Opine transport genes in the octopine (occ) and nopaline (noc) catabolic regions in Ti plasmids of Agrobacterium tumefaciens

Opportunistic Bacteria of Grapevine Crown Galls Are Equipped with the Genomic Repertoire for Opine Utilization

Young grapevines (Vitis vinifera) suffer and eventually can die from the crown gall disease caused by the plant pathogen Allorhizobium vitis (Rhizobiaceae). Virulent members of A. vitis harbor a tumor-inducing plasmid and induce formation of crown galls due to the oncogenes encoded on the transfer DNA. The expression of oncogenes in transformed host cells induces unregulated cell proliferation and metabolic and physiological changes. The crown gall produces opines uncommon to plants, which provide an important nutrient source for A. vitis harboring opine catabolism enzymes. Crown galls host a distinct bacterial community, and the mechanisms establishing a crown gall-specific bacterial community are currently unknown. Thus, we were interested in whether genes homologous to those of the tumor-inducing plasmid coexist in the genomes of the microbial species coexisting in crown galls. We isolated 8 bacterial strains from grapevine crown galls, sequenced their genomes, and tested their virulence and opine utilization ability in bioassays. In addition, the 8 genome sequences were compared with 34 published bacterial genomes, including closely related plant-associated bacteria not from crown galls. Homologous genes for virulence and opine anabolism were only present in the virulent Rhizobiaceae. In contrast, homologs of the opine catabolism genes were present in all strains including the nonvirulent members of the Rhizobiaceae and non-Rhizobiaceae. Gene neighborhood and sequence identity of the opine degradation cluster of virulent and nonvirulent strains together with the results of the opine utilization assay support the important role of opine utilization for cocolonization in crown galls, thereby shaping the crown gall community.

Comparative Transcriptome Analysis of Agrobacterium tumefaciens Reveals the Molecular Basis for the Recalcitrant Genetic Transformation of Camellia sinensis L

Tea (Camellia sinensis L.), an important economic crop, is recalcitrant to Agrobacterium-mediated transformation (AMT), which has seriously hindered the progress of molecular research on this species. The mechanisms leading to low efficiency of AMT in tea plants, related to the morphology, growth, and gene expression of Agrobacterium tumefaciens during tea-leaf explant infection, were compared to AMT of Nicotiana benthamiana leaves in the present work. Scanning electron microscopy (SEM) images showed that tea leaves induced significant morphological aberrations on bacterial cells and affected pathogen-plant attachment, the initial step of a successful AMT. RNA sequencing and transcriptomic analysis on Agrobacterium at 0, 3 and 4 days after leaf post-inoculation resulted in 762, 1923 and 1656 differentially expressed genes (DEGs) between the tea group and the tobacco group, respectively. The expressions of genes involved in bacterial fundamental metabolic processes, ATP-binding cassette (ABC) transporters, two-component systems (TCSs), secretion systems, and quorum sensing (QS) systems were severely affected in response to the tea-leaf phylloplane. Collectively, these results suggest that compounds in tea leaves, especially gamma-aminobutyrate (GABA) and catechins, interfered with plant-pathogen attachment, essential minerals (iron and potassium) acquisition, and quorum quenching (QQ) induction, which may have been major contributing factors to hinder AMT efficiency of the tea plant.

Evolutionary Relatedness and Classification of Tumor-Inducing and Opine-Catabolic Plasmids in Three Rhizobium rhizogenes Strains Isolated from the Same Crown Gall Tumor

Plasmids play a crucial role in the ecology of agrobacteria. In this study, we sequenced tumor-inducing (Ti) and opine-catabolic (OC) plasmids in three Rhizobium rhizogenes (Agrobacterium biovar 2) strains isolated from the same crown gall tumor on “Colt” cherry rootstock and conducted comparative genomic analyses. Tumorigenic strains C5.7 and C6.5 carry nopaline-type Ti plasmids pTiC5.7/pTiC6.5, whereas the nonpathogenic strain Colt5.8 carries the nopaline-type OC plasmid pOC-Colt5.8. Overall, comparative genomic analysis indicated that pTiC5.7/pTiC6.5 and related Ti plasmids described before (pTiC58 and pTi-SAKURA) originate from a common ancestor, although they have diverged during evolution. On the other hand, plasmid pOC-Colt5.8 was most closely related to the well-known OC plasmid pAtK84b; however, analysis suggested that they had different evolutionary histories and seem to share a more distant common ancestor. Although the reconstruction of the evolutionary history of Ti and OC plasmids is still speculative, we hypothesized that nopaline-type Ti plasmid might originate from the nopaline-type OC plasmid. Our results suggested that OC plasmids are widespread and closely associated with crown gall tumors. Finally, we proposed a thorough scheme for classification of Ti and OC plasmids that is based on separate comparative analysis of each functional element of the plasmid studied.

Niche Construction and Exploitation by Agrobacterium: How to Survive and Face Competition in Soil and Plant Habitats

Agrobacterium populations live in different habitats (bare soil, rhizosphere, host plants), and hence face different environmental constraints. They have evolved the capacity to exploit diverse resources and to escape plant defense and competition from other microbiota. By modifying the genome of their host, Agrobacterium populations exhibit the remarkable ability to construct and exploit the ecological niche of the plant tumors that they incite. This niche is characterized by the accumulation of specific, low molecular weight compounds termed opines that play a critical role in Agrobacterium 's lifestyle. We present and discuss the functions, advantages, and costs associated with this niche construction and exploitation.

