- Research article
- Open Access
The EH1 motif in metazoan transcription factors
© Copley; licensee BioMed Central Ltd. 2005
Received: 26 August 2005
Accepted: 27 November 2005
Published: 27 November 2005
The Engrailed Homology 1 (EH1) motif is a small region, believed to have evolved convergently in homeobox and forkhead containing proteins, that interacts with the Drosophila protein groucho (C. elegans unc-37, Human Transducin-like Enhancers of Split). The small size of the motif makes its reliable identification by computational means difficult. I have systematically searched the predicted proteomes of Drosophila, C. elegans and human for further instances of the motif.
Using motif identification methods and database searching techniques, I delimit which homeobox and forkhead domain containing proteins also have likely EH1 motifs. I show that despite low database search scores, there is a significant association of the motif with transcription factor function. I further show that likely EH1 motifs are found in combination with T-Box, Zinc Finger and Doublesex domains as well as discussing other plausible candidate associations. I identify strong candidate EH1 motifs in basal metazoan phyla.
Candidate EH1 motifs exist in combination with a variety of transcription factor domains, suggesting that these proteins have repressor functions. The distribution of the EH1 motif is suggestive of convergent evolution, although in many cases, the motif has been conserved throughout bilaterian orthologs. Groucho mediated repression was established prior to the evolution of bilateria.
The Engrailed Homology 1 (EH1) motif is a short (<10 amino acids) region, initially found in engrailed (en) and other homeobox containing proteins, that mediates transcriptional repression via interaction with the WD40 repeat containing groucho (Gro) [1, 2]. Shimeld  proposed that the EH1 motif of Smith and Jaynes was shared with various forkhead (FH/HNF-3) containing transcription factors. The short size of the motif, however, suggests that it may occur by chance in many different protein families. Shimeld did not demonstrate statistically significant sequence similarity between the motifs from the homeobox- and forkhead-containing families. However, the human orthologs of groucho (the transducin-like enhancer of split proteins) have been shown to interact with FOXA2 via a region of sequence containing an EH1 motif, clearly demonstrating the biological relevance of the sequence similarity .
In this article I search systematically for instances of the EH1 motif in homeobox and forkhead containing genes and go on to demonstrate that the EH1 motif is also found in proteins containing T-box, Doublesex Motif (DM) and Zn finger domains. I show that within metazoan genomes, the observed association of the motif with transcription factor function is statistically significant. The location of the motif in members of the same transcription factor family is often non-homologous, occurring both N- and C-terminal to the DNA binding domain, suggesting that the presence of the motif is, in part, due to convergent evolution, as proposed by Shimeld; the conservation within orthologs points to many of these convergences predating the last common ancestor of the bilateria.
Results and Discussion
Significant association of EH1 motif with transcription factor function
EH1 motifs in homeobox and forkhead containing proteins
The presence of EH1 motifs within various homeobox, and to a lesser extent, forkhead containing proteins has been widely reported, although not systematically studied . I found EH1-like motifs co-occurring with 3 major groupings of homeobox sub-types: the extended-hox class, typified by Drosophila engrailed (en); the paired class, including Drosophila goosecoid (gsc), and the NK class, including Drosophila tinman (tin) [1, 9, 10] (see  for a description of these broad classes). Related to the paired class homeobox domains, a number of genes containing PAIRED domains only (i.e. the PAX domain of SMART ) were also found to contain EH1-like motifs (see Figure 1b). With only a few exceptions, outlined below, the EH1-like motif occurs N-terminal to the homeobox domain and C-terminal to the PAIRED domain when present. A number of these proteins have been shown to interact with groucho or its orthologs e.g. C. elegans cog-1 , vertebrate Nkx proteins , Drosophila engrailed (en) and goosecoid (gsc) [2, 14], and in high throughput assays Drosophila invected (inv) and and ladybird late (Ibl) .
A handful of EH1-like motifs are found C-terminal to homeobox domains. Of these, the best characterized is C. elegans unc-4, which has been shown to interact with the groucho ortholog unc-37 ; the Drosophila ortholog unc-4 also interacts with groucho in high throughput experiments . The C-terminal EH1-like motif is conserved in the closely related Drosophila paralog OdsH. The gene prediction for the human ortholog of unc-4 (ensembl gene identifier ENSG00000164853) appears to be artefactually truncated, but the mouse ortholog (Uncx4.1 ENSMUSG00000029546) and corrected human gene models, contain EH1-like motifs both N & C-terminal to the homeobox domain. Taken together with the fact that in the majority of related homeobox containing proteins the EH1-like motifs are N-terminal, this suggests that the N-terminal motif has been lost in Drosophila and C. elegans unc-4 orthologs.
