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Journal of Bacteriology, March 2009, p. 1610-1617, Vol. 191, No. 5
0021-9193/09/$08.00+0 doi:10.1128/JB.01252-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Genetic Evidence for the Importance of Protein Acetylation and Protein Deacetylation in the Halophilic Archaeon Haloferax volcanii
,
Neta Altman-Price and
Moshe Mevarech*
Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel
Received 8 September 2008/
Accepted 16 December 2008

ABSTRACT
Protein acetylation and deacetylation reactions are involved
in many regulatory processes in eukaryotes. Recently, it was
found that similar processes occur in bacteria and archaea.
Sequence analysis of the genome of the haloarchaeon
Haloferax volcanii led to the identification of three putative protein
acetyltransferases belonging to the Gcn5 family, Pat1, Pat2,
and Elp3, and two deacetylases, Sir2 and HdaI. Intriguingly,
the gene that encodes HdaI shares an operon with an archaeal
histone homolog. We performed gene knockouts to determine whether
the genes encoding these putative acetyltransferases and deacetylases
are essential. A
sir2 deletion mutant was able to grow normally,
whereas an
hdaI deletion mutant was nonviable. The latter is
consistent with the finding that trichostatin A, a specific
inhibitor of HdaI, inhibits cell growth in a concentration-dependent
manner. We also showed that each of the acetyltransferases by
itself is dispensable for growth but that deletion of both
pat2 and
elp3 could not be achieved. The corresponding genes are
therefore "synthetic lethals," and the protein acetyltransferases
probably have a common and essential substrate.

INTRODUCTION
Protein acetylation is one of several classes of posttranslational
regulatory processes that occur in living cells. In contrast
to those of other protein modifications, such as phosphorylation,
which has been studied for over 50 years, the role of protein
acetylation in cellular events is less well understood (
23,
35). Two types of protein acetylation take place in the cell.
N acetylation is the acetylation of the amino termini of newly
synthesized proteins that mostly follows the removal of the
first methionine residue. This type of acetylation is common
in eukaryotes but also occurs to some extent in archaea (
1,
14). The second type of acetylation is a reversible modification
that occurs on lysine residues of mature proteins, resulting
in charge neutralization of these residues. The latter type
of protein acetylation is catalyzed by a family of histone acetyltransferases
(HATs) that function to transfer an acetyl group from acetyl-coenzyme
A (acetyl-CoA) to the

-amino group of certain lysine side chains.
The HAT family is composed of five enzyme subfamilies, of which
the Gcn5-related
N-acetyltransferase (GNAT) subfamily is the
best characterized (reviewed in references
11,
17,
26,
29, and
46). Acetylation reactions catalyzed by HATs can be reversed
by a family of histone deacetylases (HDACs). HDACs are divided
into three classes: class I HDACs are related to yeast Rpd3,
class II HDACs are related to yeast HdaI, and class III HDACs
are related to the yeast transcriptional repressor Sir2. Class
I and class II HDACs share some homology in their catalytic
domains and hydrolyze the acetamide bond in similar manners,
whereas class III HDACs share no homology with class I and II
HDACs and employ a different enzymatic mechanism (reviewed in
references
7,
10,
20,
39, and
47).
The reversible posttranslational protein acetylation is not restricted to eukaryotes and has been demonstrated to take place in bacteria and archaea. A metabolic role for the Sir2 homolog CobB was reported in the bacterium Salmonella enterica. It was shown that CobB activates the acetyl-CoA synthetase (ACS) via deacetylation (42), while Pat, an acetyltransferase that exhibits homology in its C-terminal 95-amino-acid-residue region to the eukaryotic Gcn5 acetyltransferases, deactivates it (43). Recent studies of the archaeon Sulfolobus solfataricus revealed that the archaeal homolog of Sir2 forms a stable complex with Alba, one of the most abundant archaeal chromatin proteins. In vitro, Sir2 is responsible for deacetylation of Alba, causing an increase in its affinity to DNA and thereby repressing transcription (5, 57). Subsequent studies showed that Alba acetylation is carried out by a homolog of S. enterica Pat (31). Sir2 homologs are not the only HDACs in archaea and are not even the most common. All archaea (with the exception of Nanoarchaeum equitans) possess homologs of HdaI. However, no genetic analysis has been performed to establish the significance of HDACs and HATs in archaea.
Haloferax volcanii is an obligate halophilic and aerobic archaeon of the euryarchaeota lineage. It has become a model organism for molecular genetic studies of archaea due to the wide range of available genetic tools (2). Also, the complete genome nucleotide sequence of the organism was recently determined and annotated by The Institute for Genomic Research. In this communication, we present bioinformatic evidence for the existence in H. volcanii of genes encoding the reversible protein acetylation/deacetylation reactions and provide genetic evidence for their essentiality.

