Regulation of the Bacillus subtilis mannitol utilization genes: promoter structure and transcriptional activation by the wild-type regulator (MtlR) and its mutants

Signaling by the EPFL-ERECTA family coordinates female germline specification through the BZR1 family in Arabidopsis

In most flowering plants, the female germline is initiated in the subepidermal L2 layer of ovule primordia forming a single megaspore mother cell (MMC). How signaling from the L1 (epidermal) layer could contribute to the gene regulatory network (GRN) restricting MMC formation to a single cell is unclear. We show that EPIDERMAL PATTERNING FACTOR-like (EPFL) peptide ligands are expressed in the L1 layer, together with their ERECTA family (ERf) receptor kinases, to control female germline specification in Arabidopsis thaliana. EPFL-ERf dependent signaling restricts multiple subepidermal cells from acquiring MMC-like cell identity by activating the expression of the major brassinosteroid (BR) receptor kinase BRASSINOSTEROID INSENSITIVE 1 and the BR-responsive transcription factor BRASSINOZOLE RESISTANT 1 (BZR1). Additionally, BZR1 coordinates female germline specification by directly activating the expression of a nucleolar GTP-binding protein, NUCLEOSTEMIN-LIKE 1 (NSN1), which is expressed in early-stage ovules excluding the MMC. Mutants defective in this GRN form multiple MMCs resulting in a strong reduction of seed set. In conclusion, we uncovered a ligand/receptor-like kinase-mediated signaling pathway acting upstream and coordinating BR signaling via NSN1 to restrict MMC differentiation to a single subepidermal cell.

From Brown Seaweed to a Sustainable Microbial Feedstock for the Production of Riboflavin

The increasing global demand for food and energy production encourages the development of new production strategies focused on sustainability. Often, microbial bioprocesses rely on food or feed competitive feedstocks; hence, there is a trending need for green substrates. Here, we have proven the potential of brown seaweed biomass as microbial feedstock on account of its content of mannitol and the glucose polymer laminarin. Our host, Corynebacterium glutamicum, was engineered to enable access to mannitol as a carbon source through the heterologous expression of the mannitol-specific phosphotransferase system and the mannitol-1-phosphate-5-dehydrogenase from Bacillus subtilis. Overproduction of riboflavin was coupled with mannitol and glucose consumption via constitutive overexpression of the biosynthetic riboflavin operon ribGCAH from C. glutamicum. Brown seaweed extract and brown seaweed hydrolysate from Laminaria hyperborea, containing mannitol and glucose, were used as a carbon source for flask and bioreactor fermentations. In a seaweed-based fed-batch fermentation, the riboflavin final titer, yield, and volumetric productivity values of 1,291.2 mg L⁻¹, 66.1 mg g⁻¹, and 17.2 mg L⁻¹ h⁻¹, respectively, were achieved.

Regulation of Mannitol Metabolism in Enterococcus faecalis and Association with parEF0409 Toxin-Antitoxin Locus Function

The par_EF0409 type I toxin-antitoxin locus is situated between genes for two paralogous mannitol family phosphoenolpyruvate phosphotransferase systems (PTSs). In order to address the possibility that par_EF0409 function was associated with sugar metabolism, genetic and phenotypic analyses were performed on the flanking genes. It was found that the genes were transcribed as two operons: the downstream operon essential for mannitol transport and metabolism and the upstream operon performing a regulatory function. In addition to genes for the PTS components, the upstream operon harbors a gene similar to mtlR, the key regulator of mannitol metabolism in other Gram-positive bacteria. We confirmed that this gene is essential for the regulation of the downstream operon and identified putative phosphorylation sites required for carbon catabolite repression and mannitol-specific regulation. Genomic comparisons revealed that this dual-operon organization of mannitol utilization genes is uncommon in enterococci and that the association with a toxin-antitoxin system is unique to Enterococcus faecalis. Finally, we consider possible links between par_EF0409 function and mannitol utilization. IMPORTANCE Enterococcus faecalis is both a common member of the human gut microbiota and an opportunistic pathogen. Its evolutionary success is partially due to its metabolic flexibility, in particular its ability to import and metabolize a wide variety of sugars. While a large number of phosphoenolpyruvate phosphotransferase sugar transport systems have been identified in the E. faecalis genome bioinformatically, the specificity and regulation of most of these systems remain undetermined. Here, we characterize a complex system of two operons flanking a type I toxin-antitoxin system required for the transport and metabolism of the common dietary sugar mannitol. We also determine the phylogenetic distribution of mannitol utilization genes in the enterococcal genus and discuss the significance of the association with toxin-antitoxin systems.

