Diverse regulatory circuits for transfer of conjugative elements

Effects of luxS gene on growth characteristics, biofilm formation, and antimicrobial resistance of multi-antimicrobial-resistant Vibrio parahaemolyticus Vp2015094 isolated from shellfish

AIMS

Vibrio parahaemolyticus is an important foodborne pathogen worldwide, which can cause gastroenteritis. This study aimed to investigate the effect of quorum sensing system LuxS/AI-2-related gene luxS on the biological characteristics and antimicrobial resistance of V. parahaemolyticus Vp2015094 from shellfish, which carried a multi-antimicrobial-resistant plasmid.

METHODS AND RESULTS

The critical gene luxS related to the synthesis of AI-2 in V. parahaemolyticus Vp2015094 was knocked out by homologous recombination with suicide plasmid. The effect of luxS on the biological characteristics of V. parahaemolyticus was determined by comparing the growth, AI-2 activity, motility, biofilm formation ability, and antibiotic resistance between the wildtype strain and the luxS deletion mutant. Compared with wildtype strain, the production of AI-2, the motility and biofilm formation ability, antimicrobial resistance, and conjugation frequency of luxS deletion mutant strain were decreased. The transcriptome sequencing showed that the transcriptional levels of many genes related to motility, biofilm formation, antimicrobial resistance, and conjugation were significantly downregulated after luxS deletion.

CONCLUSIONS

Quorum sensing system LuxS/AI-2-related gene luxS in V. parahaemolyticus Vp2015094 played an important role in growth characteristics, biofilm formation, antimicrobial resistance, and resistance genes' transfer.

The Effect of Outer Space and Other Environmental Cues on Bacterial Conjugation

Bacterial conjugation is one of the most abundant horizontal gene transfer (HGT) mechanisms, playing a fundamental role in prokaryote evolution. A better understanding of bacterial conjugation and its cross talk with the environment is needed for a more complete understanding of HGT mechanisms and to fight the dissemination of malicious genes between bacteria. Here, we studied the effect of outer space, microgravity, and additional key environmental cues on transfer (tra) gene expression and conjugation efficiency, using the under studied broad-host range plasmid pN3, as a model. High resolution scanning electron microscopy revealed the morphology of the pN3 conjugative pili and mating pair formation during conjugation. Using a nanosatellite carrying a miniaturized lab, we studied pN3 conjugation in outer space, and used qRT-PCR, Western blotting and mating assays to determine the effect of ground physicochemical parameters on tra gene expression and conjugation. We showed for the first time that bacterial conjugation can occur in outer space and on the ground, under microgravity-simulated conditions. Furthermore, we demonstrated that microgravity, liquid media, elevated temperature, nutrient depletion, high osmolarity and low oxygen significantly reduce pN3 conjugation. Interestingly, under some of these conditions we observed an inverse correlation between tra gene transcription and conjugation frequency and found that induction of at least traK and traL can negatively affect pN3 conjugation frequency in a dose-dependent manner. Collectively, these results uncover pN3 regulation by various environmental cues and highlight the diversity of conjugation systems and the different ways in which they may be regulated in response to abiotic signals. IMPORTANCE Bacterial conjugation is a highly ubiquitous and promiscuous process, by which a donor bacterium transfers a large portion of genetic material to a recipient cell. This mechanism of horizontal gene transfer plays an important role in bacterial evolution and in the ability of bacteria to acquire resistance to antimicrobial drugs and disinfectants. Bacterial conjugation is a complex and energy-consuming process, that is tightly regulated and largely affected by various environmental signals sensed by the bacterial cell. Comprehensive knowledge about bacterial conjugation and the ways it is affected by environmental cues is required to better understand bacterial ecology and evolution and to find new effective ways to counteract the threating dissemination of antibiotic resistance genes between bacterial populations. Moreover, characterizing this process under stress or suboptimal growth conditions such as elevated temperatures, high salinity or in the outer space, may provide insights relevant to future habitat environmental conditions.

Effective plasmid delivery to a plasmid-free Bacillus natto strain by a conjugational transfer system

In this study, a Bacillus natto strain named NEST141 was constructed. The strain carries no plasmids and is an authentic proline auxotroph-a feature that confers effective selection conditions for plasmids transferred from a donor, such as B. subtilis 168, via a pLS20-based conjugational transfer system. We have provided a standard effective protocol for the delivery of plasmids larger than 50 kilobase pairs. These results indicate that the B. natto NEST141 strain can become a standard model, like B. subtilis 168, for extensive genetic engineering with diverse applications.