EH1-like motifs also occur N- and C-terminal to Forkhead domains. The N-terminal class consists of the sloppy-paired genes (slp1 and slp2) of Drosophila and orthologous or closely related sequences: human FOXG1, and Drosophila CG9571; the C. elegans ortholog fkh-2 contains an EH1-like motif although a cysteine residue causes a low score. The C-terminal class consists of an apparent clade including the human FOXA, FOXB, FOXC and FOXD genes (Figure 2a), although if the EH1 motif was present in the common ancestor of this clade, multiple losses must have later occurred (see  for a Forkhead domain phylogeny). The situation is complicated somewhat by an EH1-like motif at the N-terminus of C. elegans unc-130 i.e. in the FOXD like family. The EH1 motif in slp1 has been shown to interact with groucho , and FOXA type genes have been shown to interact with human groucho orthologs .
EH1 motifs in novel domain contexts
Assuming a conservative per-domain cutoff score of 10.0 bits for true matches to the EH1hox model (see Figure 3), yields hits to proteins containing T-box domains (highest score 13.1 bits); Doublesex (DM) domains (highest score 11.6 bits) and C2H2 Zinc fingers (highest score 11.2 bits). Also of note was a further match at 9.4 bits, to an ETS domain containing protein. Prompted by these similarities I further investigated the presence of EH1-like motifs in these families, looking for high scoring matches to the EH1hox HMM that were conserved in closely related genes.
T-box containing proteins
I identified likely EH1 motifs co-occurring with T-Box domains in two distinct contexts (Figure 2b). The motif occurs C-terminal to the T-box in the Drosophila dorsocross proteins Doc1, Doc2 and Doc3. It is found N-terminal to the T-box in 11 proteins including mls-1 and mab-9 from C. elegans; H15, mid/nmr2 and bi/omd from Drosophila; in humans there are strong matches to TBX18, TBX20 and TBX22 and more marginal matches to TBX3 and TBX2. Although, to the best of my knowledge, none of these proteins has been shown to interact with groucho or its orthologs, several are known to act as transcriptional repressors: for instance, in murine heart development, Tbx20 represses Tbx2 which in turn represses Nmyc [19, 20]; the Dorsocross genes from Drosophila repress wingless and ladybird , and Doc itself is repressed by mid/nmr2 . The human proteins TBX1 and TBX10, and Drosophila org-1 which are closely related to those above, do not appear to contain EH1 motifs. The human T (brachyury) protein contains a motif broadly similar to the EH1 consensus: LQY RV DHLL SA in a comparable N-terminal location to those found in other T-box containing proteins. Although this motif scores poorly against EH1hox (-0.1 bits), the homologous regions from other T orthologs (for instance, the non-bilaterian sequences discussed below) provide a more persuasive case for the presence of a functioning EH1 motif in these proteins.
Zinc finger containing proteins
The highest scoring match of EH1hox to a C2H2 zinc finger containing protein, was ces-1 from C. elegans (bit score 11.2); this protein interacts with the groucho ortholog unc-37 [, #54] and can act as a repressor . The putative EH1 motif is at the N-terminal end of ces-1. In contrast, the Drosophila proteins bowl and odd have EH1-like motifs at their C-terminal ends (with bit scores of 10.9 & 8.4 respectively). In neither case is there direct evidence from high throughput studies of an interaction with groucho, but both can function as repressors . The human protein ZNF312 (bit score 8.6) is the ortholog of zebrafish Fezl, which contains an EH1 motif essential for repressor activity  – this motif is conserved in the human paralog ENSG00000128610 and likely Drosophila ortholog CG31670 (bit scores of 8.4 & 5.1) (Figure 2e).
Doublesex motif containing proteins
The Doublesex Motif (DM) was first found in proteins controlling sexual differentiation in Drosophila. Two DM containing proteins were confidently predicted to contain EH1-like motifs – human DMRT2 (bit score 11.6), and Drosophila dmrt11e (bit score 11.2) – these are likely orthologs; a C. elegans protein, C27C12.6 contained a weaker match (bit score 6.6) (Figure 2d). The molecular function of these proteins is unknown.