CaSWC4 regulates the immunity-thermotolerance tradeoff by recruiting CabZIP63/CaWRKY40 to target genes and activating chromatin in pepper

Pepper (Capsicum annuum) responds differently to high temperature stress (HTS) and Ralstonia solanacearum infection (RSI) but employs some shared transcription factors (TFs), such as CabZIP63 and CaWRKY40, in both cases. How the plant activates and balances these distinct responses, however, was unclear. Here, we show that the protein CaSWC4 interacts with CaRUVBL2 and CaTAF14b and they all act positively in pepper response to RSI and thermotolerance. CaSWC4 activates chromatin of immunity or thermotolerance related target genes of CaWRKY40 or CabZIP63 by promoting deposition of H2A.Z, H3K9ac and H4K5ac, simultaneously recruits CabZIP63 and CaWRKY40 through physical interaction and brings them to their targets (immunity- or thermotolerance-related genes) via binding AT-rich DNA element. The above process relies on the recruitment of CaRUVBL2 and TAF14 by CaSWC4 via physical interaction, which occurs at loci of immunity related target genes only when the plants are challenged with RSI, and at loci of thermotolerance related target genes only upon HTS. Collectively, our data suggest that CaSWC4 regulates rapid, accurate responses to both RSI and HTS by modulating chromatin of specific target genes opening and recruiting the TFs, CaRUVBL2 and CaTAF14b to the specific target genes, thereby helping achieve the balance between immunity and thermotolerance.

CaAIL1 Acts Positively in Pepper Immunity against Ralstonia solanacearum by Repressing Negative Regulators

APETALA2 (AP2) subfamily transcription factors participate in plant growth and development, but their roles in plant immunity remain unclear. Here, we discovered that the AP2 transcription factor CaAIL1 functions in immunity against Ralstonia solanacearum infection (RSI) in pepper (Capsicum annuum). CaAIL1 expression was upregulated by RSI, and loss- and gain-of-function assays using virus-induced gene silencing and transient overexpression, respectively, revealed that CaAIL1 plays a positive role in immunity to RSI in pepper. Chromatin immunoprecipitation sequencing (ChIP-seq) uncovered a subset of transcription-factor-encoding genes, including CaRAP2-7, CaGATA17, CaGtf3a and CaTCF25, that were directly targeted by CaAIL1 via their cis-elements, such as GT or AGGCA motifs. ChIP-qPCR and electrophoretic mobility shift assays confirmed these findings. These genes, encoding transcription factors with negative roles in immunity, were repressed by CaAIL1 during pepper response to RSI, whereas genes encoding positive immune regulators such as CaEAS were derepressed by CaAIL1. Importantly, we showed that the atypical EAR motif (LXXLXXLXX) in CaAIL1 is indispensable for its function in immunity. These findings indicate that CaAIL1 enhances the immunity of pepper against RSI by repressing a subset of negative immune regulators during the RSI response through its binding to several cis-elements in their promoters.

PRD-Containing Virulence Regulators (PCVRs) in Pathogenic Bacteria

Bacterial pathogens rely on a complex network of regulatory proteins to adapt to hostile and nutrient-limiting host environments. The phosphoenolpyruvate phosphotransferase system (PTS) is a conserved pathway in bacteria that couples transport of sugars with phosphorylation to monitor host carbohydrate availability. A family of structurally homologous PTS-regulatory-domain-containing virulence regulators (PCVRs) has been recognized in divergent bacterial pathogens, including Streptococcus pyogenes Mga and Bacillus anthracis AtxA. These paradigm PCVRs undergo phosphorylation, potentially via the PTS, which impacts their dimerization and their activity. Recent work with predicted PCVRs from Streptococcus pneumoniae (MgaSpn) and Enterococcus faecalis (MafR) suggest they interact with DNA like nucleoid-associating proteins. Yet, Mga binds to promoter sequences as a homo-dimeric transcription factor, suggesting a bi-modal interaction with DNA. High-resolution crystal structures of 3 PCVRs have validated the domain structure, but also raised additional questions such as how ubiquitous are PCVRs, is PTS-mediated histidine phosphorylation via potential PCVRs widespread, do specific sugars signal through PCVRs, and do PCVRs interact with DNA both as transcription factors and nucleoid-associating proteins? Here, we will review known and putative PCVRs based on key domain and functional characteristics and consider their roles as both transcription factors and possibly chromatin-structuring proteins.