A New Variant of the aadE-sat4-aphA-3 Gene Cluster Found in a Conjugative Plasmid from a MDR Campylobacter jejuni Isolate

Campylobacter jejuni is a foodborne pathogen causing bacterial gastroenteritis, with the highest incidence reported in Europe. The prevalence of antibiotic resistance in C. jejuni, as well as in many other bacterial pathogens, has increased over the last few years. In this report, we describe the presence of a plasmid in a multi-drug-resistant C. jejuni strain isolated from a gastroenteritis patient. Mating experiments demonstrated the transference of this genetic element (pCjH01) among C. jejuni by plasmid conjugation. The pCjH01 plasmid was sequenced and assembled, revealing high similarity (97% identity) with pTet, a described tetracycline resistance encoding plasmid. pCjH01 (47.7 kb) is a mosaic plasmid composed of a pTet backbone that has acquired two discrete DNA regions. Remarkably, one of the acquired sequences carried an undescribed variant of the aadE-sat4-aphA-3 gene cluster, providing resistance to at least kanamycin and gentamycin. Aside from the antibiotic resistance genes, the cluster also carries genes coding for putative regulators, such as a sigma factor of the RNA polymerase and an antisigma factor. Homology searches suggest that Campylobacter exchanges genetic material with distant G-positive bacterial genera.

Conjugation Operons in Gram-Positive Bacteria with and without Antitermination Systems

Genes involved in the same cellular process are often clustered together in an operon whose expression is controlled by an upstream promoter. Generally, the activity of the promoter is strictly controlled. However, spurious transcription undermines this strict regulation, particularly affecting large operons. The negative effects of spurious transcription can be mitigated by the presence of multiple terminators inside the operon, in combination with an antitermination system. Antitermination systems modify the transcription elongation complexes and enable them to bypass terminators. Bacterial conjugation is the process by which a conjugative DNA element is transferred from a donor to a recipient cell. Conjugation involves many genes that are mostly organized in one or a few large operons. It has recently been shown that many conjugation operons present on plasmids replicating in Gram-positive bacteria possess a bipartite antitermination system that allows not only many terminators inside the conjugation operon to be bypassed, but also the differential expression of a subset of genes. Here, we show that some conjugation operons on plasmids belonging to the Inc18 family of Gram-positive broad host-range plasmids do not possess an antitermination system, suggesting that the absence of an antitermination system may have advantages. The possible (dis)advantages of conjugation operons possessing (or not) an antitermination system are discussed.

Multiple Layered Control of the Conjugation Process of the Bacillus subtilis Plasmid pLS20

Bacterial conjugation is the main horizontal gene transfer route responsible for the spread of antibiotic resistance, virulence and toxin genes. During conjugation, DNA is transferred from a donor to a recipient cell via a sophisticated channel connecting the two cells. Conjugation not only affects many different aspects of the plasmid and the host, ranging from the properties of the membrane and the cell surface of the donor, to other developmental processes such as competence, it probably also poses a burden on the donor cell due to the expression of the large number of genes involved in the conjugation process. Therefore, expression of the conjugation genes must be strictly controlled. Over the past decade, the regulation of the conjugation genes present on the conjugative Bacillus subtilis plasmid pLS20 has been studied using a variety of methods including genetic, biochemical, biophysical and structural approaches. This review focuses on the interplay between Rco_pLS20, Rap_pLS20 and Phr*_pLS20, the proteins that control the activity of the main conjugation promoter P_c located upstream of the conjugation operon. Proper expression of the conjugation genes requires the following two fundamental elements. First, conjugation is repressed by default and an intercellular quorum-signaling system is used to sense conditions favorable for conjugation. Second, different layers of regulation act together to repress the P_c promoter in a strict manner but allowing rapid activation. During conjugation, ssDNA is exported from the cell by a membrane-embedded DNA translocation machine. Another membrane-embedded DNA translocation machine imports ssDNA in competent cells. Evidences are reviewed indicating that conjugation and competence are probably mutually exclusive processes. Some of the questions that remain unanswered are discussed.