Structural basis for high specificity of octopine binding in the plant pathogen Agrobacterium tumefaciens

Agrobacterium pathogens of octopine- and nopaline-types force host plants to produce either octopine or nopaline compounds, which they use as nutrients. Two Agrobacterium ABC-transporters and their cognate periplasmic binding proteins (PBPs) OccJ and NocT import octopine and nopaline/octopine, respectively. Here, we show that both octopine transport and degradation confer a selective advantage to octopine-type A. tumefaciens when it colonizes plants. We report the X-ray structures of the unliganded PBP OccJ and its complex with octopine as well as a structural comparison with NocT and the related PBP LAO from Salmonella enterica, which binds amino acids (lysine, arginine and ornithine). We investigated the specificity of OccJ, NocT and LAO using several ligands such as amino acids, octopine, nopaline and octopine analogues. OccJ displays a high selectivity and nanomolar range affinity for octopine. Altogether, the structural and affinity data allowed to define an octopine binding signature in PBPs and to construct a OccJ mutant impaired in octopine binding, a selective octopine-binding NocT and a non-selective octopine-binding LAO by changing one single residue in these PBPs. We proposed the PBP OccJ as a major trait in the ecological specialization of octopine-type Agrobacterium pathogens when they colonize and exploit the plant host.

Fitness costs restrict niche expansion by generalist niche-constructing pathogens

We investigated the molecular and ecological mechanisms involved in niche expansion, or generalism, versus specialization in sympatric plant pathogens. Nopaline-type and octopine-type Agrobacterium tumefaciens engineer distinct niches in their plant hosts that provide different nutrients: nopaline or octopine, respectively. Previous studies revealed that nopaline-type pathogens may expand their niche to also assimilate octopine in the presence of nopaline, but consequences of this phenomenon on pathogen dynamics in planta were not known. Here, we provided molecular insight into how the transport protein NocT can bind octopine as well as nopaline, contributing to niche expansion. We further showed that despite the ability for niche expansion, nopaline-type pathogens had no competitive advantage over octopine-type pathogens in co-infected plants. We also demonstrated that a single nucleotide polymorphism in the nocR gene was sufficient to allow octopine assimilation by nopaline-type strains even in absence of nopaline. The evolved nocR bacteria had higher fitness than their ancestor in octopine-rich transgenic plants but lower fitness in tumors induced by octopine-type pathogens. Overall, this work elucidates the specialization of A. tumefaciens to particular opine niches and explains why generalists do not always spread despite the advantage associated with broader nutritional niches.

Agrobacterium uses a unique ligand-binding mode for trapping opines and acquiring a competitive advantage in the niche construction on plant host

By modifying the nuclear genome of its host, the plant pathogen Agrobacterium tumefaciens induces the development of plant tumours in which it proliferates. The transformed plant tissues accumulate uncommon low molecular weight compounds called opines that are growth substrates for A. tumefaciens. In the pathogen-induced niche (the plant tumour), a selective advantage conferred by opine assimilation has been hypothesized, but not experimentally demonstrated. Here, using genetics and structural biology, we deciphered how the pathogen is able to bind opines and use them to efficiently compete in the plant tumour. We report high resolution X-ray structures of the periplasmic binding protein (PBP) NocT unliganded and liganded with the opine nopaline (a condensation product of arginine and α-ketoglurate) and its lactam derivative pyronopaline. NocT exhibited an affinity for pyronopaline (K_D of 0.6 µM) greater than that for nopaline (K_D of 3.7 µM). Although the binding-mode of the arginine part of nopaline/pyronopaline in NocT resembled that of arginine in other PBPs, affinity measurement by two different techniques showed that NocT did not bind arginine. In contrast, NocT presented specific residues such as M117 to stabilize the bound opines. NocT relatives that exhibit the nopaline/pyronopaline-binding mode were only found in genomes of the genus Agrobacterium. Transcriptomics and reverse genetics revealed that A. tumefaciens uses the same pathway for assimilating nopaline and pyronopaline. Fitness measurements showed that NocT is required for a competitive colonization of the plant tumour by A. tumefaciens. Moreover, even though the Ti-plasmid conjugal transfer was not regulated by nopaline, the competitive advantage gained by the nopaline-assimilating Ti-plasmid donors led to a preferential horizontal propagation of this Ti-plasmid amongst the agrobacteria colonizing the plant-tumour niche. This work provided structural and genetic evidences to support the niche construction paradigm in bacterial pathogens.

An ecological niche is defined, in a given environment, by the availability of nutritive resources, which can be specifically assimilated by certain living organisms to promote their proliferation. The bacterial pathogen Agrobacterium tumefaciens is able to engineer an ecological niche in the infected host via the transformation of the plant genome and diversion of the plant metabolism towards production of the opine nutrients. In this work, we quantified the selective advantage conferred to a member of the phytopathogenic species A. tumefaciens which is able to assimilate the opine nopaline. This opine is a condensate of arginine and α-ketoglurate that is produced both under linear and cyclic forms in the plant tumour environment. We further determined at the molecular and atomistic levels how A. tumefaciens is able to sense the nopaline molecules, and which metabolic pathways are activated in response. Overall, this work deciphered some key molecular events in the niche construction of the pathogen A. tumefaciens that is unique among living organisms and used to develop bioengineering tools.

