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Journal of Bacteriology, May 2003, p. 2980-2987, Vol. 185, No. 10
0021-9193/03/$08.00+0 DOI: 10.1128/JB.185.10.2980-2987.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Methylotrophy in Methylobacterium extorquens AM1 from a Genomic Point of View
Ludmila Chistoserdova,1 Sung-Wei Chen,1 Alla Lapidus,2 and Mary E. Lidstrom1,3*
Departments of Chemical Engineering,1
Microbiology, University of Washington, Seattle, Washington 98195,3
Integrated Genomics, Inc., Chicago, Illinois 606122

INTRODUCTION
Methylotrophy is defined as the ability to "grow at the expense
of reduced carbon compounds containing one or more carbon atoms
but containing no carbon-carbon bonds" (
3). It is an intriguing
example of microbial metabolic agility, with the use of a class
of chemicals disregarded by the majority of organisms. Even
though the ability to grow methylotrophically was first discovered
in the early 1900s (cited in reference
3), it was not until
the 1960s to 1970s that an understanding of the biochemical
nature of this capability started to emerge. Fascination with
methylotrophy in those years was fueled by the commercial interest
in single-cell protein production, and as a result, the specific
details of the biochemistry of methylotrophy began to be revealed.
Enzymes for the primary oxidation of C
1 substrates such as methanol
dehydrogenase and methylamine dehydrogenase were characterized,
and distinct modes of C
1 assimilation, such as the ribulose
monophosphate cycle and the serine cycle were discovered. The
biochemical processes involved in methylotrophy that were known
by 1982 are described in detail in the now classic book
Biochemistry of Methylotrophs by Christopher Anthony (
3). In the 20 years
following the publication of
Biochemistry of Methylotrophs,
a few additional methylotrophy biochemical pathways have been
discovered, such as the pathway for C
1 transfer linked to methanopterin
and methanofuran, which solved the long-standing mystery of
formaldehyde oxidation in many methylotrophs (
15,
53), and novel
pathways for primary C
1 oxidation, such as the pathways for
degradation of chlorinated methanes and methanesulfonic acid
(
21,
50).
The knowledge concerning the biochemistry and physiology of methylotrophic organisms accumulated over the past three decades suggests a new framework for understanding methylotrophy as a novel metabolic mode. In this framework, methylotrophy is envisioned as a set of specific metabolic functional modules, with different combinations of such modules being present in different methylotrophs (Fig. 1 for the methylotrophic metabolic modules in Methylobacterium extorquens AM1). However, until recently, a number of important details of these modules were missing, and so the picture remained incomplete. The availability of two unfinished genome sequences for the important model organisms M. extorquens AM1 (http://www.integratedgenomics.com/genomereleases.html#list6)and Methylococcus capsulatus Bath (http://tigrblast.tigr.org/ufmg/) is transforming our understanding of methylotrophy. Annotation of these two genomes combined with functional analysis will delineate the set of genes and functions that is both sufficient and necessary to define a methylotroph. Expanding genomic analyses to include other groups of methylotrophs will in turn provide clues to the origins of methylotrophy and the evolution of various methylotrophic pathways. In this publication, we summarize the existing knowledge of the genes involved in methylotrophic pathways in M. extorquens AM1, analyze its yet unfinished genome with respect to location and clustering of methylotrophy genes, and present a comprehensive list of methylotrophy genes and enzymes known at this time in M. extorquens AM1 (Table 1).