MATERIALS AND METHODS
Strains and culture conditions.
The properties of the various
H. volcanii strains used in this
work are given in Table
1.
H. volcanii was routinely grown in rich (HY) medium containing
(per liter): 150 g of NaCl, 36.9 g of MgSO
4 · 7H
2O, 5
ml of a 1 M KCl solution, 1.8 ml of a 75-mg/liter MnCl
2 solution,
5 g yeast extract (Difco), and Tris-HCl (pH 7.2) at a final
concentration of 50 mM. After autoclaving and cooling of the
medium, 5 ml of 10% (wt/vol) CaCl
2 was added. Agar plates contained
18 g of Bacto Agar (Difco) per liter. Casamino Acids (CA) medium
contained the same components as the HY medium except that the
yeast extract was replaced by 5 g/liter of CA (Difco). Minimal
medium contained (per liter) 150 g of NaCl, 36.9 g of MgSO
4 · 7H
2O, 5 ml of a 1 M KCl solution, 50 ml of 1 M NH
4Cl,
45 ml of 10% (vol/vol) glycerol, 5 ml of 10% (wt/vol) sodium
succinate, 2 ml of 0.5 M K
2HPO
4, and Tris-HCl (pH 7.2) at a
final concentration of 50 mM. After autoclaving and cooling
of the medium, the following materials were added: 5 ml of 10%
(wt/vol) CaCl
2, 1 ml trace element solution, 0.8 ml of 1-mg/ml
thiamine, and 0.1 ml of 1-mg/ml biotin.
For counterselection for uracil auxotrophs, 5-fluoroorotic acid (U.S. Biological) was added to the medium at a final concentration of 100 µg/ml. When needed, novobiocin (Sigma-Aldrich) was added to the medium at a final concentration of 2 µg/ml. When required, thymidine was added to a final concentration of 40 µg/ml, and leucine, tryptophan, and uracil were added to a final concentration of 50 µg/ml. Trichostatin A (TSA) (Sigma-Aldrich) was added to the growth media at the concentrations indicated in the text.
Gene knockouts and gene replacements.
Gene knockouts and gene replacements were performed according to the "pop-in pop-out" methodology as described previously (3, 6). In this methodology, the upstream and downstream flanking regions of the genes to be deleted are PCR amplified and cloned together into the "suicide plasmid" pGB70 or pTA131, which carries the pyrE selectable genetic marker but cannot replicate autonomously in H. volcanii. The plasmids are transformed into an H. volcanii
pyrE mutant, and transformants in which the plasmids have been integrated into the chromosome are selected on plates that lack uracil. Upon counterselection on plates containing uracil and 5-fluoroorotic acid, the only cells that survive are those in which the integrated plasmids have been excised by spontaneous intrachromosomal homologous recombination, either restoring the wild-type gene or resulting in its deletion. Gene replacements were performed according to the method of Allers et al. (3).
HY medium was used as a thymidine-minus medium for hdrB cassette selection. CA medium was used as a uracil- and tryptophan-minus medium for trpA cassette selection. Minimal medium was used as a leucine-minus medium for leuB cassette selection.
The pop-out strains were screened using pairs of external "short up" and "short down" primers located approximately 100 bp upstream and 100 bp downstream of the entire flanking construct. All the deletions were also verified by the inability to PCR amplify the coding region of the deleted genes in mutants running in parallel control reactions with the "wild-type" strains.
A list of all integrative plasmids, shuttle vectors, and other vectors that were used for this study is given in Table 2. A list of all primers used in this study is given in Table S1 in the supplemental material.
Transformation procedures.
Transformation of
H. volcanii was carried out using the polyethylene
glycol method as described previously (
6).
Determination of growth rates and TSA inhibition.
To determine growth rates, cells were grown in HY medium at 42°C to the stationary phase, diluted (1:50), grown to an optical density at 600 nm (OD600) of 0.6 to 0.7, and then diluted again to an OD600 of 0.05 in fresh medium. When the effect of TSA on the growth rate was determined, the fresh medium was supplemented with the indicated amount of TSA. OD600 measurements were taken at 2- to 4-h intervals following an overnight lag phase.
Genomic-data analyses.
H. volcanii genome sequence data were obtained from The UCSC Genome Browser (http://archaea.ucsc.edu/cgi-bin/hgGateway?db=haloVolc1.) Multiple-alignment analysis was performed using Multalin software (12; http://bioinfo.genopole-toulouse.prd.fr/multalin/multalin.html). Multiple-alignment figures were created using BoxShade 3.21 software (http://www.ch.embnet.org/software/BOX_form.html).