Other potential associations with transcription factor domains
Although scoring less highly than some non-transcription factor hits, another intriguing association is with the ETS domain. The three uncharatcerized C. elegans paralogs F19F10.5, F19F10.1 & C50A2.4 contain C-terminal matches to the EH1 motifs (bit scores 9.4, 2.3 & 7.4), and two other ETS proteins, C. elegans lin-1, and Drosophila Eip74EF, both have relatively high scoring matches (bit scores 6.5 & 6.6) (Figure 2c). A high scoring protein that is not annotated as a transcription factor (as it contains no interpro domains) is Drosophila Hairless (H) with a score of 8.3 bits. Experimental work has previously confirmed the presence of an EH1-like motif (SSY SI HSLL GG) within H that is responsible for its interaction with groucho . The Drosophila protein Dorsal has been reported to interact with groucho via an EH1-like motif  – this region (NGP TL SNLL SF) is markedly different to those reported here, having a low score against EH1hox (-10.7 bits) and so may better be regarded as a, so far, unique type of groucho interaction motif.
The EH1 motif is found N- and C-terminal to homeobox, forkhead, T-box and Zn finger protein domains. Clearly, as the locations of the EH1 motif are non-homologous, the N- and C-terminal associations must have occurred independently. The short size of the motif makes it tempting to speculate that the motif itself may have arisen independently (i.e. in repeated cases it may have evolved within sequence that was already part of the gene, rather than via a recombination event). The strongest evidence for this is that, in general, the majority of domain combinations occur in a fixed N to C orientation, suggesting that recombination events combining domains are relatively rare [29, 30]. The fact that we would here have many such events suggests that the alternative hypothesis of independent invention is more appropriate.
Pre-bilaterian origins of association with different transcription factors
Groucho is orthologous to the C. elegans unc-37 gene, and the four human paralogs TLE1-4 (Transducin Like Enhancer of split). An ortholog is also found in the cnidarian Hydra mangipapillata (e.g. the EST with gi 47137860, data not shown), and certain cnidarian homeobox containing genes also contain an EH1-like motif, suggesting groucho/EH1 mediated repression pre-dates the split between diplobasts and triplobasts; indeed, a sponge Bar/Bsh like homeobox containing protein (i.e. protein gi: 33641772)  also contains an EH1-like motif, as does paxb from the non-bilaterian placozoan Trichoplax adhaerens  and a Tlx-like protein from a ctenophore (gi: 38602653), suggesting the repression system was in place in the earliest animals (see  for a discussion of early metazoan evolution). I find high scoring EH1-like motifs in Forkhead domain containing proteins from sponges, cnidarians and ctenophores, in both the C-terminal (FOXA-D clade) (region II in ) and N-terminal (FOXG, sloppy paired clade) varieties (reported as 'HPFSI' in ). The presumed ortholog of 'T' from the Trichoplax adhaerens  contains an EH1-like motif (8.6 bits). These results suggest that groucho mediated repression using a variety of transcription factors was widespread in the last common ancestor of the metazoa.
Candidate EH1 motifs exist in combination with a variety of transcription factor domains, suggesting that these proteins have roles as repressors of transcriptional activity. The distribution of the EH1 motif is suggestive of a number of instances of convergent evolution, although in many cases the motif has been conserved throughout bilaterian orthologs. Together with the existence of a cnidarian Groucho ortholog, this leads to the conclusion that EH1/Groucho mediated repression was established prior to the evolution of bilateria.
Proteomes were derived from ensembl 32 (human NCBI 35, C. elegans wormbase 140, Drosophila BDGP 4) . In cases of multiple splice variants, the one with the most exons was included (or the longest in the case of ties). Transcription factor activity was taken as the presence of the gene ontology accession GO:0003700 associated with an interpro domain predicted for the protein . These data were also taken from ensembl. Although C2H2 subtype Zn fingers are not annotated by Interpro as transcription factors they are DNA binding and frequently have this role, so have been included in the transcription factor set. Bit scores reported in the text are for comparisons of the EH1hox HMM against the target sequence using the HMMER software package .
