Systematic Prediction of Orthologous Units of Genesin the Complete Genomes
Hidemasa Bono

bono@kuicr.kyoto-u.ac.jp

Susumu Goto

goto@kuicr.kyoto-u.ac.jp

Hiroyuki Ogata

ogata@kuicr.kyoto-u.ac.jp

Wataru Fujibuchi

wataru@kuicr.kyoto-u.ac.jp

Minoru Kanehisa

kanehisa@kuicr.kyoto-u.ac.jp

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

Abstract
In order to fully make use of the vast amount of information in the complete genome sequences,
we are developing a genome-scale system for predicting gene functions and cellular functions. The
system makes use of the information of sequence similarity, the information of positional correlationsin the genome, and the reference knowledge stored as the ortholog group tables in KEGG (Kyoto
Encyclopedia of Genes and Genomes). The ortholog group table summarizes orthologous andparalogous relations among di erent organisms for a set of genes that are considered to form afunctional unit, such as a conserved portion of the metabolic pathway or a molecular machinery
for the membrane transport. At the moment, the ortholog group table is constructed for the cases
where the genes are clustered in physically close positions in the genome for at least one organism.
In this paper, we describe the system and the actual analysis of the complete genome of Pyrococcus
horikoshii to identify ABC transporters.
1

Introduction

While an increasing number of complete genome sequences has become publically available, the biological function of roughly a half of the genes in each genome remains unknown Thus, ecient methods
still need be developed to annotate functional properties for the entire set of predicted open reading
frames (ORFs). The popular method that is widely used for functional annotation relies on searchingfor sequence similarity or motifs in the database. When a new genome is sequenced, the amino acidsequences of translated ORFs are usually searched independently against the non-redundant protein
sequence database after the ORFs are determined by some gene nding methods. There are several
problems in this approach. First, a proper threshold value cannot be predetermined to extend sequence similarity to functional similarity. Second, because some genes, such as for ABC transporters,
have many homologous genes in the genome, it is dicult to assign orthologous relations that canbe used to specify functions. Third, the so-called non-redundant database actually contains many
duplicate entries and the similarity search against it often produces a long list of similar sequencesthat is not easy to process.
Thus, it is, rst of all, desirable that additional information is incorporated to make it easier tointerpret the result of similarity searches. Especially, the positional correlation in the genome, e.g.,the operon structure, has turned out to be extremely useful information in the functional annotationof bacterial and archaeol genomes. Furthermore, it is necessary to develop a clean data set that canbe used as reference for the functional annotation process.
We started the KEGG (Kyoto Encyclopedia of Genes and Genomes) project in 1995. It aims to
make links from the gene catalogs generated by the genome sequencing projects to the biochemical
pathways that may be considered wiring-diagrams of genes and molecules [5]. Under the project we32

5'P-D-1-ribulosylformimine

Pentose phosphate cycle

HISTIDINE METABOLISM

3.5.1.2.6.1.-

PhosphoribulosylImidazolePhosphoribosyl-AMP
formimino-AICAR-P
acetol-P
2.6.1.9
L-Histidinol
5.3.1.16
3.5.4.19
2.4.2.4.2.1.19
3.1.3.15
ImidazolePhosphoribosylL-Histidinol-P
glicerol-3P
formimino-AICAR-P
1.1.1.23
5'P-Ribosyl-5-amino-4imidazole carboxamide
1-Methyl(AICAR)
3.4.13.5 L-histidine
L-Histidinal
Anserine
6.3.2.11
2.1.1.2.1.1.22
Purine metabolism
Carnosine
6.3.2.11
1.1.1.23
3.4.13.3

3.6.1.31
2.4.2.17
PRPP
Phosphoribosyl-ATP

Imidazole
Imidazolone
ImidazoleN-Formyl-Lacetaldehyde
acetate4-acetate
aspartate
3.5.2.3.5.3.5
1.14.13.5
1.2.1.3
N-Formimino-L3.5.1.15 3.5.1.8 aspartate
6.3.4.8
Aspartate

