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Molecular Microbiology (2010) 78(3), 541–544 䊏
doi:10.1111/j.1365-2958.2010.07380.x
First published online 27 September 2010
MicroCommentary
Increasing competence in the genus Streptococcus
mmi_7380 541..544
Leiv Sigve Håvarstein*
Department of Chemistry, Biotechnology and Food
Science, Norwegian University of Life Sciences, N-1432
Ås, Norway.
Summary
Streptococcus pneumoniae is one of the most important model organisms for studies on natural genetic
transformation in bacteria. The prevalence of this
gene exchange mechanism in the genus Streptococcus has not been subjected to systematic investigations, but it has been known for decades that only a
few streptococcal species develop the competent
state spontaneously when grown under laboratory
conditions. The recent discovery of a new mechanism
regulating natural transformation in Streptococcus
thermophilus suggested that this property might be
more widespread among streptococci than previously thought. This suspicion has been confirmed by
Mashburn-Warren and co-workers, who in the current
issue of Molecular Microbiology report the discovery
of a novel competence-inducing pheromone that is
conserved in Streptococcus mutans and a number of
pyogenic streptococci.
Comparative analyses of bacterial genomes have shown
that horizontal gene transfer (HGT) is widespread in the
bacterial domain of life. HGT is a major driving force of
evolution that sometimes gives rise to quantum evolutionary leaps that increase bacterial fitness and their capacity
to colonize new ecological niches. There are three recognized HGT mechanisms operating in bacteria: transduction, conjugation and natural genetic transformation. At
present, it is not known which of these is the most important, i.e. contributes most to the total exchange of genetic
material between microbes in nature. Natural transformation has been considered the least prevalent and perhaps
the least important of the three HGT mechanisms.
Whereas most bacteria are able to exchange genetic
information by conjugation and transduction, only about
Accepted 26 August, 2010. *For correspondence. E-mail sigve.
[email protected]; Tel. (+47) 64965883; Fax (+47) 64965901.
© 2010 Blackwell Publishing Ltd
65 bacterial species have so far been documented to be
naturally transformable (Johnsborg et al., 2007). Nevertheless, natural transformation is of special interest
because it represents the only form of HGT that is initiated
and fully controlled by the cell taking up foreign genetic
material. In contrast to conjugation and transduction,
which depend on mobile genetic elements such as plasmids, transposons and bacteriophages, natural transformation is part of the normal physiology of the competent
bacterium. Consequently, natural transformation must be
considered as genuine bacterial ‘sex’. Furthermore, as
the mechanism is conserved in members of at least six
different bacterial phyla, it is probably very old in evolutionary terms (Johnsborg et al., 2007).
Natural transformation was first observed by Frederick
Griffith in 1928 (Griffith, 1928) when he studied the conditions responsible for acquisition of a capsule by unencapsulated strains of Streptococcus pneumoniae. The
molecular basis of this phenomenon was not understood,
however, before DNA was identified as the transforming
agent by Avery and co-workers (Avery et al., 1944). Bacteria that are competent for natural transformation have
the ability to take up naked DNA from the environment
and incorporate it into their genomes by homologous
recombination. It has been known for decades that some
streptococcal species spontaneously and transiently differentiate into the competent state when cultivated in
certain media in the laboratory. Virtually all of these
species, of which S. pneumoniae is by far the best
studied, belong to the mitis phylogenetic group (Fig. 1).