Analysis of the Pantoea ananatis pan-genome reveals factors underlying its ability to colonize and interact with plant, insect and vertebrate hosts

Background

Pantoea ananatis is found in a wide range of natural environments, including water, soil, as part of the epi- and endophytic flora of various plant hosts, and in the insect gut. Some strains have proven effective as biological control agents and plant-growth promoters, while other strains have been implicated in diseases of a broad range of plant hosts and humans. By analysing the pan-genome of eight sequenced P. ananatis strains isolated from different sources we identified factors potentially underlying its ability to colonize and interact with hosts in both the plant and animal Kingdoms.

Results

The pan-genome of the eight compared P. ananatis strains consisted of a core genome comprised of 3,876 protein coding sequences (CDSs) and a sizeable accessory genome consisting of 1,690 CDSs. We estimate that ~106 unique CDSs would be added to the pan-genome with each additional P. ananatis genome sequenced in the future. The accessory fraction is derived mainly from integrated prophages and codes mostly for proteins of unknown function. Comparison of the translated CDSs on the P. ananatis pan-genome with the proteins encoded on all sequenced bacterial genomes currently available revealed that P. ananatis carries a number of CDSs with orthologs restricted to bacteria associated with distinct hosts, namely plant-, animal- and insect-associated bacteria. These CDSs encode proteins with putative roles in transport and metabolism of carbohydrate and amino acid substrates, adherence to host tissues, protection against plant and animal defense mechanisms and the biosynthesis of potential pathogenicity determinants including insecticidal peptides, phytotoxins and type VI secretion system effectors.

Conclusions

P. ananatis has an ‘open’ pan-genome typical of bacterial species that colonize several different environments. The pan-genome incorporates a large number of genes encoding proteins that may enable P. ananatis to colonize, persist in and potentially cause disease symptoms in a wide range of plant and animal hosts.

Electronic supplementary material

The online version of this article (doi: 10.1186/1471-2164-15-404) contains supplementary material, which is available to authorized users.

Concerted transfer of the virulence Ti plasmid and companion At plasmid in the Agrobacterium tumefaciens-induced plant tumour

The plant pathogen Agrobacterium tumefaciens C58 harbours three independent type IV secretion (T4SS) machineries. T4SST-DNA promotes the transfer of the T-DNA to host plant cells, provoking tumour development and accumulation of opines such as nopaline and agrocinopines. T4SSpTi and T4SSpAt control the bacterial conjugation of the Ti and At plasmids respectively. Expression of T4SSpTi is controlled by the agrocinopine-responsive transcriptional repressor AccR. In this work, we compared the genome-wide transcriptional profile of the wild-type A. tumefaciens strain C58 with that of its accR KO-mutant to delineate the AccR regulon. In addition to the genes that encode agrocinopine catabolism and T4SSpTi , we found that AccR also regulated genes coding for nopaline catabolism and T4SSpAt . Further opine detection and conjugation assays confirmed the enhancement of nopaline consumption and At plasmid conjugation frequency in accR. Moreover, co-regulation of the T4SSpTi and T4SSpAt correlated with the co-transfer of the At and Ti plasmids both in vitro and in plant tumours. Finally, unlike T4SSpTi , T4SSpAt activation does not require quorum-sensing. Overall this study highlights the regulatory interplays between opines, At and Ti plasmids that contribute to a concerted dissemination of the two replicons in bacterial populations colonizing the plant tumour.

Rhizosphere Bacterial Signalling: A Love Parade Beneath Our Feet

Plant roots support the growth and activities of a wide variety of microorganisms that may have a profound effect on the growth and/or health of plants. Among these microorganisms, a high diversity of bacteria have been identified and categorized as deleterious, beneficial, or neutral with respect to the plant. The beneficial bacteria, termed plant growth-promoting rhizobacteria (PGPR), are widely studied by microbiologists and agronomists because of their potential in plant production. Azospirillum, a genus of versatile PGPR, is able to enhance the plant growth and yield of a wide range of economically important crops in different soils and climatic regions. Plant beneficial effects of Azospirillum have mainly been attributed to the production of phytohormones, nitrate reduction, and nitrogen fixation, which have been subject of extensive research throughout the years. These elaborate studies made Azospirillum one of the best-characterized genera of PGPR. However, the genetic and molecular determinants involved in the initial interaction between Azospirillum and plant roots are not yet fully understood. This review will mainly highlight the current knowledge on Azospirillum plant root interactions, in the context of preceding and ongoing research on the association between plants and plant growth-promoting rhizobacteria.

Opine-based Agrobacterium competitiveness: dual expression control of the agrocinopine catabolism (acc) operon by agrocinopines and phosphate levels

Agrobacterium tumefaciens strain C58 can transform plant cells to produce and secrete the sugar-phosphate conjugate opines agrocinopines A and B. The bacterium then moves in response to the opines and utilizes them as exclusive sources of carbon, energy, and phosphate via the functions encoded by the acc operon. These privileged opine-involved activities contribute to the formation of agrobacterial niches in the environment. We found that the expression of the acc operon is induced by agrocinopines and also by limitation of phosphate. The main promoter is present in front of the first gene, accR, which codes for a repressor. This operon structure enables efficient repression when opine levels are low. The promoter contains two putative operators, one overlapping the -10 sequence and the other in the further upstream from it; two partly overlapped putative pho boxes between the two operators; and two consecutive transcription start sites. DNA fragments containing either of the operators bound purified repressor AccR in the absence of agrocinopines but not in the presence of the opines, demonstrating the on-off switch of the promoter. Induction of the acc operon can occur under low-phosphate conditions in the absence of agrocinopines and further increases when the opines also are present. Such opine-phosphate dual regulatory system of the operon may ensure maximum utilization of agrocinopines when available and thereby increase the chances of agrobacterial survival in the highly competitive environment with limited general food sources.