Alanine and Aspartate
metabolism

1-(5-Phosphoribosyl)
imidazole-4-acetate(1-Ribosylimidazole)4-acetate

Histamine
1.4.3.6

Hercynine

4.1.1.22

L-Histidine

4.1.1.28
2.1.1.8

1.4.3.4

Thiourocanic acid
Ergothioneine

4.3.1.3
4-Oxoglutaramate

2.3.1.N-Methylhistamine

L-HistidyltRNA

6.1.1.21

3.4.13.20

2.6.1.38
4-( -Acetylaminoethyl)-imidazole

3-Methylimidazole
acetaldehyde

2-Oxoglutarate

Urocanate
4.2.1.49

4-Imidazolone5-propanoate

Isoglutamate
Formylisoglutamate
N-FormiminoL-glutamate

3.5.2.7

1.2.1.5
3-Methylimidazoleacetic acid

3.5.3.13

1.14.13.N-FormylL-glutamate

Imidazolepyruvate
Hydantoin5-propionate
Imidazolelactate

2.1.2.5
3.5.3.8

3.5.1.68

N-carbamylL-glutamate

L-Glutamate

Glutamatemetabolism

00340 6/19/98

Figure 1: The KEGG standard pathway map for histidine metabolism.
have been developing a system for identifying gene functions, called GFIT (Gene Function Identication Tools), utilizing the orthologous relations among genes of the query organism and those ofreference organisms [2]. By analyzing the actual genomes with GFIT, curated orthologous relations
are stored in, what we call, the ortholog group tables in KEGG for units of genes that are functionallycorrelated.
We present here a new component of GFIT, a web-based tool that searches sequence similarity
against the ortholog group tables and that considers the positional information of the query genome.
It accepts the whole genome, i.e., the complete set of ORF sequences, and the result is presented
by integrating with the KEGG resources. Thus, the tool can be used as a workbench for functionalannotation of the whole genome.
2

Data and Methods

2.1 KEGG
Details of the KEGG systems are described elsewhere [8]. We mention here brief overview of KEGG
system. KEGG is an attempt to computerize the knowledge of the information pathways of interacting
biomolecules. Typical pathways include metabolic pathways, and an example of how KEGG represents
the metabolic pathway information is shown in Fig. 1 for histidine metabolism. KEGG also attempts tocomputerize various regulatory pathways, for example signal transduction circuits, from the biologicalknowledge currently available.
The KEGG pathway maps are implemented as clickable GIF image maps to be used in the WWW.
Thus the information about enzymes and compounds in the LIGAND chemical database [4], or genes
in the KEGG gene catalogs can be retrieved through the in-house developed database management
system, DBGET/LinkDB [3].

33

2.2 Identi cation of orthologs and paralogs
The so-called homologs of genes that share sequence similarity could be due to the following two mechanisms. Orthologs are the genes that are derived by a common ancestry; hence they are responsible forthe identical function in di erent organisms. In contrast, paralogs are generated by gene duplications
and in general have similar but not necessarily the same function.
With the availability of complete genomic sequences, practical procedures to distinguish orthologs
and paralogs were proposed [7, 10]. Given two complete lists of genes, the amino acid sequencesimilarity is examined for each gene in one organism against all genes in the other organism. Only thegene pairs that show the similarity of statistical signi cance are to be considered. If the two genes,
gene A in organism 1 and gene B in organism 2 are more closely related to each other than to any
other genes it can be paired with, we de ne that gene A and gene B are orthologous. Of course, thisis an operational de nition of orthologs, and there may be complications resulting from the existenceof high scoring paralogs within each organism, from the existence of multidomain proteins, and also
from the inconsistencies of pairwise comparisons when multiple organisms are considered.
In KEGG the functional annotation of each gene in each organism is maintained in the GENES
database [8]. For a newly sequenced organism, the EC number assignment for enzyme genes is mademanually according to the orthologous relations identi ed by comparing against all organisms in
the GENES database. The gene function annotations are continuously re-evaluated in KEGG bycomparing with the KEGG/PATHWAY database, SWISS-PROT, and other databases. Consequently,
the EC number assignment is also continuously updated.