RESULTS
H. volcanii contains genes predicted to encode putative protein acetylases and deacetylases.
To identify putative components of the
H. volcanii posttranslational
protein acetylation/deacetylation machinery, its genome database
(see Materials and Methods) was screened using sequences of
known HATs and HDACs as BLAST queries. Several putative open
reading frames that showed significant homology to known HATs
and HDACs were identified. The
H. volcanii genome was found
to contain genes belonging to two HDAC families: one gene is
a homolog of the Sir2 family (HVO_2194) (40% identity and 57%
similarly to
S. solfataricus Sir2), and another is a homolog
of the HdaI family (HVO_0522) (37% identity and 55% similarity
to
Saccharomyces cerevisiae HdaI). Figures
1 and
2 present alignments
of the yeast and archaeal Sir2 homologs and the yeast and archaeal
HdaI homologs, respectively, and show that the proteins are
closely related and conserve important functional domains.
The
hdaI gene encoding HdaI occurs in an operon containing a
histone gene (Fig.
3). The putative histone is predicted to
have a tandem H3-H4 core domain similar to that present in the
Methanopyrus kandleri HMk histone, which specifically binds
DNA (
13,
33,
41). Interestingly,
hdaI overlaps with the CCA-adding
tRNA nucleotidyltransferase gene, which is on the opposite DNA
strand. The same genome arrangement is found in other halophilic
archaea whose genomes have been sequenced but not in any other
archaea.
Three homologs of known HATs were identified in the
H. volcanii genome. The first two, Pat1 (HVO_1756) and Pat2 (HVO_1821),
belong to the Gcn5 family. They are related to
S. solfataricus Pat, and the C-terminal 95-amino-acid region shares similarity
with the
S. enterica enzyme (26% identity and 45% similarity
to
S. solfataricus Pat and 34% identity and 45% similarity to
the C terminus of
S. enterica Pat, respectively) (Fig.
4). The
third HAT (HVO_2888) is a homolog of yeast Elp3, which also
belongs to the Gcn5 family (Fig.
5) (37% identity and 58% similarity
to
S. cerevisiae Elp3). Elp3 is a subunit of the "elongator"
complex possessing acetyltransferase activity (
51,
53).
The H. volcanii sir2 homolog is dispensable, but hdaI is essential.
To determine the essentiality of the two HDACs, we employed
the "pop-in pop-out" strategy for constructing gene knockouts
previously developed for
H. volcanii (
3,
6) (see Materials and
Methods). In this procedure, if the deletion of the target gene
has no effect on the growth properties of the cells, it is expected
that in about half of the cells excision of the chromosomally
integrated plasmid leaves behind the wild-type allele of the
target gene and in about half of the cells the excision creates
the desired deletion. The
sir2 genomic deletion plasmid pMM915
was transformed into
H. volcanii strain H133 and integrated
into its chromosome to create the
sir2 "pop-in" strain WR669.
Following "pop-out" counterselection, it was found that in about
half of the cells in which the
pyrE-containing plasmid was excised,
deletion of
sir2 had occurred (to give strain WR580), as determined
by PCR analysis (see Fig. S1 in the supplemental material).
The
H. volcanii sir2 homolog is therefore not essential. The
growth rate of the mutant strain in rich HY medium was similar
to that of the parental strain, H133, grown under the same conditions.
Unlike the
