sequences have not yet been studied directly by complete sequencing of large insert clones.
The aim of this work was to search for large insert clones harboring sequences representative of barley centromeres and to study the sequence organization of cereba elements and possibly associated centromere-specific sequences within the corresponding clone in comparison with sequences of other cereal centromeres. This should provide suitable candidate sequences for gel-shift assays with kinetochoric proteins to characterize interactions between DNA and proteinaceous components of the barley centromere/kinetochore complex. We describe the insert sequence of a selected barley BAC clone which yielded similar hybridization patterns as genomic DNA using the barley centromeric sequence (BCS2) (14) and the integrase encoding region as probes and which hybridized to all barley centromeres in situ.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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BAC library screening and fluorescent in situ hybridization (FISH)
A BAC library of genomic DNA from Hordeum vulgare L. cultivar Morex (established at Clemson University) containing 313 344 clones (28) was transferred onto Hybond N+ filters (Amersham). Treatment of the filters, hybridization and washing conditions were as described (29,30). Of 10 BAC clones that hybridized with the integrase region (pGP7) of the polyprotein gene of Ty3/gypsy-like retrotransposon cereba labeled with [32P]dCTP [using a random primer extension kit (Amersham) as described previously (31)] only one (03J24, now called BAC 7) showed after FISH positive signals exclusively at the centromeric regions of all barley chromosomes.
For FISH, BAC 7 DNA was isolated using a Qiagen Plasmid Mini Kit (100) and labeled with rhodamin-5-dUTP using a nick translation kit (Roche Biochemicals) according to the manufacturer’s instructions. The primers (AGGGAG)4 and (CTCCCT)4, representing the most frequent motif within the G+C-rich domain outside the cereba elements of the BAC 7 insert, were amplified without additional template sequence and biotin labeled by PCR as described previously (32) for FISH.
Metaphase spreads from root tip meristems of the barley line MK 14/2034 (characterized by two homozygous reciprocal translocations between chromosomes 3H/4H and 7H/5H) were prepared as described (16), FISH and signal detection were performed as described (33).
BAC size determination
The size of the BAC 7 clone was measured by pulsed field gel electrophoresis (PFGE) using the CHEF-DR® II electrophoresis system (Bio-Rad) with a 5 s pulsed time (5 V/cm) for 15 h on a 1% agarose gel (Gibco BRL) at 14°C in 0.5x TBE buffer (45 mM Tris–borate, 1 mM EDTA, pH 8.0). A /HindIII ladder (MBI Fermentas) was used as a molecular weight marker.
Restriction digests, agarose gel electrophoresis and Southern blot analysis
For restriction analysis, aliquots containing 70 ng of BAC 7 DNA were completely digested for 3 h at 37°C with 10 different restriction endonucleases (BglII, BstXI, EcoRI, HindIII, KpnI, NotI, PstI, SalI, SfuI, XbaI) and 20 double combinations. The digestion products and the molecular weight markers Smartladder (EUROGENTEC) and Gene RulerTM DNA Ladder Mix (MBI Fermentas) were electrophoresed on 0.8% agarose gels (Gibco BRL, Life Technologies) in 1x TBE buffer at 78 V for 4 h.
To perform Southern blot analysis, single or double digests of BAC 7 DNA with the restriction enzymes EcoRI, HindIII, PstI, NotI, SalI were carried out. The fragments were separated on 1% agarose gels and blotted onto a Hybond-N+ nylon membrane (Amersham Life Science) in 20x SSC solution. The DNA was fixed on the membrane by exposure to UV light for 3 min. Prehybridization and hybridization were performed overnight at 68 and 58°C, respectively, in 5x SSC, 0.1% (w/v) N-laurosylsarcosine, Na-salt (Sigma), 0.02% (w/v) SDS and 0.5% (w/v) blocking reagent (Boehringer Mannheim). As probes we used pBeloBAC 11 (vector) and the following inserts of subclones of the 9 clone (accession number AF078801; 16), which represent parts of the retrotransposon cereba: pGP7 (1.5 kb, RNase H + integrase domain), pGP12 (1.6 kb, gag + RNA binding domain), pGP33 (1.6 kb, including 182 bp homologous to the barley variant of CCS1) (14), pGP5 (1.1 kb, reverse transcriptase domain) and pGP13 (0.46 kb, protease domain). The pGP inserts were obtained by digestion of the subclones pGP7 and pGP5 with XbaI and HindIII and of pGP12, pGP13 and pGP33 with EcoRI and HindIII and extraction from gels using a QIAEX Kit (Qiagen). Probes were labeled using a Dig-high prime Kit (Boehringer Mannheim), according to the supplier’s instructions. After hybridization, the membrane was washed twice in 2x SSC, 0.1% SDS for 5 min at room temperature and twice in 0.1x SSC, 0.1% SDS for 5 min at 58°C. The DNA–DNA hybrids were detected by chemiluminescence with the CSPD® Kit (Boehringer Mannheim). Prior to reuse, the membrane was stripped by boiling in 0.5% SDS.