2.3 Ortholog and paralog group tables
The sequence similarity search against the existing sequence databases, even against non-redundant
database, often generates a long list of hits, which requires human e orts to nd orthologous relations
that can be used for gene function assignments. The ortholog group tables in KEGG are curated
reference data set of orthologous relations that is intended to make this process easier. These tables
also contain the information of the group of genes that is supposed to form a functional unit, such as aregulatory unit in the metabolic pathway or a molecular unit of assembly. The data representation of
the ortholog group table is a simple HTML table, in which additional information, such as hyperlinks,
can easily be added.
We are working to maintain and expand the ortholog group tables. As of August 1998, there are
53 tables, which are manually edited from biological viewpoint. The ortholog group tables listed inTable 1 are largely categorized into two groups. One is for the tables from metabolic pathways, andthe other is for those from regulatory pathways. Fig. 2 shows the ortholog group table of histidine
metabolism, which contains orthologous genes extracted from the pathway map in Fig. 1 for di erent
organisms. In Fig. 2, the shaded cells in the same row represent genes that are closely located in thechromosome. It is supposed that they form an operon. In this gure, we can easily see that E.coli,
H.in uenzae, and B.subtilis have operon structures in the histidine metabolism pathway, but the otherspecies may not.
The tables for the metabolic pathways contain well conserved sections of the pathway, which
may be called pathway motifs, that are generated by the SIMIC (Simultaneous Linkage Clustering)
program (Ogata, H et al., manuscript in preparation) for identifying correlated clusters of genes in
the genome and the gene products in the pathway. The region is named functionally related enzyme
clusters (FRECs), and it contains an operon-like structure of genes that codes for a unit of related
enzymes in the pathway.
In contrast, the tables for the regulatory pathways are mostly collected by human e orts. The
best organized ones at the moment are for the ABC transporters [9] and the two-component signaltransducers [1] that often form large paralogous gene clusters. Other genes concerning cell processes
and cell organization are also catalogized in the tables.
34

Table 1: List of ortholog group tables.
Metabolism
Carbohydrate Metabolism
Glycolysis / Gluconeogenesis
Citrate cycle (TCA cycle)
Pentose Phosphate Cycle
Pentose and Glucuronate Interconversions
Fructose and Mannose Metabolism
Galactose Metabolism
Ascorbate and Aldarate Metabolism
Pyruvate Metabolism
Glyoxylate and Dicarboxylate Metabolism
Propanoate Metabolism
Butanoate Metabolism
Energy Metabolism
Methane Metabolism
Nitrogen Metabolism
Sulfur Metabolism
Lipid Metabolism
Fatty Acid Biosynthesis (Path 1)
Nucleotide Metabolism
Purine Metabolism
Pyrimidine Metabolism
Nucleotide Sugars Metabolism
Aminosugars Metabolism
Amino Acid Metabolism
Glutamate Metabolism
Alanine and Aspartate Metabolism
Glycine, Serine and Threonine Metabolism
Methionine Metabolism
Cysteine Metabolism
Valine, Leucine and Isoleucine Degradation
Valine, Leucine and Isoleucine Biosynthesis
Lysine Biosynthesis
Arginine and Proline Metabolism
Histidine Metabolism
Phenylalanine Metabolism
Phenylalanine, Tyrosine and Tryptophan Biosynthesis
Urea Cycle and Metabolism of Amino Groups
Metabolism of Other Amino Acidsbeta-Alanine Metabolism
Metabolism of Complex Carbohydrates
Starch and Sucrose Metabolism
Peptideglycan Biosynthesis
Metabolism of Complex Lipids
Glycerolipid Metabolism
Metabolism of Cofactors, Vitamins, and Other Substances
Thiamine Metabolism
Nicotinate and Nicotinamide Metabolism
Biotin Metabolism
Folate Biosynthesis
One Carbon Pool by Folate
Porphyrin and Chlorophyll Metabolism
Ubiquinone Biosynthesis
Metabolism of Macromolecules
Aminoacyl-tRNA Synthetase
Cell Process
Membrane Transport
ABC Transporters
PTS System
Signal Transduction
Two-Component System
Ligand-Receptor Interaction
G-protein coupled receptors
Cell Organization
Molecular Assembly
Ribosome assembly
F1F0-ATPase
Molecular Components
Translation Factors

35

Figure 2: The ortholog group table for histidine metabolism.

36

3

Genome-Scale Prediction of Biological Functions

3.1 New computational tool in GFIT
The genome-scale prediction of biological functions require a new generation of tools that examine
a complete set of genes in the genome and to return functional prediction results after considering
all dependencies. The initial version of the GFIT program provides one solution, where the program
receives the entire set of ORFs in the genome as a query, compares against each of the completely
sequenced organisms, and returns brief but informative results of similarities. As of August 1998, thecomplete genome sequences of 13 micro-organisms are available1 . GFIT tentatively assigns orthologs ofeach ORF by the operation described in the Data and Methods section. Unfortunately, the automatic
operation based on the bidirectional best hits is too strict and often misses real orthologous relations.
This becomes obvious when assigning EC numbers by GFIT. The correctness of EC number assignment
can be checked by whether the complete routes of metabolic pathways are properly reconstructed, i.e.,
whether any missing enzymes are present to make the pathway continuous [2].
With the availability of the clean data set of ortholog group tables, it is now possible to querythe entire genome sequence for, at least, a selected aspects of biological functions. In the traditional
similarity search of individual genes (or proteins) against repositories of non-redundant databases,
it has always been problematic to determine an appropriate level of sequence similarity that can beextended to functional similarity. The program to search ortholog and paralog tables bene ts from anadditional feature that is used for interpretation of sequence similarity; namely, the requirement forreconstructing a complete functional unit from a set of genes or proteins. Utilizing this feature thefunctional inference can be better performed.
The program actually searches sequences in the ortholog group tables and reports the genes abovea speci ed threshold. They can then be superimposed on the reference ortholog table with additional
coloring showing the location and the degree of similarity.