FROM GENETICS TO GENOMICS
M. extorquens AM1 is the most-well-studied methylotroph to date.
In the 1960s and 1970s, this organism served as a model to characterize
the reactions of the serine cycle for C
1 assimilation, and methanol-
and methylamine dehydrogenases from
M. extorquens AM1 were among
the first primary C
1 oxidation enzymes characterized (
3). Genetic
studies of
M. extorquens AM1 began with the isolation and characterization
of chemically induced C
1-negative mutants (
3). Later, mutants
with defects in methanol oxidation were isolated via a specific
allyl alcohol selection (
32,
33). The availability of these
C
1-negative mutants allowed identification and isolation of
the corresponding DNA regions encoding key methylotrophy genes,
and it became evident that some of these genes are clustered
together (
2,
4,
5,
6,
8,
9,
11). As sequencing became a routine
technique, these clusters were analyzed and expanded via chromosomal
walking (
5,
12,
15,
27). Concurrently, directed mutagenesis
techniques were developed and applied to the analysis of the
genes present in these methylotrophy islands (
5,
8-
15). By the
end of 1990s, classical genetic approaches in combination with
chromosomal walking and directed mutagenesis had resulted in
characterization of about 70 genes involved in methylotrophy
in
M. extorquens AM1, and these were localized to eight regions
on the chromosome (
27). One methylotrophy island spans about
60 kb and contains a number of tightly linked genes enabling
different methylotrophy metabolic modules: most of the reactions
of the serine cycle, most of the formaldehyde oxidation reactions,
and some functions involved in methanol oxidation (
27). Most
of the methanol oxidation genes were found in a different location
on the chromosome, where they formed a large operon (
1). All
of the genes enabling methylamine oxidation were found in one
location and tightly linked (
5). However, some important methylotrophy
genes were not parts of methylotrophy islands (i.e.,
gck and
glyA) (
11,
14), suggesting that further expansion of the existing
methylotrophy clusters via chromosomal walking promised limited
opportunity for finding new methylotrophy genes. Meanwhile,
a number of essential methylotrophy genes were still missing
from the picture. These included (i) two of the serine cycle
genes assumed to also participate in multicarbon metabolism,
encoding malate dehydrogenase and enolase; (ii) formate dehydrogenase
genes; and (iii) genes for the novel glyoxylate regeneration
pathway involving propionyl coenzme A (CoA) as an intermediate
(
12). In addition, no regulators were known for assimilatory
C
1 metabolism. In order to locate and study the missing methylotrophy
genes in
M. extorquens AM1, a whole-genome sequencing project
was initiated in 1998, in collaboration with the Human Genome
Sequencing Center at University of Washington and later Integrated
Genomics, Inc. (Chicago, Ill.). At the time of this writing,
a gapped sequence of the
M. extorquens AM1 genome (6.5
x coverage)
is available (
http://www.integratedgenomics.com/genomereleases.html#list6).
Data mining began at the very early stages of the sequencing
project, via BLAST analysis and key word searches against the
partial genome database. At these early stages, many genes of
interest were isolated and sequenced from a cosmid library of
M. extorquens AM1. At the later stages when analysis of larger
fragments became possible, genes of interest were PCR amplified
from the chromosome and subjected to mutation analysis. A random
(transposon-induced) mutagenesis approach was also employed,
and sites of insertions resulting in a C
1-negative phenotype
were identified (
28). The whole-genome-based gene-mining approach
outlined above has resulted in identification of about 30 new
genes involved in methylotrophy. The major outcomes of the whole-genome-analysis
approach have been twofold: (i) filling in existing metabolic
gaps in our knowledge of methylotrophy in serine cycle facultative
methylotrophs and (ii) providing, for the first time, comprehensive
knowledge on the suite of necessary genes as well as the suite
of sufficient genes for enabling methylotrophy in a given organism.
Our data at this time suggest that we have accounted for the
majority of methylotrophy genes in
M. extorquens AM1. Only a
few genes still remain unidentified: namely genes involved in
the yet unsolved reactions of the glyoxylate regeneration cycle
and a few regulatory genes participating in C
1 oxidation, C
1 assimilation, or both. Below, the current information about
each of the methylotrophic metabolic modules is presented in
more detail.

