In this study we demonstrated that mutacin IV gene expression is dramatically increased in the cell pellet, and that this regulation is mediated by the ComCDE system in a pathway independent of the ComX regulatory pathway ( and ). Further studies suggested that mutacin IV gene expression and competence development may be coordinated temporally in response to CSP ( and ). This raised an interesting question of whether this coordinated mutacin production and competence development could be used as a means for S. mutans
to acquire transforming DNA from neighbouring streptococcal species. Mixed culture assays with S. gordonii
demonstrated that increased transformation of S. mutans
by plasmid DNA originally harboured in S. gordonii
was dependent on the presence of the mutacin IV gene, suggesting a role of mutacin IV in DNA release from S. gordonii
(). Further investigation using partially purified mutacin IV demonstrated increased DNA release upon addition of mutacin IV crude extract to the S. gordonii
culture (). Based on these results, we propose the following model for the coordinated mutacin IV production and competence development (): high cell density or nutrient limitation or both triggers high-level comC
gene expression, resulting in high-level production of CSP. CSP binding to the ComDE two-component system activates transcription of a set of genes including the comX
gene and nlmA
. While expression of the late competence genes such as comY
is dependent on ComX, the nlmA
gene expression is independent of ComX. The consequence of this complex regulatory pathway () results in a co-ordinated regulation between competence and bacteriocin production. High levels of mutacin IV production would kill neighbouring streptococcal species such as S. gordonii
causing DNA release from these species. Released DNA is then internalized by the ComYA-G complex (Merritt et al., 2005
), whose expression is dependent on ComX. It is worth noting that the temporal correlation between nlmA
gene expression and competence is not strict; there is a 90 min delay of competence after nlmA
gene expression is turned on (). We speculate that this delay is probably due to a signal relay in the competence regulatory pathway to turn on the late competence genes for DNA uptake and recombination, while no such a signal relay is required to turn on the mutacin IV gene expression. This is apparently different from the findings in S. pneumoniae
where autolysis and competence strictly correlate (Steinmoen et al., 2002
). However, given the reason that mutacin IV has to be produced, secreted, bound to the sensitive cells to kill before donor DNA is released, it would make perfect sense for it to be produced first before the competence system becomes fully functional.
Model for coordinated mutacin IV production and competence in DNA release and uptake from other streptococcal species (see text for details).
Mutacin-mediated interspecies DNA acquisition may not be limited to UA140. Inspection of the clinical isolates of S. mutans
revealed that ~50% contain the mutacin IV gene in their chromosome. These mutacin IV genes are likely to be controlled by CSP as their promoter sequences could also be amplified by primers designed using the UA159 sequence (data not shown). In fact, we have tested a number of clinical isolates on a plate assay and showed that all displayed larger inhibition zones against the mitis group streptococci after CSP addition to the plate (J. Kreth et al.
unpubl.). Furthermore, mutacin IV may not be the only mutacin controlled by the ComCDE system. In a recent report by Yanezawa and Kuramitsu (Yonezawa and Kuramitsu, 2005
), a two-component lantibiotic mutacin called Smb was found to be regulated by CSP in the strain GS-5. As this Smb is active against a group C streptococcus, it would be interesting to see if this mutacin is capable of causing DNA release from the target cells during competence.
The association of bacteriocin production with competence may not be limited to S. mutans
. In Bacillus subtilis
, it was found that comS
, a small peptide required for competence, was embedded within the second open reading frame of the srfA
operon-encoding enzymes for the biosynthesis of a surfactin. This organization was suggested as a way for B. subtilis
to lyse microorganisms for the release of genetic material during competence (D’Souza et al., 1994
; Hamoen et al., 1995
). In a recent microarray study in S. pneumoniae
, Rimini et al
. and Peterson et al
. found that some genes encoding bacteriocin-like peptides were induced by CSP, although these findings were not verified by genetic studies (Rimini et al., 2000
; Peterson et al., 2004
). Although some of the bacteriocin-like peptides (e.g. genes blpA
) were later found to be regulated by a different pheromone, BlpR, which does not cross react with the ComDE system (Reichmann and Hakenbeck, 2000
; de Saizieu et al., 2000
), a recent study by Knutsen et al
. reported that an unknown ABC transporter is co-regulated by both the ComCDE and the BlpR system (Knutsen et al., 2004
). Whether this unknown ABC transporter is a link between the two quorum-sensing systems is yet to be determined. Nonetheless, findings from this study provide the first genetic evidence for a coordinated production of a bacteriocin and the development of competence as a possible mechanism for DNA acquisition from other species. This mechanism may serve as a basis for acquiring DNA from other species, which may provide the opportunity for horizontal gene transfer in multispecies microbial communities. We speculate that as more bacteriocin and competence systems are characterized in other bacterial species, this mechanism may prove to be a prevalent phenomenon among naturally competent microorganisms that inhabit multispecies environments.