Subcloning, shotgun sequencing and data analysis
BAC 7 DNA was sonicated and fragments (550 bp) were subcloned into the pBluescript II SK- vector (Stratagene) and sequenced using an ALFexpress (Pharmacia Biotech) or an ABI Prism 377 (Perkin Elmer) DNA sequencer. Of an 4.8 kb HindIII fragment, 3.9 kb flanked by G+C-rich domains could not be sequenced completely, even by two specialized Biotech companies. The shotgun-sequencing data were analyzed with the Sequencher 3.1.1. software (Gene Codes). The resulting contigs were compared with the GenBank entries for the 9 clone and the CCS1 (position 1–260) of the Hi-10 clone derived from B.sylvaticum (U52217) at NCBI using the BLASTN homology search software (http://www.ncbi.nlm.nih.gov/blast/bl2seq/bl2.html). The program TAIR BLASTTM Similarity Search (http://arabidopsis.org/blast/) was used for comparison of the BAC 7 insert sequence with other plant sequences of the GenBank database. The nucleotide sequence of BAC 7 clone has been deposited in GenBank under the accession numbers AY040832 and AY040833.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Isolation and characterization of the centromere-specific clone BAC 7
A genomic barley BAC library was screened with pGP7, a plasmid subclone of the
9 clone, which is highly homologous to the integrase region of the polyprotein gene of the Ty3/gypsy group of retrotransposons. Ten clones were selected, but only one of these (BAC 03J24, later called BAC 7) showed a positive FISH signal exclusively at the centromeric regions of all barley chromosomes (Fig. 1A). The other nine BACs also yielded dispersed signals. They either contain additionally centromere-flanking sequences, e.g. copia elements, as is the case for 9 (16) and some large insert clones of maize and sorghum (18,19) which contain gypsy- as well as copia-like elements, or represent chimeric inserts, or contain (parts of) centromeric sequences which may occur as single copies outside centromeres. FISH with genomic DNA of barley revealed uniformly dispersed signals along all chromosomes, while a 10-fold excess of unlabeled BAC 7 DNA suppressed signals at centromeres, suggesting that BAC 7 contains the major sequence components of barley centromeres. BAC 7 showed a hybridization pattern similar to that of genomic DNA after digestion with DraI and Southern hybridization with pGP7 and BCS2, the barley homolog of the CCS1 family (14). These observations and the fact that all sequence components of BAC 7 are detectable by FISH exclusively at all centromeres (see below and 16) indicates that the insert of BAC 7 is indeed representative for barley centromeric sequences.
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To determine the size of BAC 7, its DNA was isolated, digested with NotI and XhoI, respectively, and separated by PFGE (Fig. 1B). Digestion with XhoI yielded only one band corresponding to the linearized plasmid (
30 kb), while NotI yielded two fragments, one (6.9 kb) comprising most of the vector and the other one (23 kb) the insert flanked by short stretches of vector DNA at both sides. Therefore, the insert size was estimated to be 22 500 bp, which together with the vector pBeloBAC 11 (7507 bp) constitute BAC 7.