PRIMARY C1 OXIDATION
M. extorquens AM1 possesses two primary oxidation metabolic
modules for methylotrophy, which function in the oxidation of
methanol and methylamine, respectively. All 11 of the known
methylamine utilization genes (
mauFBEDACJGIMN)i.e., the
genes for the catalytic subunits for methylamine dehydrogenase,
the specific electron acceptor amicyanin, and the specific cofactor
(TTQ) biosynthesis enzymesare located in a single 8.4-kb
gene cluster with all genes transcribed in the same direction
(
5), except the regulatory gene or genes, which remain unknown.
The genes enabling methanol oxidation in M. extorquens AM1 are found in three different locations on the chromosome. One 12.5-kb cluster (cluster 1) contains 14 genes (mxaFJGIRSACKLDEHB), all transcribed in the same direction. These genes encode the structural polypeptides of methanol dehydrogenase, the specific cytochrome c that accepts electrons from methanol dehydrogenase, the proteins essential for calcium insertion into the apoprotein, one regulatory protein, and a few proteins whose functions are still unknown (1, 2, 27, 29-31, 41, 44). One gene (mxaW) is located immediately upstream of this cluster, transcribed in the opposite direction by a methanol-inducible promoter, but its function is unknown (41). One pair of genes involved in transcriptional regulation of MeDH (mxbMD) are a part of the 60-kb methylotrophy island (cluster 2) (42), and another pair (mxcQE) are located elsewhere on the chromosome. The six genes for PQQ biosynthesis are located in two different clusters: one of them (pqqABC/DE) is located in the large methylotrophy island immediately downstream of mxbMD (45), while another cluster (pqqFG) is not linked to other methylotrophy genes (cluster 2) (43). In this work, we view PQQ biosynthesis as a separate metabolic module, because it is a cofactor of other dehydrogenases in M. extorquens AM1, based on genomic predictions.

C1 TRANSFER BETWEEN FORMALDEHYDE AND FORMATE
Two distinct metabolic modules operate in
M. extorquens AM1
for transferring C
1 units between the oxidation levels of formaldehyde
and formate, both linked to folate cofactors. One module uses
tetrahydrofolate (H
4F) as a cofactor. The enzymes converting
methylene-H
4F to formyl-H
4F, methylene-H
4F dehydrogenase, and
methenyl-H
4F cyclohydrolase are encoded by genes (
mtdA and
fch)
(
9,
36,
51) unique to methylotrophs, while in most known bacteria,
both reactions are performed by a bifunctional enzyme encoded
by
folD. While
mtdA and
fch are cotranscribed and are a part
of the serine cycle gene cluster (
9,
20,
36), the gene encoding
formyl-H
4F ligase (
ftfL) is located elsewhere on the
M. extorquens AM1 chromosome, and the encoded polypeptide shows high similarity
to known formyl-H
4F ligases (
28).
mtdA and
fch are regulated
coordinately with the serine cycle enzymes, suggesting a potential
link between the H
4F-linked C
1 transfer module in
M. extorquens AM1 and C
1 assimilation (
20).
In this study, we place the genes for H4F biosynthesis into a separate metabolic module, because H4F is involved not only methylotrophy functions, but also in general metabolism during growth on multicarbon compounds. fol genes (involved in folate synthesis) are therefore essential, and no null mutants can be isolated in these genes (13). Three fol genes (folKBP) are linked together, transcribed in the same direction, and are a part of a methylotrophy island (cluster 2); one gene (dyr) is loosely linked to the gene for fumarase (fumA; cluster 13); and two other genes (folC and folE) are not linked to other fol or methylotrophy genes.
The H4MPT-linked metabolic module that involves "archaeal-like" genes and enzymes appears to be the pathway responsible for the majority of formaldehyde oxidation (15). Seventeen of the genes involved in this module (fhcCDAB-orf4-mtdB-orfY-mch-orf5-orf7-fae-orf17-orf9-[3 non-"archaeal" genes]-orf19-orf20-orf21-orf22) are clustered together in a 20-kb region on the chromosome, located at one end of the largest methylotrophy island (cluster 2). Some of these genes are in the opposite orientation with respect to others, and therefore the region must be composed of a number of transcriptional units. One gene involved in this module, dmrA, encoding putative dihydromethanopterin reductase, was found in a different location on the chromosome, and was not linked to any other known methylotrophy genes (28). One other gene has been identified as a putative methanopterin biosynthesis gene, orf4, encoding the first enzyme in the methanopterin biosynthesis pathway, ß-ribofuranosylaminobenzene 5'-phosphate synthase (40). dmrA and orf4 remain, at this point, the only genes proposed to participate in archaeal cofactor (H4MPT) biosynthesis in M. extorquens AM1. It is likely, however, that many of the archaeal-like genes of yet unknown function in the "archaeal-like" gene cluster are involved in biosynthesis of H4MPT or methanofuran or are involved in regulation of these biosynthetic pathways. As such, we have assigned these genes to the H4MPT-linked C1 transfer metabolic module.