S. enterica cobB knockout mutant (
45,
48), the
H. volcanii sir2 deletion mutant grew normally on low concentrations
of acetate as the sole energy source. The
H. volcanii
sir2 mutant
also showed no apparent growth impairment compared to the wild-type
strain when cultured at salt concentrations ranging from 4 M
to 1.5 M NaCl and at temperatures ranging from 37°C to 45°C
(data not shown).
The same "pop-in pop-out" strategy was employed in attempts to inactivate hdaI. Since the hdaI gene partially overlaps the essential CCA-adding transferase gene that is involved in tRNA maturation (Fig. 3), attempts were made to delete nucleotides 71 to 920 of the nonoverlapping region of the hdaI gene. Plasmid pMM1028 was used to create the "pop-in" strain WR671. However, no hdaI deletions were obtained following the "pop-out" counterselection, suggesting that hdaI is essential.
To confirm that hdaI is essential, a plasmid carrying the complete hdaI gene was cloned into the pRV1-ptna-bgaH plasmid (28) between the NdeI and EcoRI sites, placing the hdaI gene under the control of the tryptophanase promoter (pMM1142). Plasmid pMM1142 transformed into the "pop-in" strain, WR671. In this genetic background, it was possible to knock out the chromosomal gene and create strain WR718 (see Fig. S2 in the supplemental material). The novobiocin resistance gene present on the pMM1142 plasmid was used to confirm the presence of the plasmid in the cell, together with the genomic knockout background. Other evidence to support the essentiality of hdaI was obtained from the effect of the HdaI-specific inhibitor TSA (16, 50, 55) on the growth rate of H. volcanii. Figure 6 shows that TSA inhibits the growth of H. volcanii in a concentration-dependent manner.
A pat1 and pat2 double-knockout strain is viable.
H. volcanii contains genes coding for two homologs of the
S. solfataricus Pat. Plasmid pMM956 was designed to allow replacement
of
pat1 by the
trpA cassette, and pMM1033 was designed to allow
replacement of
pat2 by the
hdrB cassette (see Fig. S3 in the
supplemental material). The growth rates in rich HY media of
the two single-deletion mutants (WR643 for
pat1 replacement
with the
trpA cassette and WR644 for
pat2 replacement with the
hdrB cassette) and the double-deletion mutant (WR645) were comparable
to that of the parental strain, H133 (data not shown).
The elp3 knockout is viable, but elp3 and pat2 are "synthetic lethals."
The elp3 gene was replaced by the leuB selectable marker using the pMM1108 plasmid. It was possible to create an elp3 knockout in the wild-type background (WR660), as well as in the background of the
pat1 strain (WR709). However, attempts to create an elp3 null strain in the
pat2 single-mutant or the
pat1
pat2 double-mutant strain failed. These results imply that elp3 and pat2 are "synthetic lethals," namely, their products may share the same target(s).
Acetyltransferase knockout strains show no resistance to TSA.
The growth rates of all acetyltransferase knockout strains (
pat1,
pat2,
pat1
pat2,
elp3, and
pat1
elp3) in rich HY medium were found to be comparable to those of the parental H133 strain (data not shown). We also examined the growth properties of the mutants in rich media containing TSA. However, none of the single-mutant strains were resistant to TSA. Similarly, attempts to knock out hdaI in strains carrying the various acetyltransferase gene knockouts also failed.

