| Nucleic Acids Research | Pages |
Construction of a variability map for eukaryotic large subunit ribosomal RNA
Introduction
Materials And Methods
Sequence alignment and estimation of substitution rates
Results And Discussion
Variability map of eukaryotic LSU rRNA
Acknowledgements
References
Construction of a variability map for eukaryotic large subunit ribosomal RNA
Received May 14, 1999; Accepted May 27, 1999
ABSTRACT In this paper, we present a variability map of the eukaryotic large subunit ribosomal RNA, showing the distribution of variable and conserved sites in this molecule. The variability of each site in this map is indicated by means of a colored dot. Construction of the variability map was based on the substitution rate calibration (SRC) method, in which the substitution rate of each nucleotide site is computed by looking at the frequency with which sequence pairs differ at that site as a function of their evolutionary distance. Variability maps constructed by this method provide a much more accurate and objective description of site-to-site variability than visual inspection of sequence alignments.
INTRODUCTION
A few years ago, quantitative substitution rate maps of the 5S rRNA, the small subunit ribosomal RNA (SSU rRNA), and the large subunit ribosomal RNA (LSU rRNA) of bacteria (1) and of the SSU rRNA of eukaryotes (2) were published. These maps were constructed by applying the substitution rate calibration (SRC) method that defines the variability or substitution rate of each nucleotide site as its evolutionary rate relative to the average evolutionary rate of all the nucleotide sites of the molecule (3,4). The variability maps constructed in this way clearly showed the distribution of variable and conserved positions in the different rRNA molecules.
Until recently, a reliable variability map for the eukaryotic LSU rRNA could not be constructed because too few complete sequences were available. However, due to recent sequencing efforts, the number of complete eukaryotic LSU rRNA sequences has increased significantly (5,6, unpublished results) and currently about 80 complete sequences are available for organisms belonging to the so-called crown of evolution (7), which now allows the construction of a reliable and detailed map showing the substitution rates in the LSU rRNA of eukaryotes.
Detailed information about the variability or conservation of nucleotide positions in ribosomal RNA is important for several reasons. The variability maps can be interpreted in terms of higher order structure and function and they facilitate the selection of areas suitable for the design of PCR primers and hybridization probes. In addition, the measurement of site variability is important from a phylogenetic point of view. While conserved areas can be used to unravel old relationships, the more variable regions can be used to study evolution between closely related organisms (e.g. 8). Regarding phylogenetic tree construction, the study of site variability in molecules has gained much interest lately. Newly developed tree construction methods take into account differences in nucleotide substitution rates, which leads to more consistent tree topologies (9-11).
MATERIALS AND METHODS
Sequence alignment and estimation of substitution rates
The LSU rRNA database, established at the University of Antwerp (UIA) in 1994, is continuously updated by scanning the international nucleotide sequence libraries such as GenBank and EMBL for corrected or new ribosomal RNA sequences. In general, only complete or nearly complete sequences are compiled. All ribosomal RNA sequences are stored in the form of an alignment that is based on the secondary structure adopted for the molecule (e.g. 12-14). All sequences in the database are aligned with the DCSE sequence editor (15). Beside the primary and secondary structure information, literature references, accession numbers and detailed taxonomic information about the organism from which the sequence was derived are also compiled. For more information about the LSU rRNA database and its contents, we refer to the latest database issue of Nucleic Acids Research (16). The easiest way to obtain data is through the World Wide Web at URL http://rrna.uia.ac.be/lsu/
Estimation of substitution rates and construction of a variability map was done as described previously (1,3,4). In short, substitution rates are estimated by looking at the frequency with which sequence pairs differ at each site as a function of the distance between the sequence pairs (3). Substitution rates or variabilities v are estimated for every site in the sequence alignment that is not absolutely conserved and contains a nucleotide in at least 25% of the aligned sequences. Then, after estimation of all substitution rates, alignment positions are grouped into sets of similar rate. A spectrum of relative nucleotide substitution rates is thus obtained (1,4). A color map, superimposed on the secondary structure model of the LSU rRNA, can then be constructed by dividing the nucleotides into different variability subsets and assigning a different color to each of these.
Once the shape of the spectrum is known, it is also possible to derive a new equation describing the evolutionary distance between two sequences as a function of the observed number of differences, i.e. the dissimilarity. This new equation can then be applied to tree construction, and several successful applications taking into account among-site rate variation in ribosomal RNAs have been described elsewhere (e.g. 10,17).