Reversible regulation of conjugation of Bacillus subtilis plasmid pLS20 by the quorum sensing peptide responsive anti-repressor RappLS20

Quorum sensing plays crucial roles in bacterial communication including in the process of conjugation, which has large economical and health-related impacts by spreading antibiotic resistance. The conjugative Bacillus subtilis plasmid pLS20 uses quorum sensing to determine when to activate the conjugation genes. The main conjugation promoter, Pc, is by default repressed by a regulator RcopLS20 involving DNA looping. A plasmid-encoded signalling peptide, Phr*pLS20, inactivates the anti-repressor of RcopLS20, named RappLS20, which belongs to the large group of RRNPP family of regulatory proteins. Here we show that DNA looping occurs through interactions between two RcopLS20 tetramers, each bound to an operator site. We determined the relative promoter strengths for all the promoters involved in synthesizing the regulatory proteins of the conjugation genes, and constructed an in vivo system uncoupling these regulatory genes to show that RappLS20 is sufficient for activating conjugation in vivo. We also show that RappLS20 actively detaches RcopLS20 from DNA by preferentially acting on the RcopLS20 molecules involved in DNA looping, resulting in sequestration but not inactivation of RcopLS20. Finally, results presented here in combination with our previous results show that activation of conjugation inhibits competence and competence development inhibits conjugation, indicating that both processes are mutually exclusive.

Group II intron as cold sensor for self-preservation and bacterial conjugation

Group II introns are self-splicing ribozymes and mobile genetic elements. Splicing is required for both expression of the interrupted host gene and intron retromobility. For the pRS01 plasmid-encoded Lactococcus lactis group II intron, Ll.LtrB, splicing enables expression of the intron's host relaxase protein. Relaxase, in turn, initiates horizontal transfer of the conjugative pRS01 plasmid and stimulates retrotransposition of the intron. Little is known about how splicing of bacterial group II introns is influenced by environmental conditions. Here, we show that low temperatures can inhibit Ll.LtrB intron splicing. Whereas autocatalysis is abolished in the cold, splicing is partially restored by the intron-encoded protein (IEP). Structure profiling reveals cold-induced disruptions of key tertiary interactions, suggesting that a kinetic trap prevents the intron RNA from assuming its native state. Interestingly, while reduced levels of transcription and splicing lead to a paucity of excised intron in the cold, levels of relaxase mRNA are maintained, partially due to diminished intron-mediated mRNA targeting, allowing intron spread by conjugal transfer. Taken together, this study demonstrates not only the intrinsic cold sensitivity of group II intron splicing and the role of the IEP for cold-stress adaptation, but also maintenance of horizontal plasmid and intron transfer under cold-shock.

Widespread Utilization of Peptide Communication in Phages Infecting Soil and Pathogenic Bacteria

Temperate phages can adopt either a lytic or lysogenic lifestyle within their host bacteria. It was recently shown that Bacillus-subtilis-infecting phages of the SPbeta group utilize a peptide-based communication system called arbitrium to coordinate the lysogeny decision. The occurrence of peptide-based communication systems among phages more broadly remains to be explored. Here, we uncover a wide array of peptide-based communication systems utilized by phages for lysogeny decisions. These arbitrium-like systems show diverse peptide codes and can be detected in numerous genetically distant phage types and conjugative elements. The pathogens Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis are commonly infected by arbitrium-carrying mobile elements, which often carry toxins essential for pathogenicity. Experiments with phages containing these arbitrium-like systems demonstrate their involvement in lysogeny decisions. Finally, our results suggest that the peptide-based decision is executed by an antisense RNA that controls the regulator of the lysogenic state.

Surface Exclusion Revisited: Function Related to Differential Expression of the Surface Exclusion System of Bacillus subtilis Plasmid pLS20

During conjugation a genetic element is transferred from a bacterial donor to a recipient cell via a connecting channel. It is the major route responsible for the spread of antibiotic resistance. Conjugative elements can contain exclusion system(s) that inhibit its transfer to a cell already harboring the element. Our limited knowledge on exclusion systems is mainly based on plasmids of Gram-negative bacteria. Here we studied the conjugative plasmid pLS20 of the Gram-positive Bacillus subtilis. We demonstrate that pLS20 contains an exclusion system and identified the single gene responsible for exclusion, named sespLS20 , which is embedded in the conjugation operon. SespLS20 is the founding member of a novel family of surface exclusion proteins encoded by conjugative elements of Gram-positive origin. We show that the extent of surface exclusion correlates with the level of sespLS20 expression, and that sespLS20 is expressed at basal low-levels in all donor cells but becomes highly expressed in conjugating cells. Accordingly, the transfer of pLS20 from a conjugation-primed donor cell to an un-primed or conjugation-primed donor is inhibited moderately and very efficiently, respectively. The consequences of this differential regulation, which appears to be a conserved feature of surface exclusion systems of Gram-positive and Gram-negative origin, are discussed.