DISCUSSION
Posttranslational protein modification plays a key role in many
cellular processes. The natures of these modifications cannot,
in most cases, be deduced from genomic information and have
to be determined by elaborate procedures. While archaeal genomic
data are accumulating rapidly with innovations in genome-sequencing
techniques, progress regarding archaeal protein modification
is much slower. The present study describes a genetic approach
to explore the potential of protein acetylation and deacetylation
in
H. volcanii by identifying the putative protein acetylase
and protein deacetylase genes, followed by attempts to delete
these genes. The results of this study are summarized in Table
3.
The plethora of acetyltransferases and deacetylases in yeasts
is not observed in archaea in general and in
H. volcanii in
particular. Unlike many organisms (
8,
9),
H. volcanii (and other
archaea [
27,
30]) contains only one putative Sir2 homolog. The
second
H. volcanii histone deacetylase is an HdaI homolog. Interestingly,
Sir2 is widely distributed among bacteria and eukarya (
9,
15),
whereas in archaea, HdaI is more common. In fact, HdaI is present
in all archaeal orders and is missing only from the
Nanoarchaeota genome. Similarly,
elp3 is widely distributed and highly conserved
among all archaeal genomes (
37,
49).
S. cerevisiae sir2 knockout mutants, as well as those in other eukaryotes, display a wide range of phenotypes, whereas the single H. volcanii sir2 knockout mutant had no recognizable phenotype. In contrast, the yeast hdaI knockout mutant had a mild phenotype and the hdaI rpd3 double mutant was viable (39). Since addition of TSA to the culture medium causes a severe defect in the growth of H. volcanii and deletion of the hdaI gene is lethal, it seems likely that HdaI is the main protein deacetylase in H. volcanii.
Single null mutations of S. cerevisiae gcn5 or elp3, homologs of H. volcanii pat2 and elp3, are viable, and the growth properties of the single-mutant strains are comparable to those of the wild-type strain under most conditions (53). However, S. cerevisiae elp3 and gcn5 double mutants, though viable, have more severe growth defects. These defects can be partly relieved by an accompanying hdaI deletion (52).
Similarly, single-deletion mutations in each of the three putative H. volcanii acetyltransferase genes had no discernible effect on growth or on TSA resistance. Nevertheless, it was not possible to delete both pat2 and elp3. These results imply that Pat2 and Elp3 share a substrate whose acetylation is essential and that this substrate cannot be acetylated by Pat1. The fact that hdaI is also essential indicates that indiscriminate acetylation of lysine residues is harmful and necessitates selective removal by protein deacetylases. So far, we have been unable to create a strain in which pat2, elp3, and hdaI are all deleted in analogy to the situation described above for S. cerevisiae.
Given that the acetylation/deacetylation machinery is an essential cellular process, we can address the issue of the nature of the possible protein targets at which it acts. The archaeal chromatin protein Alba was shown to undergo acetylation/deacetylation (31). Alba is found in many archaea but is missing in all halophilic archaea (40, 49). ACS was recently shown in S. enterica to be acetylated on residue K609 by Pat and deacetylated by CobB (a Sir2 homolog). Residue L641 was found to be important for enzymatic acetylation (44). H. volcanii has four ACS homologs. Two ACS genes (HVO_1585 and HVO_0894) are on the chromosome, while the other two genes (HVO_A0158 and HVO_A0156) are on an extrachromosomal megaplasmid (pHV4). The four enzymes show considerable similarity to the bacterial ACS, and their acetylation sites are also well preserved. Many other archaea have ACS homologs and have preserved the acetylation site (some examples are given in Fig. 7). Nevertheless, the H. volcanii sir2 deletion mutant displays no phenotypic growth impairments and, unlike the cobB mutant, can grow on minimal media using low concentrations of acetate as the sole carbon source. The only haloarchaeal protein known so far to be acetylated in vivo is the 2Fe-2S ferredoxin. The amino acid sequences of the Halobacterium salinarum (18) and Haloarcula marismortui (19) ferredoxins were determined and shown to contain a unique acetylated lysine close to their carboxyl termini. The conservation of the acetylated lysines in the two distantly related halophilic archaea might indicate their functional significance.
Among the eukaryotes, the best-studied protein acetylation process
is that of the amino-terminal histone tails. Their acetylation,
and other posttranslational modifications, plays an important
role in the regulation of gene expression (some models for regulation
are reviewed in references
22,
24,
25,
34, and
36).
H. volcanii histone lacks the tail extension and consists only of sequences
homologous to the core sequence of the eukaryotic H3-H4 histones.
Evidently, the potential target of the acetylation-deacetylation
machinery in
H. volcanii cannot be the histone tail. While acetylation
of the histone tail is well documented, evidence regarding histone
core modifications is gradually accumulating. Recently, a core
acetylation modification was observed in Lys79 of histone H4
in yeasts (
21,
56). This lysine residue is conserved in the
core H3-H4 histone of most archaea and also in
H. volcanii histone.
This conservation, along with the physical location of the halophilic
histone gene in an operon with the essential HdaI deacetylase
gene, suggests that the histone is a possible target for acetylation.

ACKNOWLEDGMENTS
This work was supported by a grant from the Israel Science Foundation
(ISF-998-07).
We thank Thorsten Allers from the University of Nottingham for the generous donation of plasmids, Gerald Cohen for critical reading of the manuscript, and Adit Naor for technical assistance.

FOOTNOTES
* Corresponding author. Mailing address: Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel. Phone: 972-3-6408715. Fax: 972-3-6409407. E-mail:
mevarech{at}post.tau.ac.il 
Published ahead of print on 29 December 2008. 
Supplemental material for this article may be found at http://jb.asm.org/. 

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Journal of Bacteriology, March 2009, p. 1610-1617, Vol. 191, No. 5
0021-9193/09/$08.00+0 doi:10.1128/JB.01252-08
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