Deciphering the Regulation of the Mannitol Operon Paves the Way for Efficient Production of Mannitol in Lactococcus lactis

Lactococcus lactis has great potential for high-yield production of mannitol, which has not yet been fully realized. In this study, we characterize how the mannitol genes in L. lactis are organized and regulated and use this information to establish efficient mannitol production. Although the organization of the mannitol genes in L. lactis was similar to that in other Gram-positive bacteria, mtlF and mtlD, encoding the enzyme IIA component (EIIA^mtl) of the mannitol phosphotransferase system (PTS) and the mannitol-1-phosphate dehydrogenase, respectively, were separated by a transcriptional terminator, and the mannitol genes were found to be organized in two transcriptional units: an operon comprising mtlA, encoding the enzyme IIBC component (EIIBC^mtl) of the mannitol PTS, mtlR, encoding a transcriptional activator, and mtlF, as well as a separately expressed mtlD gene. The promoters driving expression of the two transcriptional units were somewhat similar, and both contained predicted catabolite responsive element (cre) genes. The presence of carbon catabolite repression was demonstrated and was shown to be relieved in stationary-phase cells. The transcriptional activator MtlR (mtlR), in some Gram-positive bacteria, is repressed by phosphorylation by EIIA^mtl, and when we knocked out mtlF, we indeed observed enhanced expression from the two promoters, which indicated that this mechanism was in place. Finally, by overexpressing the mtlD gene and using stationary-phase cells as biocatalysts, we attained 10.1 g/liter mannitol with a 55% yield, which, to the best of our knowledge, is the highest titer ever reported for L. lactis. Summing up, the results of our study should be useful for improving the mannitol-producing capacity of this important industrial organism. IMPORTANCE Lactococcus lactis is the most studied species of the lactic acid bacteria, and it is widely used in various food fermentations. To date, there have been several attempts to persuade L. lactis to produce mannitol, a sugar alcohol with important therapeutic and food applications. Until now, to achieve mannitol production in L. lactis with significant titer and yield, it has been necessary to introduce and express foreign genes, which precludes the use of such strains in foods, due to their recombinant status. In this study, we systematically characterize how the mannitol genes in L. lactis are regulated and demonstrate how this impacts mannitol production capability. We harnessed this information and managed to establish efficient mannitol production without introducing foreign genes.

ERECTA signaling regulates plant immune responses via chromatin-mediated promotion of WRKY33 binding to target genes

The signaling pathway mediated by the receptor-like kinase ERECTA (ER) plays important roles in plant immune responses, but the underlying mechanism is unclear. Genetic interactions between ER signaling and the chromatin remodeling complex SWR1 in the control of plant immune responses were studied. Electrophoretic mobility shift assay and yeast one-hybrid analysis were applied to identify ER-WRKY33 downstream components. Chromatin immunoprecipitation analyses were further investigated. In this study, we show that the chromatin remodeling complex SWR1 enhances resistance to the white mold fungus Sclerotinia sclerotiorum in Arabidopsis thaliana via a process mediated by ER signaling. We identify a series of WRKY33 target YODA DOWNSTREAM (YDD) genes and demonstrate that SWR1 and ER signaling are required to enrich H2A.Z histone variant and H3K4me3 histone modification at YDDs and the binding of WRKY33 to YDD promoters upon S. sclerotiorum infection. We also reveal that the binding of WRKY33 to YDD promoters in turn promotes the enrichment of H2A.Z and H3K4me3 at YDD genes, thereby forming a positive regulatory loop to activate YDDs expression. Our study reveals how H2A.Z, H3K4me3 and ER signaling mutually regulate YDDs gene expression upon pathogen infection, highlighting the critical role of chromatin structure in ER-signaling-mediated plant immune responses.