For the present study, 77 complete LSU rRNA sequences were analyzed, listed in Table 1. Duplicate sequences belonging to the same species were omitted from the analysis. The color map for eukaryotic LSU rRNA presented in this study can also be consulted on-line at URL http://bioc-www.uia.ac.be/u/yvdp/ . Color maps of bacterial 5S rRNA, bacterial SSU rRNA, bacterial LSU rRNA and eukaryotic SSU rRNA, published previously (1,2), can be found there as well. Secondary structures were drawn with the software tool RNAViz (18).
Table 1. Eukaryotic LSU rRNA sequences (and accession numbers) used for the nucleotide substitution rate calibration
| Animals (18) | |
| Acipenser brevirostrum | U34340 |
| Aedes albopictus | L22060 |
| Anguilla rostrata | U34342 |
| Anopheles albimanus | L78065 |
| Caenorhabditis elegans | X03680 |
| Drosophila melanogaster | M29800 |
| Dugesia tigrina | U78718 |
| Herdmania momus | X53538 |
| Homo sapiens | M11167, J01866 |
| Latimeria chalumnae | U34336 |
| Lepidosiren paradoxa | U34337 |
| Mus musculus | X00525, J00623 |
| Neoceratodus forsteri | U34338 |
| Oncorhynchus mykiss | U34341 |
| Protopterus aethiopicus | U34339 |
| Rattus norvegicus | V01270 |
| Xenopus borealis | X59733 |
| Xenopus laevis | X02995 |
| Fungi (11) | |
| Arxula adeninivorans | Z50840 |
| Blastocladiella emersonii | X90411, X90410 |
| Candida albicans | X70659, L07796 |
| Cryptococcus neoformans | L14067 |
| Entomophaga aulicae | U35394 |
| Pneumocystis carinii | M86760 |
| Saccharomyces cerevisiae | J01355, K01048 |
| Saccharomycopsis fibuligera | U09238, U10409 |
| Schizosaccharomyces japonicus | Z32848 |
| Schizosaccharomyces pombe | Z19578 |
| Tricholoma matsutake | U62964 |
| Land plants (21) | |
| Acorus gramineus | AF036490 |
| Arabidopsis thaliana | X52320 |
| Brassica napus | D10840 |
| Drimys winteri | AF036491 |
| Ephedra distachya | AF036489 |
| Eucryphia lucida | AF036494 |
| Fragaria ananassa | X58118, X15589 |
| Funaria hygrometrica | X99331, X74114 |
| Gnetum gnemon | AF036488 |
| Hamamelis virginiana | AF036495 |
| Jepsonia parryi | AF036497 |
| Lithophragma trifoliata | AF036501 |
| Mitella pentandra | AF036502 |
| Oryza sativa | M11585,M16845 |
| Parnassia fimbriata | AF036496 |
| Peltoboykinia tellimoides | AF036499 |
| Plumbago auriculata | AF036492 |
| Saxifraga mertensiana | AF036498 |
| Sinapis alba | X66325 |
| Tellima grandiflora | AF036500 |
| Tragopogon dubius | AF036493 |
| Green algae (1) | |
| Chlorella ellipsoidea | D17810 |
| Heterokont algae and relatives (8) | |
| Hyphochytrium catenoides | X80345, X80346 |
| Nannochloropsis salina | Y07975, Y07974 |
| Ochromonas danica | Y07977, Y07976 |
| Phytophthora megasperma | X75631, X75632 |
| Prorocentrum micans | X16108, M14649 |
| Scytosiphon lomentaria | D16558 |
| Skeletonema pseudocostatum | Y11512, Y11511 |
| Tribonema aequale | Y07979, Y07978 |
| Apicomplexans (6) | |
| Cryptosporidium parvum | AF040725 |
| Eimeria tenella | AF026388 |
| Neospora caninum | AF001946 |
| Plasmodium falciparum | U21939 |
| Theileria parva | AF013419 |
| Toxoplasma gondii | X75429 |
| Ciliates (4) | |
| Tetrahymena pyriformis | X54004, M10752 |
| Tetrahymena thermophila | X54512 |
| Spathidium amphoriforme | Unpublished |
| Euplotes aediculatus | Unpublished |
| Haptophytes (2) | |
| Phaeocystis antarctica | Unpublished |
| Prymnesium patelliferum | Unpublished |
| Red algae (2) | |
| Gracilaria verrucosa | Y11508, Y11507 |
| Palmaria palmata | Y11506 |
| Other (4) | |
| Chlorarachnion sp. | Unpublished |
| Pedinomonas minutissima | U58510 |
| Guillardia theta | Unpublished |
| Guillardia theta nucleomorph | Y11510, Y11509 |
RESULTS AND DISCUSSION
Variability map of eukaryotic LSU rRNA
The substitution rate spectrum obtained for eukaryotic LSU rRNA is shown in Figure 1. A different color is assigned to each of the different subsets. The relative rate limits of the subsets, and the corresponding colors used in the variability map, are as follows:
Figure 1. Distribution of the relative substitution rates, estimated from an alignment of 77 eukaryotic LSU rRNAs. The species used are specified in Table 1. Rates were estimated for 2153 alignment positions. Not included are 923 positions that are identical in all known sequences and 474 positions that contain a nucleotide in <25% of the aligned sequences. Sets of nucleotides are indicated in the same colors as used on the variability map of Figure 3.