The exception is Streptococcus mutans, the principal
microbial aetiological agent of dental caries, which is the
only known naturally transformable member of the
mutans group (Perry and Kuramitsu, 1981). In the mid90s, it was discovered that competence in mitis and anginosus groups are regulated by secreted competence
stimulating peptides (CSPs) in a quorum sensing-like
manner (Håvarstein et al., 1995; 1997). A few years later,
a signalling peptide of the CSP type was found to be
involved in the regulation of natural transformation in S.
mutans as well (Li et al., 2001). Mature CSP is derived
from a ribosomally synthesized precursor, ComC, which is
processed concomitant with export by the ComAB secretion apparatus (Hui and Morrison, 1991; Håvarstein et al.,
542 L. S. Håvarstein 䊏
Fig. 1. Phylogenetic tree based on
streptococcal 16S rRNA sequences. The
colour indicates the nature of the competence
regulatory system found in each phylogenetic
group. Red, ComCDE; green, ComRS and
yellow, both ComCDE and ComRS. In case of
the mutans group, only S. mutans has been
investigated. The illustration is adapted from
Kawamura et al. (1995) with permission from
the publishers.
1995). The extracellular concentration of CSP is monitored by a two-component regulatory system consisting of
the transmembrane histidine kinase ComD and its
cognate response regulator ComE (Pestova et al., 1996).
Sequencing of a number of comC genes from mitis streptococci revealed that a large variety of CSPs are produced by different strains and species belonging to this
phylogenetic group. In general, CSP pheromones are
highly specific, and are not recognized by non-cognate
receptors. As a result, only streptococci belonging to the
same pherogroup are able to communicate through CSP
signalling. When the external CSP concentration reaches
a threshold level phosphorylated ComE directly activates
transcription of the early competence genes, including the
gene encoding the alternative sigma factor ComX. ComX
directs expression of the late competence genes, most of
which are involved in DNA uptake and processing (Lee
and Morrison, 1999). Orthologs of ComX, designated
SigX in the pyogenic streptococci, have been detected in
all streptococcal genomes sequenced so far. In fact, they
appear to be present in the genomes of all species
belonging to the order Lactobacillales (Martin et al.,
2006).
Genes involved in natural transformation, so called
competence genes, are not found in a single locus, but
are dispersed throughout the chromosome of bacteria
containing them. Interestingly, homologues of competence genes are quite widespread among bacteria, and
are present in the genomes of a large number of species
that have never been observed to be naturally
transformable. The role of these genes in seemingly noncompetent bacteria has been subject to much speculation
but little systematic study. It has been proposed that they
serve a different function in non-competent bacteria, or
represent remnants of once functional competence
genes. A third and more interesting possibility is that most
apparently non-competent bacteria, possessing the core
competence genes essential for DNA uptake and processing, are in reality naturally transformable when the
right growth conditions and/or specific environmental
cues are present. This was indeed shown to be the case
for Vibrio cholerae, which was found to be become competent when subjected to chitin, increasing cell density or
stress (Meibom et al., 2005). In the genus Streptococcus
recent developments tell a similar story. A species within
the salivarius group, the dairy starter bacterium Streptococcus thermophilus LMG 18311, turned out to become
naturally transformable when ComX was overexpressed
from an inducible promoter (Blomqvist et al., 2006). This
finding proved that the ComX regulon, i.e. the core competence genes of S. thermophilus LMG 18311, is functional despite the fact that spontaneous competence
development had never been observed in this species.
A few years later it was discovered that cultivation of S.
thermophilus strain LMD-9 in a synthetic medium resulted
in spontaneous competence development during early
exponential phase, and that competence development
depended on the Ami oligopeptide transport system
(Gardan et al., 2009). Considering that the comCDE
genes are missing in S. thermophilus, these findings
strongly suggested that competence in this species is
regulated by an alternative mechanism involving the Ami
transporter. In other Gram-positive bacteria, such as
Enterococcus faecalis and Bacillus subtilis, oligopeptide
transporters are known to be part of signalling pathways
regulating conjugation and competence development
respectively (Leonard et al., 1996; Pottathil and
Lazazzera, 2003). Their role is to import oligopeptide
© 2010 Blackwell Publishing Ltd, Molecular Microbiology, 78, 541–544
Competence in the genus Streptococcus 543
signals that bind to and modify the activity of intracellular
effector molecules (Shi et al., 2005). Very recently, Fontaine et al. (2010) succeeded in identifying the S. thermophilus competence pheromone as well as its intracellular
target protein. The novel oligopeptide pheromone, ComS,
turned out to be a short hydrophobic peptide (~7 amino
acids), which is cleaved by an unknown mechanism from
the C-terminal end of a 24 amino acid ribosomally synthesized precursor. Although not yet formally proven, it is
very likely that intracellular ComS interacts with and activates ComR, a transcriptional regulator belonging to the
Rgg family. The current working model predicts that activated ComR binds to an inverted repeat motif upstream
the comX gene and initiates expression of this gene followed by ComX-mediated competence induction.