SocA is a novel periplasmic binding protein for fructosyl amino acid

Bacterial periplasmic proteins (bPBPs) undergo drastic conformational changes upon binding substrate, making them appealing as novel molecular recognition tools for biosensing. A putative bPBP-encoding gene, socA, belongs to the soc operon responsible for santhopine (fructosyl glutamine, FQ) catabolism of Agrobacterium tumefaciens. The socA gene was isolated and expressed in Escherichia coli as a soluble 28.8kDa periplasmic protein to investigate its properties as a potential bPBP for fructosyl amino acid (FA). The autofluorescence of SocA was used to monitor the protein's conformational change resulting from substrate binding. The fluorescence intensity changed upon binding FQ in a concentration dependent manner with a calculated K(d) of 2.1muM, but was unaffected by the presence of sugars or amino acid. Our results demonstrate that SocA is a novel FA bPBP that can be utilized as a novel molecular recognition element for the monitoring of FA.

Engineered rhizosphere: the trophic bias generated by opine-producing plants is independent of the opine type, the soil origin, and the plant species

In a previous study, we demonstrated that transgenic Lotus plants producing opines (which are small amino acid and sugar conjugates) specifically favor growth of opine-degrading rhizobacteria. The opine-induced bias was repeated and demonstrated with another soil type and another plant species (Solanum nigrum). This phenomenon is therefore independent of both soil type and plant species.

Two opines control conjugal transfer of an Agrobacterium plasmid by regulating expression of separate copies of the quorum-sensing activator gene traR

Conjugal transfer of Ti plasmids from Agrobacterium spp. is controlled by a hierarchical regulatory system designed to sense two environmental cues. One signal, a subset of the opines produced by crown gall tumors initiated on plants by the pathogen, serves to induce production of the second, an acyl-homoserine lactone quorum-sensing signal, the quormone, produced by the bacterium itself. This second signal activates TraR, and this transcriptional activator induces expression of the tra regulon. Opines control transfer because the traR gene is a member of an operon the expression of which is regulated by the conjugal opine. Among the Ti plasmid systems studied to date, only one of the two or more opine families produced by the associated tumor induces transfer. However, two chemically dissimilar opines, nopaline and agrocinopines A and B, induce transfer of the opine catabolic plasmid pAtK84b found in the nonpathogenic Agrobacterium radiobacter isolate K84. In this study we showed that this plasmid contains two copies of traR, and each is associated with a different opine-regulated operon. One copy, traR(noc), is the last gene of the nox operon and was induced by nopaline but not by agrocinopines A and B. Mutating traR(noc) abolished induction of transfer by nopaline but not by the agrocinopines. A mutation in ocd, an upstream gene of the nox operon, abolished utilization of nopaline and also induction of transfer by this opine. The second copy, traR(acc), is located in an operon of four genes and was induced by agrocinopines A and B but not by nopaline. Genetic analysis indicated that this gene is required for induction of transfer by agrocinopines A and B but not by nopaline. pAtK84b with mutations in both traR genes was not induced for transfer by either opine. However, expression of a traR gene in trans to this plasmid resulted in opine-independent transfer. The association of traR(noc) with nox is unique, but the operon containing traR(acc) is related to the arc operons of pTiC58 and pTiChry5, two Ti plasmids inducible for transfer by agrocinopines A-B and C-D, respectively. We conclude that pAtK84b codes for two independently functioning copies of traR, each regulated by a different opine, thus accounting for the activation of the transfer system of this plasmid by the two opine types.

Bacterial ABC transporters of amino acids

There are two subfamilies of ABC uptake systems for amino acids in bacteria, the polar amino acid transport family and the hydrophobic amino acid transport family. We consider the general properties of these families and we examine the specific transporters. Focusing on some of the best-studied ATP binding cassette transporters we also examine the mechanism of amino acid uptake, paying particular attention to the question of bidirectionality of solute movement.

Phylogenetic approaches to the identification and characterization of protein families and superfamilies

With the advent of megabase genome sequencing, the need for computational analyses increases exponentially. Sequencing errors must be corrected, encoded proteins must be identified, functions must be assigned to these proteins, and distant phylogenetic relationships must be recognized in order to maximize the yield of information obtainable from genome sequencing projects. Both the computer and the human brain have their limitations, but using them in combination, the biologist can vastly extend his or her analytic capabilities. Computer techniques can be used to estimate protein structure, function, biogenesis, and evolution. In this review, the application of available computer programs to several protein families, particularly transport, receptor, and transcriptional regulatory protein families, illustrate our current capabilities and limitations. Although some multidomain protein families are evolutionarily homogeneous, others have mosaic origins. Evidence concerning the nature and frequency of occurrence of domain shuffling, splicing, fusion, deletion, and duplication during evolution of specific protein families is evaluated. It is shown that specific families of enzymes, receptors, transport proteins, and transcriptional regulatory proteins share a common evolutionary origin, frequently diverging in function because of domain splicing and ligation. Some large families arose gradually over evolutionary time, whereas others developed suddenly, due to bursts of intragenic or intergenic (or both) duplication events occurring over relatively short periods of time. It is argued that energy coupling to transport was a late occurrence, superimposed on preexisting mechanisms of solute facilitation. It is also shown that several transport protein families have evolved independently of each other, employing different routes, at different times in evolutionary history, to give topologically similar transmembrane protein complexes.