Ca14-3-3 Interacts With CaWRKY58 to Positively Modulate Pepper Response to Low-Phosphorus Starvation

Low-phosphorus stress (LPS) and pathogen attack are two important stresses frequently experienced by plants in their natural habitats, but how plant respond to them coordinately remains under-investigated. Here, we demonstrate that CaWRKY58, a known negative regulator of the pepper (Capsicum annuum) response to attack by Ralstonia solanacearum, is upregulated by LPS. Virus-induced gene silencing (VIGS) and overexpression of CaWRKY58 in Nicotiana benthamiana plants in combination with chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays (EMSA) demonstrated that CaWRKY58 positively regulates the response of pepper to LPS by directly targeting and regulating genes related to phosphorus-deficiency tolerance, including PHOSPHATE STARVATION RESPONSE1 (PHR1). Yeast two-hybrid assays revealed that CaWRKY58 interacts with a 14-3-3 protein (Ca14-3-3); this interaction was confirmed by pull-down, bimolecular fluorescence complementation (BiFC), and microscale thermophoresis (MST) assays. The interaction between Ca14-3-3 and CaWRKY58 enhanced the activation of PHR1 expression by CaWRKY58, but did not affect the expression of the immunity-related genes CaNPR1 and CaDEF1, which are negatively regulated by CaWRKY58 in pepper upon Ralstonia solanacearum inoculation. Collectively, our data indicate that CaWRKY58 negatively regulates immunity against Ralstonia solanacearum, but positively regulates tolerance to LPS and that Ca14-3-3 transcriptionally activates CaWRKY58 in response to LPS.

The LysR-Type Transcriptional Regulator YeeY Plays Important Roles in the Regulatory of Furazolidone Resistance in Aeromonas hydrophila

Aeromonas hydrophila is an aquatic pathogen of freshwater fish. The emergence of widespread antimicrobial-resistance strains of this pathogen has caused increasing rates of fish infections. Our previous research reported that A. hydrophila yeeY, a LysR-type transcriptional regulator (LTTR), negatively regulated furazolidone (FZ) resistance. Although, it’s intrinsic regulatory mechanism is still unclear. In this study, a data-independent acquisition (DIA) quantitative proteomics method was used to compare the differentially expressed proteins (DEPs) between the ΔyeeY and wild-type strain under FZ treatment. When compared to the control, a total of 594 DEPs were identified in ΔyeeY. Among which, 293 and 301 proteins were substantially increased and decreased in abundance, respectively. Bioinformatics analysis showed that several biological pathways such as the secretion system and protein transport were mainly involved in FZ resistance. Subsequently, the antibiotics susceptibility assays of several gene deletion strains identified from the proteomics results showed that YeeY may regulate some important genes such as cysD, AHA_2766, AHA_3195, and AHA_4275, which affects the FZ resistance in A. hydrophila. Furthermore, 34 antimicrobial resistance genes (ARGs) from the bacterial drug resistance gene database (CARD) were found to be directly or indirectly regulated by YeeY. A subsequent assay of several ARGs mutants showed that ΔAHA_3222 increased the susceptibility of A. hydrophila to FZ, while ΔcysN and ΔAHA_3753 decreased the susceptibility rate. Finally, the chromatin immunoprecipitation (ChIP) PCR and an electrophoretic mobility shift assay (EMSA) have revealed that the genes such as AHA_3222 and AHA_4275 were directly and transcriptionally regulated by YeeY. Taken together, our findings demonstrated that YeeY may participate in antimicrobial resistance of A. hydrophila to FZ, which provides a new target for the development of novel antimicrobial agents in the future.

CaCBL1 Acts as a Positive Regulator in Pepper Response to Ralstonia solanacearum

Bacterial wilt caused by Ralstonia solanacearum is an important disease of pepper (Capsicum annuum), an economically important solanaceous vegetable worldwide, in particular, under high temperature (HT) conditions. However, the molecular mechanism underlying pepper immunity against bacterial wilt remains poorly understood. Herein, CaCBL1, a putative calcineurin B-like protein, was functionally characterized in the pepper response to R. solanacearum inoculation (RSI) under HT (RSI/HT). CaCBL1 was significantly upregulated by RSI at room temperature (RSI/RT), HT, or RSI/HT. CaCBL1-GFP fused protein targeted to whole epidermal cells of Nicotiana benthamiana when transiently overexpressed. CaCBL1 silencing by virus-induced gene silencing significantly enhanced pepper susceptibility to RSI under RT or HT, while its transient overexpression triggered hypersensitive response mimic cell death and upregulation of immunity-associated marker genes, including CabZIP63, CaWRKY40, and CaCDPK15, the positive regulators in the pepper response to RSI or HT found in our previous studies. In addition, by chromatin immunoprecipitation PCR and electrophoretic mobility shift assay, CaCBL1 was found to be directly targeted by CaWRKY40, although not by CaWRKY27 or CaWRKY58, via the W-box-2 within its promoter, and its transcription was found to be downregulated by silencing of CaWRKY40 while it was enhanced by its transient overexpression. These results suggest that CaCBL1 acts as a positive regulator in pepper immunity against R. solanacearum infection, constituting a positive feedback loop with CaWRKY40.