0 < vi < 10-0.925 (blue); 10-0.925 < vi < 10-0.425 (green); 10-0.425 < vi < 10+0.075 (yellow); 10+0.075 < vi < 10+0.575 (orange); vi [ge] 10+0.575 (red). Since the rate distribution is not rectangular (Fig. 1), some colors are more abundant than others.
Figure 2 shows the secondary structure model of the LSU rRNA of the yeast Saccharomyces cerevisiae while Figure 3 shows the variability of the nucleotide sites of LSU rRNA mapped in the same shape. Colors attributed to different sites are as described above. Absolutely conserved (invariant) positions (vi = 0) are indicated in purple while sites colored in pink belong to areas that are very variable, but that are deleted in too many sequences to allow a sufficiently accurate measurement of their relative evolutionary rate. The color map for LSU rRNA gives a much more detailed and quantitative description of positional variability than the crude distinction between variable and conserved areas that is often made by visual inspection of sequence alignments.
Figure 2. Secondary structure model for the LSU rRNA of S.cerevisiae. The sequence is written clockwise from 5[prime] to 3[prime], sites are numbered in red every 100 nucleotides. Helix numbering is according to De Rijk et al. (16).
Figure 3. Variability map superimposed on the LSU rRNA secondary structure model of S.cerevisiae. Nucleotides are subdivided into five groups of increasing variability (see text for details). The most variable positions are in red, the least variable ones in blue. Absolutely conserved positions in all structures hitherto known are indicated in purple. Hypervariable regions that were not taken into consideration for rate calibration, because they are absent in >75% of the eukaryotic sequences considered, are indicated in pink. These include C1_1 to C1_3; E20_1 to E20_2; H1_1 to H1_3, the hairpin loops of D20 and E9_1, as well as individual nucleotides peculiar to the S.cerevisiae LSU rRNA.
It can be seen that in general the two nucleotides of a base pair have the same or a neighboring color, i.e. they are about equally variable. This is as expected, since the substitution of a base-paired nucleotide generally requires a compensating substitution in the opposite strand. Exceptions are mostly due to the fact that in some cases a particular base, usually a G or a U, seems to be required in one strand, but the existence of G·U pairs aside from G·C and A·U pairs allows the complementary base to change more freely.
As can be seen in Figure 3, 10 highly variable areas (orange, red and pink) can be distinguished in the eukaryotic LSU rRNA. These are formed by the following helices: B8; B13_1 to B16; the whole area denoted as C; D5 and D5_1; D14_1; D20; E20_1 to E20_2; G5_1 to G5_2; and H1_1 to H1_3. Many of the variable areas are characterized by major size variations. For example, the areas enclosed by helices C1 and E20, the entire area H1_n, and to a lesser extent helix D20, are also hot-spots for extremely variable insertions. These insertions were first described by Hassouna et al. (19) who referred to them as D(ivergent)-domains. As a rule, strong length heterogeneity seems to be most common in apical helices, i.e. those ending in a hairpin loop. Helices formed by long distance interactions, i.e. those bounded by multibranched loops, have less freedom to change in length. It should also be noted that the LSU rRNA molecule contains a number of potential branching points that bear additional helices in a limited set of species. For example, helices B14 and B15, though separated only by an internal loop in S.cerevisiae, were numbered differently because a potential branching point separates them.