Plant ABC transporters

The ATP binding cassette (ABC) superfamily is a large, ubiquitous and diverse group of proteins, most of which mediate transport across biological membranes. ABC transporters have been shown to function not only as ATP-dependent pumps, but also as ion channels and channel regulators. Whilst members of this gene family have been extensively characterised in mammalian and microbial systems, the study of plant ABC transporters is a relatively new field of investigation. Sequences of over 20 plant ABC proteins have been published and include homologues of P-glycoprotein, MRP, PDR5 and organellar transporters. At present, functions have been assigned to a small proportion of these genes and only the MRP subclass has been extensively characterised. This review aims to summarise literature relevant to the study of plant ABC transporters, to review methods of cloning, to discuss the utility of yeast and mammalian systems as models and to speculate on possible roles of uncharacterised ABC transporters in plants.

Mannopinic acid and agropinic acid catabolism region of the octopine-type Ti plasmid pTi15955

Octopine-type Ti plasmids such as pTi15955, pTiA6 and pTiR10 direct the catabolism of at least eight compounds called opines that are released from crown gall tumours. Four of these compounds are denoted mannityl opines, each of which possesses a D-mannityl substituent on the nitrogen atom of either glutamate or glutamine. We have analysed a 20 kb region of the Ti plasmid pTi15955 that is required for the catabolism of two such opines, mannopinic acid and agropinic acid. A total of 12 genes in four operons were identified by DNA sequence analysis. Transposons Tn5lacZ and MudK were used to mutagenize these genes and to create aga-lacZ and moa-lacZ translational fusions. The expression of all fusions was induced by agropinic acid and by mannopinic acid. One of these four operons encodes an agropinic acid permease, whereas a second one encodes a mannopinic acid permease. A third operon contains three genes encoding probable catabolic enzymes, two of which (AgaF and AgaG) are thought to convert agropinic acid to mannopinic acid, while the third (AgaE) probably converts mannopinic acid to mannose and glutamate. AgaE resembles a bacterial amino acid deaminase, whereas AgaF and AgaG resemble two bacterial proteins that together catabolize substituted hydantoins, whose chemical structure resembles that of agropinic acid. The remaining operon encoded the MoaR protein, a negative regulator of itself and of the other three operons.

Opine catabolic loci from Agrobacterium plasmids confer chemotaxis to their cognate substrates

Opines are carbon compounds produced by crown galls and hairy roots induced by Agrobacterium tumefaciens and A. rhizogenes, respectively. These novel condensation products of plant metabolic intermediates are utilized as nutritional sources by the Agrobacterium strains that induced the growths. Thus, opines are thought to favor the propagation of agrobacteria in the tumorsphere. Certain Agrobacterium strains were chemoattracted to opines. The chemotactic activities to octopine, to nopaline, to mannopine, and to agrocinopines A + B were dependent on the type of the Ti plasmid present in the bacterium. The determinants for chemotaxis to these opines were localized to the regions of the octopine- and nopaline-type Ti plasmids coding for transport and catabolism of that opine. An insertion in accA, which encodes the periplasmic binding protein for agrocinopines A + B, abolished chemotaxis while an insertion in accC, which encodes a component of the transport system, and an insertion in accF, which encodes a function required for agrocinopine catabolism, did not affect chemotaxis to this opine. Thus, transport and catabolism of these opines are not required for the chemotactic activity. Analyses of subclones of the acc region confirmed that accA is the only gene required from the Ti plasmid for chemotaxis to agrocinopines A + B.

Octopine-type Ti plasmids code for a mannopine-inducible dominant-negative allele of traR, the quorum-sensing activator that regulates Ti plasmid conjugal transfer

Conjugal transfer of Agrobacterium tumefaciens Ti plasmids is regulated by two hierarchical signalling systems. Transfer is dependent on a subset of opines produced by the plant tumours induced by the bacterium. Induction also requires an acyl-homoserine lactone signal, called AAI, that is produced by the bacteria themselves. AAI is the co-inducer for TraR, the transcriptional activator required for expression of the tra regulon. Octopine induces conjugation of the octopine-mannityl opine-type Ti plasmids by regulating the expression of traR via OccR, the octopine-dependent activator of the opine regulon. We have discovered a second traR-like gene, trlR, on the octopine-mannityl opine-type Ti plasmids pTi15955 and pTiR10. This gene is located in an operon coding for a mannopine transport system and is expressed as part of the mannityl opine regulon. Sequence analysis indicated that trlR is a frameshift allele of traR, and the resulting protein lacks the carboxy-terminal domain thought to constitute the DNA-binding region of TraR. Expression of trlR inhibited octopine-induced conjugation of pTi15955 and pTiR10 by suppressing the TraR-mediated transcription of the tra and trb operons. Although TrlR had no effect on the expression of traR, TraR activated the expression of trlR. Southern hybridizations indicated that several other Ti and opine-catabolic plasmids contain more than one copy of genes homologous to traR. We propose that trlR is a dominant negative allele of traR and that TrlR inhibits conjugation by forming inactive heteromultimers with TraR.