The association of transcription factor function (coded as a dichotomous variable, t, taking the values 1 [transcription factor] or 0 [non-transcription factor]) with the bit score, x, of the EH1hox HMM, was tested using a logistic regression model implemented in the glm() function of the R package ). I fitted the model
Prob(t = 1) = exp(a + bx)/(1 + exp(a + bx))
The coefficients a, b were estimated from the data by maximum-likelihood. The hypothesis of no association is equivalent to testing if b = 0.
Where inferences of orthology are made, they are based on clear-cut separation of BLAST scores or alignment-based phylogenies.
I am grateful to an anonymous referee for comments on TBX15 & brachyury. I thank the Wellcome Trust for financial support, Dr. Richard Mott for statistical advice, Drs. Martin Taylor and William Valdar for helpful suggestions.
- Smith ST, Jaynes JB: A conserved region of engrailed, shared among all en-, gsc-, Nk1-, Nk2- and msh-class homeoproteins, mediates active transcriptional repression in vivo. Development. 1996, 122 (10): 3141-3150.PubMedPubMed CentralGoogle Scholar
- Tolkunova EN, Fujioka M, Kobayashi M, Deka D, Jaynes JB: Two distinct types of repression domain in engrailed: one interacts with the groucho corepressor and is preferentially active on integrated target genes. Mol Cell Biol. 1998, 18 (5): 2804-2814.PubMedPubMed CentralView ArticleGoogle Scholar
- Shimeld SM: A transcriptional modification motif encoded by homeobox and fork head genes. FEBS Lett. 1997, 410 (2–3): 124-125. 10.1016/S0014-5793(97)00632-7.PubMedView ArticleGoogle Scholar
- Wang JC, Waltner-Law M, Yamada K, Osawa H, Stifani S, Granner DK: Transducin-like enhancer of split proteins, the human homologs of Drosophila groucho, interact with hepatic nuclear factor 3beta. J Biol Chem. 2000, 275 (24): 18418-18423. 10.1074/jbc.M910211199.PubMedView ArticleGoogle Scholar
- Bateman A, Coin L, Durbin R, Finn RD, Hollich V, Griffiths-Jones S, Khanna A, Marshall M, Moxon S, Sonnhammer EL: The Pfam protein families database. Nucleic Acids Res. 2004, D138-141. 10.1093/nar/gkh121. 32 DatabaseGoogle Scholar
- Bailey TL, Elkan C: Fitting a mixture model by expectation maximization to discover motifs in biopolymers. Proc Int Conf lntell Syst Mol Biol. 1994, 2: 28-36.Google Scholar
- SMART – Simple Modular Architecture Research Tool. [http://smart.embl.de]
- Letunic I, Copley RR, Schmidt S, Ciccarelli FD, Doerks T, Schultz J, Ponting CP, Bork P: SMART 4.0: towards genomic data integration. Nucleic Acids Res. 2004, D142-144. 10.1093/nar/gkh088. 32 DatabaseGoogle Scholar
- Galliot B, de Vargas C, Miller D: Evolution of homeobox genes: Q50 Paired-like genes founded the Paired class. Dev Genes Evol. 1999, 209 (3): 186-197. 10.1007/s004270050243.PubMedView ArticleGoogle Scholar
- Jagla K, Bellard M, Frasch M: A cluster of Drosophila homeobox genes involved in mesoderm differentiation programs. Bioessays. 2001, 23 (2): 125-133. 10.1002/1521-1878(200102)23:2<125::AID-BIES1019>3.0.CO;2-C.PubMedView ArticleGoogle Scholar
- Banerjee-Basu S, Baxevanis AD: Molecular evolution of the homeodomain family of transcription factors. Nucleic Acids Res. 2001, 29 (15): 3258-3269. 10.1093/nar/29.15.3258.PubMedPubMed CentralView ArticleGoogle Scholar
- Chang S, Johnston RJ, Hobert O: A transcriptional regulatory cascade that controls left/right asymmetry in chemosensory neurons of C. elegans. Genes Dev. 2003, 17 (17): 2123-2137. 10.1101/gad.1117903.PubMedPubMed CentralView ArticleGoogle Scholar