Sequencing and restriction fragment mapping of BAC 7
Shotgun sequencing of 150 subclones of BAC 7 with an average size of 550 bp was performed. Because the occurrence of repetitive sequences was to be expected for the insert of BAC 7 and sequencing and alignment into contigs is difficult for such sequences, a restriction map of BAC 7 was constructed in parallel. For that purpose, DNA of BAC 7 was completely digested with 10 restriction enzymes and 20 pairwise combinations and electrophoresed on agarose gels. Southern blots were hybridized consecutively with five subclones of 9 as well as with the vector pBeloBAC 11 as probes. A compilation of the resulting fragments is given in Table 1. As expected, all fragments per digest amounted to 30 kb, the size of the entire BAC 7. Double or triple bands were determined by comparing band intensities to that of the molecular weight markers. The restriction map has been designed manually by assembling restriction fragments from single and multiple digests in comparison with the sequence alignment obtained from shotgun sequencing data (Fig. 2D1 and D2). This led to mutual control and confirmation of data (sequence alignment versus fingerprinting). The entire insert of BAC 7 (22 500 bp) revealed a contig of 14 993 bp, separated from a second contig of 3603 bp by a fragment of 3900 bp, flanked on either side by G+C-rich sequences of 349 and 776 bp, respectively (Fig. 2D). The internal part of this fragment could not be sequenced completely by two specialized Biotech companies. Subclones of this fragment revealed mainly the motif AGGGAG and degenerated versions of it, but no new sequences. Tetrameres of the AGGGAG motif and its complementary sequence were used as primers for PCR with only nucleotides and Taq polymerase. The primer-multimer products yielded a smear on agarose gels and strong FISH signals exclusively at all centromeres of barley (Fig. 3) but not on rye and wheat centromeres.
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From sequence comparison of BAC 7 with the components of cereba of
9 a high degree of similarity became evident in spite of some rearrangements outside the polygene region (Table 2 and Fig. 2C). Within both contigs the RNA binding domains of the BAC 7 cereba elements show insertions of 119 bp (position 1640–1759) and 110 bp (position 9460–9570 and position 2349–2459 within the shorter contig, respectively) in addition to insertions within the 5' and 3' untranslated regions between the LTRs and the polygene regions as compared to the corresponding sequence of cereba of 9 (Table 2 and Fig. 2C).
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The contig of 14 993 bp contains two cereba elements in tandem
The left contig of the BAC 7 insert is formed by two almost identical and complete Ty3/gypsy-like retroelements of high similarity to cereba of
9. Both contained all five catalytic regions (RNA binding site/protease/reverse transcriptase/RNase H/integrase) including gag, primer binding sites (PBSs) and polypurine tracts (PPTs) and are flanked by LTRs on both sides. The upstream LTR of the first element (position 1–257) lacks the first 665 bp. The PBS follows immediately at its 3' end (position 258–274). The downstream LTR (922 bp) with a terminal TGAT/ATCA inverted repeat is preceded by a PPT at position 6242–6254 (Fig. 2D1). The second copy of the cereba element is complete except for the first 23 bp lacking at the upstream LTR. The 5'-regions of the (almost) complete LTRs show similarity (50%) with the LTR sequence of the RIRE 7 gypsy-type retrotransposon in rice (24) and the last third of LTRs (260 bp) with the CCS1 sequence (80%). Both retrotransposons show extended homology to the sequence RCB 11 (AB013613; 21), the gypsy-type retrotransposon RIRE 7 (AB033235; 24) and the repeat RCS 1 (AF078903; 19,20) of rice, to the repeats pSau3A9 (SBU68165; 15) and pHind22 (AF078901; 19) of sorghum, as well as to the retrotransposon-like repeat CentA of maize (AF078917; 18), which all occupy centromeric positions (Table 3 and Fig. 2E).