Novel regulatory mechanism of establishment genes of conjugative plasmids

The principal route for dissemination of antibiotic resistance genes is conjugation by which a conjugative DNA element is transferred from a donor to a recipient cell. Conjugative elements contain genes that are important for their establishment in the new host, for instance by counteracting the host defense mechanisms acting against incoming foreign DNA. Little is known about these establishment genes and how they are regulated. Here, we deciphered the regulation mechanism of possible establishment genes of plasmid p576 from the Gram-positive bacterium Bacillus pumilus. Unlike the ssDNA promoters described for some conjugative plasmids, the four promoters of these p576 genes are repressed by a repressor protein, which we named Reg576. Reg576 also regulates its own expression. After transfer of the DNA, these genes are de-repressed for a period of time until sufficient Reg576 is synthesized to repress the promoters again. Complementary in vivo and in vitro analyses showed that different operator configurations in the promoter regions of these genes lead to different responses to Reg576. Each operator is bound with extreme cooperativity by two Reg576-dimers. The X-ray structure revealed that Reg576 has a Ribbon-Helix-Helix core and provided important insights into the high cooperativity of DNA recognition.

Regulation of Gram-Positive Conjugation

Type IV Secretion Systems (T4SSs) are membrane-spanning multiprotein complexes dedicated to protein secretion or conjugative DNA transport (conjugation systems) in bacteria. The prototype and best-characterized T4SS is that of the Gram-negative soil bacterium Agrobacterium tumefaciens. For Gram-positive bacteria, only conjugative T4SSs have been characterized in some biochemical, structural, and mechanistic details. These conjugation systems are predominantly encoded by self-transmissible plasmids but are also increasingly detected on integrative and conjugative elements (ICEs) and transposons. Here, we report regulatory details of conjugation systems from Enterococcus model plasmids pIP501 and pCF10, Bacillus plasmid pLS1, Clostridium plasmid pCW3, and staphylococcal plasmid pSK41. In addition, regulation of conjugative processes of ICEs (ICEBs1, ICESt1, ICESt3) by master regulators belonging to diverse repressor families will be discussed. A special focus of this review lies on the comparison of regulatory mechanisms executed by proteins belonging to the RRNPP family. These regulators share a common fold and govern several essential bacterial processes, including conjugative transfer.

Stable and efficient delivery of DNA to Bacillus subtilis (natto) using pLS20 conjugational transfer plasmids

Bacillus subtilis (natto) is generally regarded as a safe bacterium and used as a host for the production of several materials. However, genetic engineering of B. subtilis (natto) is not well established because of poor DNA delivery methods and the lack of a standard strain for the aim. Here, we developed a genetic delivery tool in B. subtilis (natto) using the pLS20 conjugational plasmid (65 kbp). Transmission of pLS20 from B. subtilis 168 to wild-type B. subtilis (natto) did not occur via established mating protocols. We isolated B. subtilis (natto) mutants showing dramatically increased recipient activity. Whole-genome sequence analyses revealed three common alterations: mutations in the restriction endonuclease gene and in the methyl-accepting chemotaxis protein gene, and a 43-kbp deletion at the genome replication termination locus. A representative strain named NEST116 was generated as the first B. subtilis (natto) strain suitable for exploring pLS20-based genetic engineering.

Group II intron inhibits conjugative relaxase expression in bacteria by mRNA targeting

Group II introns are mobile ribozymes that are rare in bacterial genomes, often cohabiting with various mobile elements, and seldom interrupting housekeeping genes. What accounts for this distribution has not been well understood. Here, we demonstrate that Ll.LtrB, the group II intron residing in a relaxase gene on a conjugative plasmid from Lactococcus lactis, inhibits its host gene expression and restrains the naturally cohabiting mobile element from conjugative horizontal transfer. We show that reduction in gene expression is mainly at the mRNA level, and results from the interaction between exon-binding sequences (EBSs) in the intron and intron-binding sequences (IBSs) in the mRNA. The spliced intron targets the relaxase mRNA and reopens ligated exons, causing major mRNA loss. Taken together, this study provides an explanation for the distribution and paucity of group II introns in bacteria, and suggests a potential force for those introns to evolve into spliceosomal introns.