Beside highly variable regions, several regions of conserved nature can be distinguished in the LSU rRNA. For the LSU rRNA of prokaryotes and for the SSU rRNA of prokaryotes and eukaryotes a stronger sequence conservation in single-strands than in double-strands has been reported (1,8,20-22). In order to examine this quantitatively in the case of eukaryotic LSU rRNA, separate substitution rate spectra were measured for nucleotide sites involved in base pairing and for those forming part of each type of single-stranded structural elements: multibranched-, hairpin-, internal- and bulge-loops. These spectra (not shown) were not very different from that measured for the entire molecule (Fig. 1). However, it should be remembered that the spectra show only the distribution of sites with a measurable substitution rate and do not include sites that are identical in all hitherto sequenced molecules. The latter have vi = 0 and therefore cannot be represented on a logarithmic scale. In order to demonstrate the greater tendency of conservation in single-stranded areas, sites were divided arbitrarily in more conserved ones (vi < 10-0.925, i.e. purple and blue in Figs 1 and 3) and more variable ones (vi [ge] 10-0.925, i.e. green to red in Figs 1 and 3). Table 2 shows that the fraction of more conserved sites is larger in single-stranded structural elements than in helices. In Figure 4 the fraction of more conserved sites (vi < 10-0.925) is calculated in both single- and double-stranded regions over the entire molecule in a sliding window of 50 nucleotides. In this calculation positions that contain a nucleotide in <25% of the sequences (pink in Fig. 3) were included with the most variable ones (vi [ge] 10-0.925). This graph shows more clearly that, overall, single-stranded regions have a higher fraction of conserved positions than double-stranded regions of the LSU rRNA molecule. One notable exception is the area between positions 2000 and 2400, which corresponds approximately to the region covered by helices E16-E25.
Figure 4. Graph showing the fraction of conserved sites, counted with a sliding window of 50 nucleotides. Each red dot represents the fraction of positions with vi < 10-0.925 in single stranded sites, each green dot represents this fraction in helical sites. The location of structural areas B to I is indicated by bars on top of the figure. Long areas with a fraction of conserved sites equal to 0 around positions 700, 2200 and 3400 consist of sites of presumably high but unmeasurable variability (pink dots in Fig. 3).
Table 2. Fraction of more conserved and less conserved sites in double-stranded areas and in different types of loops
| Relative substitution | No. of sites in different structural elements | |||||
| rate vi | multi-branched loops | hairpin loops | internal- and bulge loopsa | all single stranded elements | helices | entire moleculeb |
| vi < 10-0.925 | 252 | 180 | 181 | 613 | 548 | 1161 |
| vi [ge] 10-0.925 | 322 | 201 | 268 | 791 | 1124 | 1915 |
| Total | 574 | 381 | 449 | 1404 | 1672 | 3076 |
| % with vi < 10-0.925 | 43.9 | 47.2 | 40.3 | 43.7 | 32.8 | 37.7 |
| % with vi [ge] 10-0.925 | 56.1 | 52.8 | 59.7 | 56.3 | 67.2 | 62.3 |
bThis includes all sites for which a relative substitution rate was computed, i.e. all but those deleted in >75% of the sequences and indicated in pink on the color map of Figure 3.
Very often, specific functions can be ascribed to regions of the molecule that are conserved in structure (e.g. 23,24). One of the most conserved areas, both in bacterial (1) and eukaryotic LSU rRNA is the large multibranched loop in area G and the helices surrounding it (Fig. 3). This structure is generally considered to be the major element of the peptidyl transferase center of the ribosome, which is the catalytic center responsible for the peptide bond formation (25,26). Other highly conserved structures in the LSU rRNA are helices D18-D19 which are part of the so-called GTPase center of the ribosome, helices E21-E28 and the hairpin loop of helix H2 (25).
ACKNOWLEDGEMENTS
We want to thank Linda Medlin for DNA of haptophytes. Our research is supported by the Special Research Fund of the University of Antwerp and by the Fund for Scientific Research, Flanders. Yves Van de Peer is Research Fellow of the Fund for Scientific Research, Flanders.
REFERENCES
*To whom correspondence should be addressed at present address: Department of Biology, University of Konstanz, D-78457 Konstanz, Germany. Tel: +49 7531 88 2763; Fax: +49 7531 88 3018; Email: yves.vandepeer{at}uni-konstanz.de
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