The melREDCA Operon Encodes a Utilization System for the Raffinose Family of Oligosaccharides in Bacillus subtilis

Bacillus subtilis is a heterotrophic soil bacterium that hydrolyzes different polysaccharides mainly found in the decomposed plants. These carbohydrates are mainly cellulose, hemicellulose, and the raffinose family of oligosaccharides (RFOs). RFOs are soluble α-galactosides, such as raffinose, stachyose, and verbascose, that rank second only after sucrose in abundance. Genome sequencing and transcriptome analysis of B. subtilis indicated the presence of a putative α-galactosidase-encoding gene (melA) located in the msmRE-amyDC-melA operon. Characterization of the MelA protein showed that it is a strictly Mn²⁺- and NAD⁺-dependent α-galactosidase able to hydrolyze melibiose, raffinose, and stachyose. Transcription of the msmER-amyDC-melA operon is under control of a σ^A-type promoter located upstream of msmR (P _msmR ), which is negatively regulated by MsmR. The activity of P _msmR was induced in the presence of melibiose and raffinose. MsmR is a transcriptional repressor that binds to two binding sites at P _msmR located upstream of the -35 box and downstream of the transcriptional start site. MsmEX-AmyCD forms an ATP-binding cassette (ABC) transporter that probably transports melibiose into the cell. Since msmRE-amyDC-melA is a melibiose utilization system, we renamed the operon melREDCA IMPORTANCE Bacillus subtilis utilizes different polysaccharides produced by plants. These carbohydrates are primarily degraded by extracellular hydrolases, and the resulting oligo-, di-, and monosaccharides are transported into the cytosol via phosphoenolpyruvate-dependent phosphotransferase systems (PTS), major facilitator superfamily, and ATP-binding cassette (ABC) transporters. In this study, a new carbohydrate utilization system of B. subtilis responsible for the utilization of α-galactosides of the raffinose family of oligosaccharides (RFOs) was investigated. RFOs are synthesized from sucrose in plants and are mainly found in the storage organs of plant leaves. Our results revealed the modus operandi of a new carbohydrate utilization system in B. subtilis.

Cross Talk among Transporters of the Phosphoenolpyruvate-Dependent Phosphotransferase System in Bacillus subtilis

The phosphoenolpyruvate-dependent phosphotransferase system (PTS) is the main carbohydrate uptake system in Bacillus subtilis A typical PTS consists of two general proteins, enzyme I (EI) and a histidine-containing protein (HPr), as well as a specific carbohydrate transporter (or enzyme II [EII]), all of which transfer the phosphoryl group from phosphoenolpyruvate to the transported carbohydrate. The specific PTS transporters are formed by multidomain proteins or single-domain subunits. These domains are domain C (EIIC), the transmembrane channel for the carbohydrate transport; domain B (EIIB), the membrane-bound domain responsible for phosphorylation of the carbohydrate; and domain A (EIIA), the mediator between HPr(H15∼P) and EIIB. There are 16 PTS transporters in B. subtilis, 6 of which, i.e., NagP, MalP, MurP, TreP, SacP, and SacX, contain no EIIA domain. Deletion of the single-EIIA-containing transporters showed that there is cross talk between the noncognate EIIA and EIIB domains in PTS. By deletion of all EIIA-containing proteins, strain KM455 (ΔEIIA) was constructed, and the EIIA-containing proteins were individually introduced into the strain. In this way, the PTS transporters of the glucose family, namely, PtsG, GamP, and PtsA (also known as YpqE), enabled growth with maltose, N-acetylglucosamine, sucrose, or trehalose as the sole carbon source. Construction of TkmA-EIIA fusion proteins confirmed the probable interaction between the EIIAs of the glucose family of PTS transporters and the EIIA-deficient PTS transporters. Likewise, we have shown that SacX is mainly phosphorylated by PtsA and GamP. PtsG and GmuA were also able to phosphorylate SacX, albeit less well than GamP and PtsA.IMPORTANCE The phosphoenolpyruvate-dependent phosphotransferase system (PTS) not only is a carbohydrate uptake system in B. subtilis but also plays an important role in sensing the nutrient fluctuation in the medium. This sensing system enables the cells to respond to these fluctuations properly. The PTS transporters have a pivotal role in this sensing system since they are carbohydrate specific. In this study, we tried to understand the interactions among these transporters which revealed the cross talk among PTSs. Three PTS proteins, namely, PtsG (the specific transporter of glucose), GamP (the specific transporter of glucosamine), and PtsA (a cytoplasmic single-domain EIIA protein) were shown to play the major role in the interaction among the PTSs.