- Muhr J, Andersson E, Persson M, Jessell TM, Ericson J: Groucho-mediated transcriptional repression establishes progenitor cell pattern and neuronal fate in the ventral neural tube. Cell. 2001, 104 (6): 861-873. 10.1016/S0092-8674(01)00283-5.PubMedView ArticleGoogle Scholar
- Jimenez G, Verrijzer CP, Ish-Horowicz D: A conserved motif in goosecoid mediates groucho-dependent repression in Drosophila embryos. Mol Cell Biol. 1999, 19 (3): 2080-2087.PubMedPubMed CentralView ArticleGoogle Scholar
- Giot L, Bader JS, Brouwer C, Chaudhuri A, Kuang B, Li Y, Hao YL, Ooi CE, Godwin B, Vitols E: A protein interaction map of Drosophila melanogaster. Science. 2003, 302 (5651): 1727-1736. 10.1126/science.1090289.PubMedView ArticleGoogle Scholar
- Winnier AR, Meir JY, Ross JM, Tavernarakis N, Driscoll M, Ishihara T, Katsura I, Miller DM: UNC-4/UNC-37-dependent repression of motor neuron-specific genes controls synaptic choice in Caenorhabditis elegans. Genes Dev. 1999, 13 (21): 2774-2786. 10.1101/gad.13.21.2774.PubMedPubMed CentralView ArticleGoogle Scholar
- Mazet F, Yu JK, Liberles DA, Holland LZ, Shimeld SM: Phylogenetic relationships of the Fox (Forkhead) gene family in the Bilateria. Gene. 2003, 316: 79-89. 10.1016/S0378-1119(03)00741-8.PubMedView ArticleGoogle Scholar
- Andrioli LP, Oberstein AL, Corado MS, Yu D, Small S: Groucho-dependent repression by sloppy-paired 1 differentially positions anterior pair-rule stripes in the Drosophila embryo. Dev Biol. 2004, 276 (2): 541-551. 10.1016/j.ydbio.2004.09.025.PubMedView ArticleGoogle Scholar
- Stennard FA, Costa MW, Lai D, Biben C, Furtado MB, Solloway MJ, McCulley DJ, Leimena C, Preis JI, Dunwoodie SL: Murine T-box transcription factor Tbx20 acts as a repressor during heart development, and is essential for adult heart integrity, function and adaptation. Development. 2005, 132 (10): 2451-2462. 10.1242/dev.01799.PubMedView ArticleGoogle Scholar
- Cai CL, Zhou W, Yang L, Bu L, Qyang Y, Zhang X, Li X, Rosenfeld MG, Chen J, Evans S: T-box genes coordinate regional rates of proliferation and regional specification during cardiogenesis. Development. 2005, 132 (10): 2475-2487. 10.1242/dev.01832.PubMedView ArticleGoogle Scholar
- Reim I, Lee HH, Frasch M: The T-box-encoding Dorsocross genes function in amnioserosa development and the patterning of the dorsolateral germ band downstream of Dpp. Development. 2003, 130 (14): 3187-3204. 10.1242/dev.00548.PubMedView ArticleGoogle Scholar
- Reim I, Mohler JP, Frasch M: Tbx20-related genes, mid and H15, are required for tinman expression, proper patterning, and normal differentiation of cardioblasts in Drosophila. Mech Dev. 2005Google Scholar
- Li S, Armstrong CM, Bertin N, Ge H, Milstein S, Boxem M, Vidalain PO, Han JD, Chesneau A, Hao T: A map of the interactome network of the metazoan C. elegans. Science. 2004, 303 (5657): 540-543. 10.1126/science.1091403.PubMedPubMed CentralView ArticleGoogle Scholar
- Thellmann M, Hatzold J, Conradt B: The Snail-like CES-1 protein of C. elegans can block the expression of the BH3-only cell-death activator gene egl-1 by antagonizing the function of bHLH proteins. Development. 2003, 130 (17): 4057-4071. 10.1242/dev.00597.PubMedView ArticleGoogle Scholar
- Campbell G: Regulation of gene expression in the distal region of the Drosophila leg by the Hox11 homolog, C15. Dev Biol. 2005, 278 (2): 607-618. 10.1016/j.ydbio.2004.12.009.PubMedView ArticleGoogle Scholar