3.2 Identi cation of ABC transporters
In this section, we show the result of using the new GFIT program. We performed the analysis of
ABC transporters in newly sequenced bacterium, Pyrococcus horikoshii [6]. All ORFs of Pyrococcus
horikoshii were searched against the ortholog (and paralog) group tables of ABC transporters [9].
Fig. 3 shows the top part of the whole result, in which the columns correspond to the threecomponents of ABC transporters (binding protein, membrane protein, and ATP-binding protein) andthe annotation in the original database (last column). The rows correspond to the ORFs of Pyrococcus
horikoshii that have homology to at least one of these components. The numeral in each cell is thehighest FASTA opt score between the ORF sequence and the database sequence, also showing towhich components the similarity was found. The background color shows the percentage range of thedatabase hits among paralogs. This representation of the result also contains the information aboutclustering of genes in the chromosome. The rows separated by thin lines are the genes that are locatednext to each other in the genome. The rows separated by thick lines are the genes that are apart.
Therefore, a cluster of genes not separated by thick lines contain hits to all necessary components,
then it is considered to be the functional unit of, in this case, the ABC transporter. One of the results
we obtained is the cluster of ORFs from pho:PHBC018 to pho:PHBC015 (check boxes in Fig. 3).
A detailed picture of matches can be examined for these genes and Table 2 shows the summaryof best hits (only the top three hits are indicated here) according to the FASTA opt scores. Because
the database hits exist in the reference ortholog group table, they can be displayed by superimposing
on the reference table. A portion of the superimposed table is shown in Fig. 4. Except for b1123 alldatabase hits are in the subgroup of `Maltose / sn-Glycerol-3-phosphate' although the table of ABC
transporter contains more than 250 gene clusters.
1

http://www.genome.ad.jp/kegg/java/org list.html

37

Figure 3: Genome scale identi cation of ABC transporters.

Table 2: A detailed content of the database hits.
ORF
pho:PHBC018
pho:PHBC017
pho:PHBC016
pho:PHBC015

database hits (opt scores) in ABC transporter ortholog group tableyurJ(1134) slr0747(1081) b3450(1049) ...
yurM(366)
b1312(359)slr0531(346) ...
slr1202(377) b1311(343)yurN(330)...
b1123(164) yurO(162)b1310(140) ...

38

Figure 4: The portion of the ortholog group table used for functional prediction.
In the annotation by the original authors of Pyrococcus horikoshii, pho:PHBC018 was tentatively
assigned to be `sugar-binding transport ATP-binding protein', but others were all left as hypothetical. We predict pho:PHBC016 and pho:PHBC017 are the membrane proteins and pho:PHBC015 is thesubstrate binding protein. We also predict that the transporter is not for simple sugar (such as ribose
and galactose) but for multiple sugar (maltose) or sn-glycerol-3-phosphate.
4

Summary and Perspective

KEGG organizes the knowledge of metabolic and regulatory pathways eciently and usefully. The tool
presented here is a rst attempt to incorporate the information of well curated ortholog (and paralog)group tables and the information of chromosomal neighbors, as well as the information of sequencesimilarity, for functional prediction of ORFs. The KEGG ortholog group table representation is more
informative than the KEGG pathway representation because it contains the positional information in
the genome, it represents a multiple alignment of organisms, and it is far better curated in contrast
to the automatically reconstructed pathway maps that contain many missing enzymes. However, themajor drawback of the ortholog group table is that it covers only a small fraction of the pathway
information that is present in KEGG. By comparative genomics, especially for identifying conserved
gene gene clusters, we hope to identify more functional units that can be represented by the ortholog
group tables.
Acknowledgments

This work was supported in part by the Grant-in-Aid for Scienti c Research on the Priority Areas `Genome Science' from the Ministry of Education, Science, Sports and Culture in Japan. The
computation time was provided by the Supercomputer Laboratory, Institute for Chemical Research,
Kyoto University. Hidemasa Bono was supported by Research Fellowships of the Japan Society for
the Promotion of Science for Young Scientists.

39

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