Sophisticated Regulation of Transcriptional Factors by the Bacterial Phosphoenolpyruvate: Sugar Phosphotransferase System

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) is a carbohydrate transport and phosphorylation system present in bacteria of all different phyla and in archaea. It is usually composed of three proteins or protein complexes, enzyme I, HPr, and enzyme II, which are phosphorylated at histidine or cysteine residues. However, in many bacteria, HPr can also be phosphorylated at a serine residue. The PTS not only functions as a carbohydrate transporter but also regulates numerous cellular processes either by phosphorylating its target proteins or by interacting with them in a phosphorylation-dependent manner. The target proteins can be catabolic enzymes, transporters, and signal transduction proteins but are most frequently transcriptional regulators. In this review, we will describe how PTS components interact with or phosphorylate proteins to regulate directly or indirectly the activity of transcriptional repressors, activators, or antiterminators. We will briefly summarize the well-studied mechanism of carbon catabolite repression in firmicutes, where the transcriptional regulator catabolite control protein A needs to interact with seryl-phosphorylated HPr in order to be functional. We will present new results related to transcriptional activators and antiterminators containing specific PTS regulation domains, which are the phosphorylation targets for three different types of PTS components. Moreover, we will discuss how the phosphorylation level of the PTS components precisely regulates the activity of target transcriptional regulators or antiterminators, with or without PTS regulation domain, and how the availability of PTS substrates and thus the metabolic status of the cell are connected with various cellular processes, such as biofilm formation or virulence of certain pathogens.

Trigger Enzymes: Coordination of Metabolism and Virulence Gene Expression

Virulence gene expression serves two main functions, growth in/on the host, and the acquisition of nutrients. Therefore, it is obvious that nutrient availability is important to control expression of virulence genes. In any cell, enzymes are the components that are best informed about the availability of their respective substrates and products. It is thus not surprising that bacteria have evolved a variety of strategies to employ this information in the control of gene expression. Enzymes that have a second (so-called moonlighting) function in the regulation of gene expression are collectively referred to as trigger enzymes. Trigger enzymes may have a second activity as a direct regulatory protein that can bind specific DNA or RNA targets under particular conditions or they may affect the activity of transcription factors by covalent modification or direct protein-protein interaction. In this chapter, we provide an overview on these mechanisms and discuss the relevance of trigger enzymes for virulence gene expression in bacterial pathogens.

The bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system: regulation by protein phosphorylation and phosphorylation-dependent protein-protein interactions

The bacterial phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) carries out both catalytic and regulatory functions. It catalyzes the transport and phosphorylation of a variety of sugars and sugar derivatives but also carries out numerous regulatory functions related to carbon, nitrogen, and phosphate metabolism, to chemotaxis, to potassium transport, and to the virulence of certain pathogens. For these different regulatory processes, the signal is provided by the phosphorylation state of the PTS components, which varies according to the availability of PTS substrates and the metabolic state of the cell. PEP acts as phosphoryl donor for enzyme I (EI), which, together with HPr and one of several EIIA and EIIB pairs, forms a phosphorylation cascade which allows phosphorylation of the cognate carbohydrate bound to the membrane-spanning EIIC. HPr of firmicutes and numerous proteobacteria is also phosphorylated in an ATP-dependent reaction catalyzed by the bifunctional HPr kinase/phosphorylase. PTS-mediated regulatory mechanisms are based either on direct phosphorylation of the target protein or on phosphorylation-dependent interactions. For regulation by PTS-mediated phosphorylation, the target proteins either acquired a PTS domain by fusing it to their N or C termini or integrated a specific, conserved PTS regulation domain (PRD) or, alternatively, developed their own specific sites for PTS-mediated phosphorylation. Protein-protein interactions can occur with either phosphorylated or unphosphorylated PTS components and can either stimulate or inhibit the function of the target proteins. This large variety of signal transduction mechanisms allows the PTS to regulate numerous proteins and to form a vast regulatory network responding to the phosphorylation state of various PTS components.