- Levkowitz G, Zeller J, Sirotkin HI, French D, Schilbach S, Hashimoto H, Hibi M, Talbot WS, Rosenthal A: Zinc finger protein too few controls the development of monoaminergic neurons. Nat Neurosci. 2003, 6 (1): 28-33. 10.1038/nn979.PubMedView ArticleGoogle Scholar
- Barolo S, Stone T, Bang AG, Posakony JW: Default repression and Notch signaling: Hairless acts as an adaptor to recruit the corepressors Groucho and dCtBP to Suppressor of Hairless. Genes Dev. 2002, 16 (15): 1964-1976. 10.1101/gad.987402.PubMedPubMed CentralView ArticleGoogle Scholar
- Flores-Saaib RD, Jia S, Courey AJ: Activation and repression by the C-terminal domain of Dorsal. Development. 2001, 128 (10): 1869-1879.PubMedGoogle Scholar
- Apic G, Gough J, Teichmann SA: Domain combinations in archaeal, eubacterial and eukaryotic proteomes. J Mol Biol. 2001, 310 (2): 311-325. 10.1006/jmbi.2001.4776.PubMedView ArticleGoogle Scholar
- Gough J: Convergent evolution of domain architectures (is rare). Bioinformatics. 2005, 21 (8): 1464-1471. 10.1093/bioinformatics/bti204.PubMedView ArticleGoogle Scholar
- Hill A, Tetrault J, Hill M: Isolation and expression analysis of a poriferan Antp-class Bar-/Bsh-like homeobox gene. Dev Genes Evol. 2004, 214 (10): 515-523.PubMedGoogle Scholar
- Hadrys T, Desalle R, Sagasser S, Fischer N, Schierwater B: The Trichoplax PaxB Gene: A Putative Proto-PaxA/B/C Gene Predating the Origin of Nerve and Sensory Cells. Mol Biol Evol. 2005, 22 (7): 1569-1578. 10.1093/molbev/msi150.PubMedView ArticleGoogle Scholar
- Medina M, Collins AG, Silberman JD, Sogin ML: Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA. Proc Natl Acad Sci USA. 2001, 98 (17): 9707-9712. 10.1073/pnas.171316998.PubMedPubMed CentralView ArticleGoogle Scholar
- Adell T, Muller WE: Isolation and characterization of five Fox (Forkhead) genes from the sponge Suberites domuncula. Gene. 2004, 334: 35-46. 10.1016/j.gene.2004.02.036.PubMedView ArticleGoogle Scholar
- Yamada A, Martindale MQ: Expression of the ctenophore Brain Factor 1 forkhead gene ortholog (ctenoBF-1) mRNA is restricted to the presumptive mouth and feeding apparatus: implications for axial organization in the Metazoa. Dev Genes Evol. 2002, 212 (7): 338-348. 10.1007/s00427-002-0248-x.PubMedView ArticleGoogle Scholar
- Martinelli C, Spring J: Distinct expression patterns of the two T-box homologues Brachyury and Tbx2/3 in the placozoan Trichoplax adhaerens. Dev Genes Evol. 2003, 213 (10): 492-499. 10.1007/s00427-003-0353-5.PubMedView ArticleGoogle Scholar
- Hubbard T, Andrews D, Caccamo M, Cameron G, Chen Y, Clamp M, Clarke L, Coates G, Cox T, Cunningham F: Ensembl 2005. Nucleic Acids Res. 2005, D447-453. 33 DatabaseGoogle Scholar
- Mulder NJ, Apweiler R, Attwood TK, Bairoch A, Bateman A, Binns D, Bradley P, Bork P, Bucher P, Cerutti L: InterPro, progress and status in 2005. Nucleic Acids Res. 2005, D201-205. 33 DatabaseGoogle Scholar
- HMMER: sequence analysis using profile hidden Markov models. [http://hmmer.wustl.edu]
- The R project for statistcal computing. [http://www.r-project.org]
- Goodstadt L, Ponting CP: CHROMA: consensus-based colouring of multiple alignments for publication. Bioinformatics. 2001, 17 (9): 845-846. 10.1093/bioinformatics/17.9.845.PubMedView ArticleGoogle Scholar
- Bairoch A, Apweiler R, Wu CH, Barker WC, Boeckmann B, Ferro S, Gasteiger E, Huang H, Lopez R, Magrane M: The Universal Protein Resource (UniProt). Nucleic Acids Res. 2005, D154-159. 33 DatabaseGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.