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The contig of 3603 bp contains a truncated cereba element
The right contig of the BAC 7 insert between the G+C-rich stretch and the vector covers a cereba element extending from the 5' LTR (position 1–920) with the terminal TGAT/ATCA inverted repeat up to the end of the gag+RB region. This element is nearly identical to the corresponding parts of the complete cereba elements of the left contig.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We have shown that the gypsy-like retroelement cereba occurs in three copies within 23 kb and that apparently complete and autonomous cereba elements including LTRs of nearly 1 kb constitute together with the G+C-rich satellite sequences, the major DNA component of barley centromeres. The density of retroelements is higher in barley centromeres than calculated for wheat (one gypsy-like element per 55 kb; 26) or sorghum centromeres (two such elements within a 90 kb BAC; 19). According to a previous estimation of about 200 cereba elements per barley centromere (16), this would amount to at least 1.4 Mb of centromeric DNA per chromosome. Also the completeness of the cereba elements is a novelty when compared to those within centromeric clones of other cereals (22). The assumption that CCS1 sequences form (parts of) LTRs (16) could be confirmed. The sequence and restriction analyses of BAC 7 are in accordance with the previous assumption that the
9 insert is not representative for the sequence organization within barley centromeres, albeit large insert clones combining centromeric gypsy-like and non-centromeric copia-like elements were also reported for sorghum (19) and maize (18). These clones are either chimeric or originate from centromere-flanking regions. Obviously, related elements are conserved within the centromeres of all cereals since their radiation 60 million years ago, because a horizontal transfer between the contemporary cereal species is rather improbable (35). Furthermore, it is suggested that these types of retrotransposons do not frequently invade non-centromeric positions within their host genomes. Probably, they represent favoured targets for kinetochore assembling. This may be tested in future by gel shift assays with suitable plant kinetochoric proteins. The absence of internally deleted elements or solo LTRs from BAC 7 is in contrast to observations made for rice (20,24), maize (18) and sorghum (34). It might be possible that centromere-specific satellites such as the G+C-rich sequence motif of barley have originated during evolution by nested transposition (36); their redundancy may depend on species- and position-specific transposition frequencies of certain types of mobile elements which may (22) or may not be identical with those found to be clustered at cereal centromeres. Provided there is indeed a trend to substitute the centromeric gypsy-like elements by satellite sequences (22), then barley represents an early stage of this development due to its high density of centromeric retroelements.
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ACKNOWLEDGEMENTS |
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We thank Jörg Plieske for advice with PFGE, Stephan Streng for his help with the BAC library screening, Paul Fransz and Dagmar Schmidt for stimulating discussion, Barbara Hildebrandt for technical assistance and Rigomar Rieger for critical reading of the manuscript. This work was supported in part by a grant from the DFG (Schu 951/6-1).
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FOOTNOTES |
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* To whom correspondence should be addressed. Tel: +49 39482 5239; Fax: +49 39482 5137; Email: schubert{at}ipk-gatersleben.de Present addresses:Gernot G. Presting, Novartis Agriculture Discovery Institute, 3115 Marryfield Row, San Diego, CA 92121-U25, USAKarla dos Santos, Institut für Pflanzenbau und Pflanzenzüchtung, Universtät Göttingen, von Siebold Strasse 8, 37075 Göttingen, Germany +AY040832, AY040833
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R. Kalendar, C. M. Vicient, O. Peleg, K. Anamthawat-Jonsson, A. Bolshoy, and A. H. Schulman Large Retrotransposon Derivatives: Abundant, Conserved but Nonautonomous Retroelements of Barley and Related Genomes Genetics, March 1, 2004; 166(3): 1437 - 1450. [Abstract] [Full Text] [PDF]
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R. J. Mroczek and R. K. Dawe Distribution of Retroelements in Centromeres and Neocentromeres of Maize Genetics, October 1, 2003; 165(2): 809 - 819. [Abstract] [Full Text] [PDF]
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Z.-J. Cheng and M. Murata A Centromeric Tandem Repeat Family Originating From a Part of Ty3/gypsy-Retroelement in Wheat and Its Relatives Genetics, June 1, 2003; 164(2): 665 - 672. [Abstract] [Full Text] [PDF]
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K. Nagaki, J. Song, R. M. Stupar, A. S. Parokonny, Q. Yuan, S. Ouyang, J. Liu, J. Hsiao, K. M. Jones, R. K. Dawe, et al. Molecular and Cytological Analyses of Large Tracks of Centromeric DNA Reveal the Structure and Evolutionary Dynamics of Maize Centromeres Genetics, February 1, 2003; 163(2): 759 - 770. [Abstract] [Full Text] [PDF]
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 R. K. Dawe RNA Interference, Transposons, and the Centromere PLANT CELL, February 1, 2003; 15(2): 297 - 301. [Full Text] [PDF]
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C. X. Zhong, J. B. Marshall, C. Topp, R. Mroczek, A. Kato, K. Nagaki, J. A. Birchler, J. Jiang, and R. K. Dawe Centromeric Retroelements and Satellites Interact with Maize Kinetochore Protein CENH3 PLANT CELL, November 1, 2002; 14(11): 2825 - 2836. [Abstract] [Full Text] [PDF]
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