Treatment with Cefotaxime Affects Expression of Conjugation Associated Proteins and Conjugation Transfer Frequency of an IncI1 Plasmid in Escherichia coli

Horizontal gene transfer (HGT) is the major mechanism responsible for spread of antibiotic resistance. Antibiotic treatment has been suggested to promote HGT, either by directly affecting the conjugation process itself or by selecting for conjugations subsequent to DNA transfer. However, recent research suggests that the effect of antibiotic treatment on plasmid conjugation frequencies, and hence the spread of resistance plasmids, may have been overestimated. We addressed the question by quantifying transfer proteins and conjugation frequencies of a blaCTX-M-1 encoding IncI1 resistance plasmid in Escherichia coli MG1655 in the presence and absence of therapeutically relevant concentrations of cefotaxime (CTX). Analysis of the proteome by iTRAQ labeling and liquid chromatography tandem mass spectrometry revealed that Tra proteins were significantly up-regulated in the presence of CTX. The up-regulation of the transfer machinery was confirmed at the transcriptional level for five selected genes. The CTX treatment did not cause induction of the SOS-response as revealed by absence of significantly regulated SOS associated proteins in the proteome and no significant up-regulation of recA and sfiA genes. The frequency of plasmid conjugation, measured in an antibiotic free environment, increased significantly when the donor was pre-grown in broth containing CTX compared to growth without this drug, regardless of whether blaCTX-M-1 was located on the plasmid or in trans on the chromosome. The results shows that antibiotic treatment can affect expression of a plasmid conjugation machinery and subsequent DNA transfer.

Intercellular communication and conjugation are mediated by ESX secretion systems in mycobacteria

Communal bacterial processes require intercellular communication mediated by secretion systems to coordinate appropriate molecular responses. Intercellular communication has not been described previously in mycobacteria. Here we show that the ESX secretion-system family member ESX-4 is essential for conjugal recipient activity in Mycobacterium smegmatis Transcription of esx4 genes in the recipient requires coculture with a donor strain and a functional ESX-1 apparatus in the recipient. Conversely, mutation of the donor ESX-1 apparatus amplifies the esx4 transcriptional response in the recipient. The effect of ESX-1 on esx4 transcription correlates with conjugal DNA transfer efficiencies. Our data show that intercellular communication via ESX-1 controls the expression of its evolutionary progenitor, ESX-4, to promote conjugation between mycobacteria.

Mechanisms of Conjugative Transfer and Type IV Secretion-Mediated Effector Transport in Gram-Positive Bacteria

Conjugative DNA transfer is the most important means to transfer antibiotic resistance genes and virulence determinants encoded by plasmids, integrative conjugative elements (ICE), and pathogenicity islands among bacteria. In gram-positive bacteria, there exist two types of conjugative systems, (i) type IV secretion system (T4SS)-dependent ones, like those encoded by the Enterococcus, Streptococcus, Staphylococcus, Bacillus, and Clostridia mobile genetic elements and (ii) T4SS-independent ones, as those found on Streptomyces plasmids. Interestingly, very recently, on the Streptococcus suis genome, the first gram-positive T4SS not only involved in conjugative DNA transfer but also in effector translocation to the host was detected. Although no T4SS core complex structure from gram-positive bacteria is available, several structures from T4SS protein key factors from Enterococcus and Clostridia plasmids have been solved. In this chapter, we summarize the current knowledge on the molecular mechanisms and structure-function relationships of the diverse conjugation machineries and emerging research needs focused on combatting infections and spread of multiple resistant gram-positive pathogens.