FORMATE OXIDATION
Until recently, the formate oxidation step was believed to be
essential for energy generation during methylotrophic growth
(
3). However, no randomly generated mutants were available with
lesions in formate oxidation. The whole-genome approach has
revealed the presence of three gene clusters unlinked to each
other, encoding three nonhomologous formate dehydrogenases in
M. extorquens AM1 designated
fdh1AB, fdh2ABCD, and
fdh3ABC (
25;
L. Chistoserdova, M. Laukel, J. A. Vorholt, and M. E. Lidstrom,
unpublished observations). In each case, the genes are transcribed
in the same orientation with respect to each other. Mutation
analysis has shown that the formate oxidation step is not essential
for energy generation during growth on methanol or methylamine.
It is essential, however, for growth on formate, but any of
the three formate oxidation modules can fulfill this energy-generating
function (Chistoserdova et al., unpublished).

SERINE CYCLE
The serine cycle is the pathway for formaldehyde assimilation
during methylotrophic growth of
M. extorquens AM1. Even though
the role of the pathway, the net production of one C
3 molecule
(phosphoglycerate) from two molecules of formaldehyde and one
molecule of CO
2, is uniquely methylotrophic, little is unique
about the 11 genes involved in this module. The first enzyme
in the pathway, serine hydroxymethyltransferase (GlyA) is a
traditional enzyme found in most known organisms, where it functions
in supplying C
1 units in the form of methylene-H
4F for biosynthetic
pathways, for instance, purine biosynthesis. Mutants in this
enzyme are normally deficient in the biosynthesis of purines
(
37,
47). GlyA in
M. extorquens AM1, however, is specialized
to methylotrophy and is not required for growth on multicarbon
compounds (
11), so an alternative source of C
1 units must exist
for purine biosynthesis. Homologs of other enzymes of the serine
cycle are also found in nonmethylotrophic bacteria. It therefore
seems that the functionality of the serine cycle must be determined
by subtle substrate specificity adjustments for the enzymes
involved and by common regulation. The
glyA gene is not linked
to other serine cycle genes. However, six of the serine cycle
genes are clustered at one end of the large methylotrophy island
(the end opposite to that containing the H
4MPT module genes),
together with two of the H
4F-linked C
1 transfer module genes
(
mtdA and
fch), and these are transcribed in two units,
sga-hpr-mtdA-fch and
mtkA-mtkB-ppc-mcl (
20). The serine cycle enzymes encoded
by this gene cluster are serine-glyoxylate aminotransferase
(
sga), hydroxypyruvate reductase (
hpr), the two subunits of
malate thiokinase (
mtkAB), an acetyl-CoA-independent phosphoenolpyruvate
(PEP) carboxylase (
ppc), and malyl-CoA lyase (
mcl). Another
serine cycle gene,
gck, encoding glycerate kinase, is not linked
to other serine cycle genes. Most of the serine cycle genes
are regulated coordinately, but so far, only one regulator is
known, QscR (
20), the gene for which has been discovered via
random mutagenesis (
28). QscR is a LysR-type regulator with
high identity to CbbR, a regulator of autotrophy and photosynthesis
in other bacteria (
39). The location of
qscR on the chromosome,
adjacent to the fructose-1,6-bisphosphatase and phosphoribulokinase
genes, might be indicative of a relatively recent acquisition
from an autotrophic bacterium. While most of the reactions of
the serine cycle are enabled by genes specific to the methylotrophic
mode of metabolism, and mutants with mutations in these genes
grow normally on multicarbon compounds, two enzymes in the pathway
are directly borrowed from other metabolic modules and therefore
have dual functions. The genome of
M. extorquens AM1 contains
only one gene for malate dehydrogenase (
mdh), and therefore
the same enzyme functions in the serine cycle during growth
on C
1 compounds and in the tricarboxylic acid (TCA) cycle during
growth on multicarbon compounds. Likewise, only one gene is
found for enolase (
eno); therefore, one and the same enzyme
functions in both the serine cycle and gluconeogenesis. Mutant
analysis has confirmed that both genes,
mdh and
eno, are essential,
and no null mutants can be obtained on either C
1 or multicarbon
substrates (L. Chistoserdova and M. E. Lidstrom, unpublished
observations).