Remarkably, in this issue of Molecular Microbiology
Mashburn-Warren et al. (2010) provide convincing evidence that competence in a number of streptococci
belonging to the pyogenic and bovis groups (Fig. 1),
which so far have proven refractory to transformation
even though they possess the core competence genes
(Martin et al., 2006; Woodbury et al., 2006), is regulated
by a system closely related to the one operating in S.
thermophilus. Consequently, it is very likely that the pyogenic and bovis streptococci are naturally transformable
after all, and that they will develop the competent state
spontaneously when grown under the appropriate
conditions.
Curiously, the discoveries described above show that
competence development in the genus Streptococcus is
regulated by two completely different quorum sensing-like
systems. ComCDE controls competence in members of
the mitis and probably also in the anginosus group,
whereas ComRS perform the same function in the pyogenic, salivarius and bovis groups (Fig. 1). Even more
curious is the fact that S. mutans possesses both systems.
Almost a decade ago it was observed that unlike S. pneumoniae, inactivation of each of the comCDE genes
reduced but did not eliminate competence in S. mutans (Li
et al., 2001). This puzzling observation can now be
explained by taking into account the new parallel ComRS
regulatory system reported in the current edition of this
journal (Mashburn-Warren et al., 2010). In contrast, knocking out the ComRS system in S. mutans renders the
bacterium completely non-competent. Thus, while the
ComRS system is able to function in the absence of
ComCDE, the reverse is not true. This indicates that ComR
and not ComE is the proximal regulator of ComX in S.
mutans (Mashburn-Warren et al., 2010). A possible explanation for the existence of two different mechanisms controlling competence development in streptococci is that
ComRS represents an ancestral system that during evolution has been replaced by ComCDE in the mitis and
anginosus groups. If this is what happened, where did the
ComCDE system come from? Most bacteria in the order
Lactobacillales produce bacteriocins, antimicrobial peptides that kill or inhibit the growth of other bacteria. In
streptococci production of bacteriocins is frequently regulated by two-component signal transduction pathways of
the ComCDE type. Indeed, the ComCDE systems of S.
pneumoniae and S. mutans both regulate production of
class II bacteriocins in addition to competence development (Guiral et al., 2005; van der Ploeg, 2005). The major
pneumococcal system governing bacteriocin production,
BlpCHR, operates independently of the closely related
ComCDE system. ComCDE-mediated competence development takes place at early logarithmic phase, whereas
BlpCHR controlled synthesis of bacteriocins starts when
pneumococcal cultures approach the stationary phase (de
Saizieu et al., 2000). It is conceivable that a recombination
event could have replaced a ComR-type binding site with a
ComE/BlpR-type binding site upstream the ComX gene in
the common ancestor of bacteria belonging to the mitis and
anginosus groups. This would have resulted in an immediate shift from a ComRS to a ComCDE-driven competence regulation system. At present, however, the opposite
process cannot be ruled out. Interestingly, the two systems
seem to be activated by different environmental condition.
Whereas rich complex media stimulate ComCDE expression and competence in mitis group streptococci, these
conditions appear to inhibit spontaneous competence
development in streptococci carrying the ComRS system.
Consequently, the ComRS/ComCDE shift discussed
above may represent an example of an evolutionary leap
that had profound and lasting consequences for the
affected streptococcus and its descendants.
Acknowledgements
This work was supported by The Research Council of
Norway.
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