Characterization of the acc operon from the nopaline-type Ti plasmid pTiC58, which encodes utilization of agrocinopines A and B and susceptibility to agrocin 84

The acc locus from the Ti plasmid pTiC58 confers utilization of and chemotaxis toward agrocinopines A and B (A+B), as well as susceptibility to a highly specific antiagrobacterial antibiotic, agrocin 84. DNA sequence analyses revealed that acc is composed of eight open reading frames, accR and accA through accG. Previous work showed that accR encodes the repressor which regulates this locus, and accA codes for the periplasmic binding protein of the agrocinopine transport system (S. Beck Von Bodman, G. T. Hayman, and S. K. Farrand, Proc. Natl. Acad. Sci. USA 89:643-647, 1992; G. T. Hayman, S. Beck Von Bodman, H. Kim, P. Jiang, and S. K. Farrand, J. Bacteriol. 175:5575-5584, 1993). The predicted proteins from accA through accE, as a group, have homology to proteins that belong to the ABC-type transport system superfamily. The predicted product of accF is related to UgpQ of Escherichia coli, which is a glycerophosphoryl diester phosphodiesterase, and also to agrocinopine synthase coded for by acs located on the T-DNA. The translated product of accG is related to myoinositol 1 (or 4) monophosphatases from various eucaryotes. Analyses of insertion mutations showed that accA through accE are required for transport of both agrocin 84 and agrocinopines A+B, while accF and accG are required for utilization of the opines as the sole source of carbon. Mutations in accF or accG did not abolish transport of agrocin 84, although we observed slower removal of the antibiotic from the medium by the accF mutant compared to the wild type. However, the insertion mutation in accF abolished detectable uptake of agrocinopines A+B. A mutation in accG had no effect on transport of the opines. The accF mutant was not susceptible to agrocin 84 although it took up the antibiotic. This finding suggests that agrocin 84 is activated by AccF after being transported into the bacterial cell.

Identification and characterization of a basic cell surface-located protein from Lactobacillus fermentum BR11

Extraction of Lactobacillus fermentum BR11 cells with 5 M LiCl yielded a preparation containing a single predominant polypeptide with an apparent molecular mass of 32 kDa. A clone encoding an immunoreactive 32-kDa polypeptide was isolated from a pUC18 library of L. fermentum BR11 DNA by screening with an antiserum raised against whole cells of L. fermentum BR11. Sequence determination of the insert in the clone revealed a complete 795-bp open reading frame (ORF) that defines a 28,625-Da polypeptide (BspA). N-terminal sequencing of the LiCl-extracted polypeptide from L. fermentum BR11 confirmed that it is the same as the cloned BspA. BspA was found to have a sequence similar to those of family III of the bacterial solute-binding proteins. The sequences of two ORFs upstream of bspA are consistent with bspA being located in an operon encoding an ATP-binding cassette-type uptake system. Unusually, BspA contains no lipoprotein cleavage and attachment motif (LXXC), despite its origin in a gram-positive bacterium. Biotin labelling and trypsin digestion of whole cells indicated that this polypeptide is exposed on the cell surface. The isoelectric point as predicted from the putative mature sequence is 10.59. It was consequently hypothesized that the positively charged BspA is anchored by electrostatic interaction with acidic groups on the cell surface. It was shown that BspA could be selectively removed from the surface by extraction with an acidic buffer, thus supporting this hypothesis.

The Agrobacterium tumefaciens virulence gene chvE is part of a putative ABC-type sugar transport operon

The Agrobacterium tumefaciens virulence determinant ChvE is a periplasmic binding protein which participates in chemotaxis and virulence gene induction in response to monosaccharides which occur in the plant wound environment. The region downstream of the A. tumefaciens chvE gene was cloned and sequenced for nucleotide and expression analysis. Three open reading frames transcribed in the same direction as chvE were revealed. The first two, together with chvE, encode putative proteins of a periplasmic binding protein-dependent sugar uptake system, or ABC-type (ATP binding cassette) transporter. The third open reading frame encodes a protein of unknown function. The deduced transporter gene products are related on the amino acid level to bacterial sugar transporters and probably function in glucose and galactose uptake. We have named these genes gguA, -B, and -C, for glucose galactose uptake. Mutations in gguA, gguB, or gguC do not affect virulence of A. tumefaciens on Kalanchoe diagremontiana; growth on 1 mM galactose, glucose, xylose, ribose, arabinose, fucose, or sucrose; or chemotaxis toward glucose, galactose, xylose, or arabinose.

Transcriptional regulation and locations of Agrobacterium tumefaciens genes required for complete catabolism of octopine

By screening for octopine-inducible gene expression, we previously identified all the genes required for utilization of octopine as a source of carbon, nitrogen, and energy. They are (i) octopine oxidase, which converts octopine to arginine and pyruvate and is encoded by the ooxAB operon, (ii) arginase, which converts arginine to ornithine and urea and is encoded by arcA, (iii) ornithine cyclodeaminase, which converts ornithine to proline and ammonia and is encoded by the homologous arcB and ocd genes, and (iv) proline dehydrogenase, which converts proline to glutamate and is encoded by putA. Here we describe the regulation and localization of each of these genes. The ooxA-ooxB-ocd operon was previously shown to reside on the Ti plasmid and to be directly inducible by octopine. The arcAB operon is directly inducible by arginine, while it is induced by octopine only in strains that can convert octopine to arginine. Ornithine may also be a direct inducer of arcAB. putA is directly inducible by proline, while induction by octopine and by arginine (and probably by ornithine) requires their conversion to proline. Genetic studies indicate that arcAB and putA are localized on a conjugal genetic element. This element can be transferred to other Agrobacterium tumefaciens strains by a mechanism that does not require recA-dependent homologous recombination. Transfer of this genetic element from A. tumefaciens R10 requires at least one tra gene found on its Ti plasmid, indicating that this element is not self-transmissible but is mobilizable by the Ti plasmid. The DNA containing the arcAB and putA genes comigrates with a 243-kb linear molecular weight standard on field inversion electrophoretic gels.