GLYOXYLATE REGENERATION CYCLE
The glyoxylate regeneration cycle (GRC) in serine cycle methylotrophs
not containing isocitrate lyase has remained a mystery for three
decades. The pregenomic efforts resulted in identification of
four genes loosely linked on the chromosome (cluster 22); two
genes encoding polypeptides of unknown function, MeaA (
12,
40)
and MeaB (
23); and the genes encoding crotonyl-CoA reductase
and propionyl-CoA carboxylase (
crr,
pccB) (
12,
23). The genomic
approach combined with gene-specific, as well as random mutagenesis
and metabolite analysis, has resulted in the identification
of most of the reactions of the GRC (
23,
24). The pathway involves
an elaborate series of reactions proceeding via the CoA derivatives
of C
3, C
4, and C
5-carboxylic acids and involves two carboxylation
and two decarboxylation reactions and at least two mutase reactions
(
23). A few pieces of the puzzle are still missing: i.e., the
two enzymes participating in conversion of ethylmalonyl-CoA
into isobutyryl-CoA (a putative mutase and a putative decarboxylase)
are yet to be identified, and the substrate for MeaA, a putative
mutase, remains unknown, as do the other enzymes involved in
the conversion of ß-hydroxyisobutyryl-CoA into propionyl-CoA
(Fig.
1). Mutant analysis shows that early steps of the GRC
overlap with the pathway for poly-ß-hydroxbutyrate
(PHB) biosynthesis (
24), and the late steps overlap with late
steps of the TCA cycle and the serine cycle (
23). A total of
12 genes are known that are specific to this pathway (
croR,
crr,
pccAB,
ibd2,
meaABCD,
mcmAB, and
epm), 3 that overlap with
PHB biosynthesis (
phaABR), 5 that overlap with the TCA cycle
(
sdhABCD, fumA), and 3 that overlap with the serine cycle (
mtkAB, mcl). With the exception of the four genes that are part of
cluster 22 and the
sdh and the
pha genes, the genes for the
GRC are not linked to each other or to other known methylotrophy
genes. Why such an elaborate pathway is employed by many serine
cycle methylotrophs instead of the classic glyoxylate shunt
remains unknown. However, it may in part reflect the need for
low carbon flux through the initial steps of the TCA cycle during
methylotrophic growth. The GRC is viewed here as a separate
metabolic module, as opposed to a part of the serine cycle due
to its role not only in C
1, but also in C
2 metabolism in
M. extorquens AM1 and possibly in other bacteria. The presence
of homologs for the GRC genes in other

-proteobacterial genomes
points towards this pathway being widespread in non-methylotrophs,
and at least in
Streptomyces, the pathway has been shown to
be involved in C
2 metabolism (
19).

PHB CYCLE
Serine cycle methylotrophs accumulate PHB as a reserve material.
Metabolism of PHB in
M. extorquens AM1 is intimately interlinked
with its C
1 metabolism. The first two reactions of the PHB cycle
(catalyzed by PhaA and PhaB) are also the first reactions of
the GRC (
23,
24). Besides the catalytic steps, there are common
regulatory mechanisms controlling both PHB production and C
1 metabolism. These are not completely understood at this point,
but one such common regulator, PhaR, seems to be involved in
directing flows of acetyl-CoA between C
1 assimilation and PHB
accumulation (
23).