Plasmid transfer in biofilms: a perspective on limitations and opportunities

Biofilms dominate microbial life, and their importance for human health and the environment can no longer be dismissed. Nevertheless many of the processes governing this form of microbial growth are still poorly understood. This includes the horizontal exchange of genetic information, which is a major driver in bacterial evolution and rapid adaptation, exemplified by the alarming spread of multi-drug resistance among pathogens mediated by plasmids. Biofilms are often considered hot spot for horizontal gene transfer, yet several studies have shown that plasmid transfer is limited to the outer layers. On the basis of results from decades of research we analyse this paradox and discuss the mechanisms by which biofilm growth can promote the initial transfer of some plasmids, but also limit further plasmid invasion into the population or community. If we want to adequately promote or combat horizontal gene spread in biofilms, we need to gain better insight into the physicochemical and biological mechanisms that control this process.

Molecular and cellular factors control signal transduction via switchable allosteric modulator proteins (SAMPs)

Background

Rap proteins from Bacilli directly target response regulators of bacterial two-component systems and modulate their activity. Their effects are controlled by binding of signaling peptides to an allosteric site. Hence Raps exemplify a class of monomeric signaling receptors, which we call switchable allosteric modulator proteins (SAMPs). These proteins have potential applications in diverse biomedical and biotechnical settings, but a quantitative understanding of the impact of molecular and cellular factors on signal transduction is lacking. Here we introduce mathematical models that elucidate how signals are propagated though the network upon receptor stimulation and control the level of active response regulator.

Results

Based on a systematic parameter analysis of the models, we show that key features of the dose-response behavior at steady state are controlled either by the molecular properties of the modulator or the signaling context. In particular, we find that the biochemical activity (i.e. non-enzymatic vs. enzymatic) and allosteric properties of the modulator control the response amplitude. The Hill coefficient and the EC₅₀ are controlled in addition by the relative ligand affinities. By tuning receptor properties, either graded or more switch-like (memory-less) response functions can be fashioned. Furthermore, we show that other contextual factors (e.g. relative concentrations of network components and kinase activity) have a substantial impact on the response, and we predict that there exists a modulator concentration which is optimal for response amplitude.

Conclusion

We discuss data on Rap-Phr systems in B. subtilis to show how our models can contribute to an integrated view of SAMP signaling by combining biochemical, structural and physiological insights. Our results also suggest that SAMPs could be evolved or engineered to implement diverse response behaviors. However—without additional regulatory controls—they can generate rather variable cellular outputs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0274-3) contains supplementary material, which is available to authorized users.

Control of biofilm-associated infections by signaling molecules and nanoparticles

As the severe infections caused by resistant pathogens and biofilm embedded bacteria continue to emerge, alternative antimicrobial strategies could represent a solution. Recent studies support the development of molecular approaches (through signaling molecules) aiming to fight infections by modulating the virulence, behavior and formation of resistance structures such as biofilms. The utilization of such formulations would offer the advantage of reducing the selection of resistant isolates, since most of the proposed molecules do not interfere with the population fitness if utilized in low amounts. Despite the promising results, these therapies are delaying to be applied in the clinical context mainly because of the following: (i) limited knowledge regarding their long and medium term effect, (ii) specific properties that make most of these molecules difficult to be utilized in pharmacological formulations, (iii) low stability, (iv) difficulty to reach a target within the host body, and (v) limited availability. For reducing most of these disadvantages, nanotechnology seem to offer the best option through the development of nanostructured materials and nanoparticles able to improve the efficiency of molecular virulence modulators and novel antimicrobial compounds and to ensure their targeted delivery and controlled release.

Silencing quorum sensing and ICE mobility through antiactivation and ribosomal frameshifting

Mobile genetic elements run an evolutionary gauntlet to maintain their mobility in the face of selection against their selfish dissemination but, paradoxically, they can accelerate the adaptability of bacteria through the gene-transfer events that they facilitate. These temporally conflicting evolutionary forces have shaped exquisite regulation systems that silence mobility and maximize the competitive fitness of the host bacterium, but maintain the ability of the element to deliver itself to a new host should the opportunity arise. Here we review the excision regulation system of the Mesorhizobium loti symbiosis island ICEMlSym^R7A, a 502-kb integrative and conjugative element (ICE) capable of converting non-symbiotic mesorhizobia into plant symbionts. ICEMlSym^R7A excision is activated by quorum sensing, however, both quorum sensing and excision are strongly repressed in the vast majority of cells by dual-target antiactivation and programmed ribosomal-frameshifting mechanisms. We examine these recently discovered regulatory features under the light of natural selection and discuss common themes that can be drawn from recent developments in ICE biology.