Requirement for genes with homology to ABC transport systems for attachment and virulence of Agrobacterium tumefaciens

Transposon mutants of Agrobacterium tumefaciens which were avirulent and unable to attach to plant cells were isolated and described previously. A clone from a library of Agrobacterium tumefaciens DNA which was able to complement these chromosomal att mutants was identified. Tn3HoHo1 insertions in this clone were made and used to replace the wild-type genes in the bacterial chromosome by marker exchange. The resulting mutants were avirulent and showed either no or very much reduced attachment to carrot suspension culture cells. We sequenced a 10-kb region of this clone and found a putative operon containing nine open reading frames (ORFs) (attA1A2BCDEFGH). The second and third ORFs (attA2 and attB) showed homology to genes encoding the membrane-spanning proteins (potB and potH; potC and potI) of periplasmic binding protein-dependent (ABC) transport systems from gram-negative bacteria. The homology was strongest to proteins involved in the transport of spermidine and putrescine. The first and fifth ORFs (attA1 and attE) showed homology to the genes encoding ATP-binding proteins of these systems including potA, potG, and cysT from Escherichia coli; occP from A. tumefaciens; cysA from Synechococcus spp.; and ORF-C from an operon involved in the attachment of Campylobacte jejuni. The ability of mutants in these att genes to bind to host cells was restored by addition of conditioned medium during incubation of the bacteria with host cells.

Localization of OccR-activated and TraR-activated promoters that express two ABC-type permeases and the traR gene of Ti plasmid pTiR10

Conjugation of Agrobacterium tumefaciens wide-host-range octopine-type Ti plasmids is regulated by the LuxR-type transcriptional activator TraR in conjunction with an acylated homoserine lactone designated AAI. Expression of traR in octopine-type Ti plasmids is stimulated by OccR in response to octopine, an opine released from crown gall tumours, and is also positively autoregulated by TraR and AAI. Genetic and physical mapping of these promoters indicates that the OccR-activated promoter lies 14.5 kb upstream of traR, while the TraR-activated promoter lies 6 kb upstream. The upstream portion of the 14.5 kb operon contains seven previously characterized genes that direct the uptake and catabolism of octopine. The TraR-activated promoter lies just downstream from the octopine catabolic genes, and transcribes six genes in addition to traR, including five genes (ophABCDE) that show strong homology to oligo-peptide permeases of Salmonella typhimurium and Bacillus subtilis. Several TraR-regulated promoters overlap with 18 bp inverted repeats called tra boxes. In contrast, the traR autoregulatory promoter is not associated with a consensus tra box.

Identification of Agrobacterium tumefaciens genes that direct the complete catabolism of octopine

Agrobacterium tumefaciens R10 was mutagenized by using the promoter probe transposon Tn5-gusA7, and a library of approximately 5,000 transcriptional fusions was screened for octopine-inducible patterns of gene expression. Twenty-one mutants carrying strongly inducible gusA fusions, 20 of which showed defects in the catabolism of octopine or its metabolites, were obtained. One group of mutants could not use octopine as a carbon source, while a second group of mutants could not utilize arginine or ornithine and a third group could not utilize octopine, arginine, ornithine, or proline as a carbon source. Utilization of these compounds as nitrogen sources showed similar but not identical patterns. Fifteen fusions were subcloned together with adjacent DNA. Sequence analysis and further genetic analysis indicated that insertions of the first group are localized in the occ region of the Ti plasmid. Insertions of the second group were localized to a gene encoding ornithine cyclodeaminase. This gene is very similar to, but distinct from, a homolog located on the Ti plasmid. This gene is located immediately downstream from a gene encoding an arginase. Genetic experiments indicated that this arginase gene is essential for octopine and arginine catabolism. Insertions of the third group was localized to a gene whose product is required for degradation of proline. We therefore have identified all steps required for the catabolism of octopine to glutamate.

Ti plasmid-encoded octopine and nopaline catabolism in Agrobacterium: specificities of the LysR-type regulators OccR and NocR, and protein-induced DNA bending

The occ and noc regions in octopine and nopaline Ti plasmids, respectively, are responsible for the catabolism of octopine and nopaline in Agrobacterium. The functions are activated in the presence of the opines by OccR and NocR, two related regulatory proteins, and the promoters contain common sequence motifs. We have investigated heterologous interactions between the regulators and the promoters. Previous experiments using all possible heterologous combinations of opines, regulators, and promoters in vivo had demonstrated that only the combination of nopaline, NocR, and the occ promoter led to limited promoter activation. We now show that OccR and NocR bind to the heterologous promoters in vitro and in vivo. The weak or non-existent promoter activation actually observed could be explained by the assumption that OccR and NocR use different activation mechanisms; we investigated protein-induced DNA bending because of reports that the two regulators differ in this respect. Analysis with a bending vector showed that both OccR and NocR induced a DNA bend that is relaxed in the presence of the respective opine. The data suggest that subtle differences in regulator/promoter interactions are responsible for the inactivity of the heterologous combinations. Investigations with a chimeric NocR/OccR protein indicated that it induced a DNA bend in both promoters. No opine-induced relaxation was detectable with the hybrid, and the inducible promoter was not activated. These findings suggest that bend relaxation may be an integral part of promoter activation.