TCA CYCLE
During growth on multicarbon compounds, the TCA cycle plays
its classical role in carbon and energy metabolism in
M. extorquens AM1 (
3). In contrast, it is not involved in energy generation
during growth on C
1 compounds. Instead,

-ketoglutarate dehydrogenase
is repressed, and the incomplete cycle plays a strictly assimilatory
role (
3,
48,
49). However, many of the TCA cycle enzymes are
involved in C
1 assimilation. Besides malate dehydrogenase, a
series of the TCA cycle reactions converting succinyl-CoA into
malate form a part of the GRC. Genes encoding two enzyme systems
capable of converting succinyl-CoA into succinate have been
identified in the genome of
M. extorquens AM1: for succinyl-CoA
synthase (the genes
scsA and
scsB are linked to
mdh) and keto
acid succinyl-CoA transferase (
kst). In addition, cell extracts
contain a succinyl-CoA hydrolase activity, but the gene responsible
for this activity is unknown (Chistoserdova and Lidstrom, unpublished).
Null mutations in
scsB and
kst caused no effect on growth of
M. extorquens AM1 on C
1 or multicarbon compounds (Chistoserdova
and Lidstrom, unpublished). This result points toward either
succinyl-CoA hydrolase being the essential enzyme for this conversion
or the three systems being degenerate for this function. The
genes for succinate dehydrogenase (SDH) are all linked on the
chromosome (cluster 29), and SDH null mutants cannot be obtained
either on methanol or on succinate (
48,
49). However, they can
be obtained on methanol supplemented with glyoxylate (N. Korotkova
and M. E. Lidstrom, unpublished observations). This growth condition
circumvents the GRC, confirming the hypothesis that SDH is involved
in glyoxylate regeneration during growth on C
1 compounds. Two
genes showing homology to fumarases (
fumA and
fumB) are present
in the genome. One of the potential fumarase genes (
fumA) has
been subjected to mutagenesis, and, as in the case of SDH, null
mutants could only be obtained on methanol supplemented with
glyoxylate (Korotkova and Lidstrom, unpublished), confirming
the dual function of this enzyme in C
1 and multicarbon metabolism.
The role of the second homolog of fumarase remains unknown.

CONCLUSIONS
We have presented here the genome-based analysis of methylotrophic
metabolism in one model facultative methylotroph,
M. extorquens AM1. This analysis presents a comprehensive picture of the complex
genetic and biochemical makeup of methylotrophy in a given organism.
A little over 100 genes participate in C
1 metabolism in
M. extorquens AM1, and these belong to a few specialized metabolic modules.
Some genes involved in these modules are located in "methylotrophy
islands," while others are scattered around the chromosome and
are present as singular entities. While some genes are specialized
in methylotrophic metabolism, others are shared with nonmethylotrophic
pathways. Such a complex framework of methylotrophy in
M. extorquens AM1 may reflect a complex and nonlinear history of this metabolic
capability. Because a nearly complete set of genes involved
in the different steps of methylotrophic metabolism in
M. extorquens AM1 is now defined, the stage is now set for studies directed
at understanding how this complex network of genes and enzymes
is coordinated and what are the important mechanisms for switching
between C
1 and multicarbon metabolic modes in this organism.

FOOTNOTES
* Corresponding author. Mailing address: Department of Chemical Engineering, University of Washington, Box 352125, Seattle, WA 98195-2125. Phone: (206) 616-5282. Fax: (206) 616-5721. E-mail:
lidstrom{at}u.washington.edu.


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Journal of Bacteriology, May 2003, p. 2980-2987, Vol. 185, No. 10
0021-9193/03/$08.00+0 DOI: 10.1128/JB.185.10.2980-2987.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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