Convergent sensing pathways mediate response to two extracellular competence factors in Bacillus subtilis

Development of genetic competence in Bacillus subtilis is regulated by extracellular signaling molecules, including the ComX pheromone, a modified 9- or 10-amino-acid peptide. Here, we present characterization of a second extracellular competence stimulating factor (CSF). CSF appears to be, at least in part, a small peptide of between 520 and 720 daltons. Production of CSF requires several genes that are needed both for initiation of sporulation and development of competence (spo0H, spo0A, spo0B, and spo0F). Although both peptide factors regulate competence, two different sensing pathways mediate the response to the ComX pheromone and CSF. Analysis of double mutants indicated that ComX pheromone is on the same genetic pathway as the membrane-bound histidine protein kinase encoded by comP and that CSF is on the same genetic pathway as the oligopeptide permease encoded by spo0K. Furthermore, the cellular response to partly purified ComX pheromone requires the ComP histidine protein kinase, whereas the response to partly purified CSF requires the Spo0K oligopeptide permease. These two sensing pathways converge to activate competence genes. Both factors and their convergent sensing pathways are required for normal development of competence and might function to integrate different physiological signals.

Octopine and nopaline oxidases from Ti plasmids of Agrobacterium tumefaciens: molecular analysis, relationship, and functional characterization

The occ and noc regions of pTiAch5 (octopine) and pTiC58 (nopaline) Ti plasmids are responsible for the catabolic utilization of octopine and nopaline in Agrobacterium spp. The first enzymatic step is the oxidative cleavage into L-arginine and pyruvate or 2-ketoglutarate, respectively, by membrane-bound opine oxidases requiring two polypeptides (subunits B and A) for function. The DNA sequences showed that the subunits of pTiAch5 and pTiC58 are related, but none of the proteins revealed significant similarities to the biosynthetic enzymes expressed in transformed plant cells. The four proteins had no extensive overall similarity to other proteins, but the 35 N-terminal amino acids contained motifs found in many enzymes utilizing flavin adenine dinucleotide, flavin mononucleotide, or NAD(P)+ as cofactors. However, the activities were completely independent of added cofactors, and the nature of the electron acceptor remained unclear. Membrane solubilization led to complete loss of enzyme activity. The nopaline oxidase accepted nopaline and octopine (Vmax ratio, 5:1) with similar Km values (1.1 mM). The octopine oxidase had high activity with octopine (Km = 1 mM) and barely detectable activity with nopaline. The subunits from the occ and the noc regions were exchangeable. The combinations ooxB-noxA and noxB-ooxA both produced active enzymes which oxidized octopine and nopaline at similar rates, suggesting that both subunits contributed to the substrate specificity. These experiments also showed that the formation of functional enzyme required close proximity of the subunit genes on the same plasmid and that even a reversal of the gene order (A-B instead of B-A) led to reduced activity.

Five Listeria monocytogenes genes preferentially expressed in infected mammalian cells: plcA, purH, purD, pyrE and an arginine ABC transporter gene, arpJ

Listeria monocytogenes is a bacterial pathogen that multiplies within the cytosol of eukaryotic cells. To identify Listeria genes with preferentially intracellular expression (pic genes), a library of Tn917-lac insertion mutants was screened for transcriptional fusions to lacZ with higher expression inside a macrophage-like cell line than in a rich broth medium. Five pic genes with up to 100-fold induction inside cells were identified. Three of them (purH, purD and pyrE) were involved in nucleotide biosynthesis. One was part of an operon encoding an ABC (ATP-binding cassette) transporter for arginine. The corresponding mutants were not affected in intracellular growth, cell-to-cell spread or virulence, except for the transporter mutant, whose LD50 after intravenous infection of mice was twofold higher than the wild-type. The fifth gene was plcA, a previously identified virulence gene that encodes a phosphatidylinositol-phospholipase C, and is cotranscribed with prfA, a gene encoding a pleiotropic transcriptional activator of known virulence genes. Although plcA expression is known to depend on PrfA, a prfA promoter-lacZ fusion was highly expressed both inside and outside cells. Furthermore, in the presence of cellobiose, a disaccharide recently shown to repress plcA and hly expression, plcA and hly mRNA levels were dramatically reduced without any decrease in the monocistronic prfA mRNA levels. These results demonstrate that virulence gene activation does not depend only on prfA transcript accumulation.

Bacterial binding protein-dependent permeases: characterization of distinctive signatures for functionally related integral cytoplasmic membrane proteins

Bacterial binding protein-dependent transport systems belong to the superfamily of ABC transporters, which is widely distributed among living organisms. Their hydrophobic membrane proteins are the least characterized components. The primary structures of 61 integral membrane proteins from 35 uptake systems were compared in order to characterize a short conserved hydrophilic segment, with a consensus EAA---G---------I-LP, located approximately 100 residues from the C-terminus. Secondary structure predictions indicated that this conserved region might be formed by two amphipathic alpha-helices connected by a loop containing the invariant G residue. We classified the conserved motifs and found that membrane proteins from systems transporting structurally related substrates specifically display a greater number of identical residues in the conserved region. We determined a consensus for each class of membrane protein and showed that these can be considered as signatures.

Sequence relationships between integral inner membrane proteins of binding protein-dependent transport systems: evolution by recurrent gene duplications

Periplasmic binding protein-dependent transport systems are composed of a periplasmic substrate-binding protein, a set of 2 (sometimes 1) very hydrophobic integral membrane proteins, and 1 (sometimes 2) hydrophilic peripheral membrane protein that binds and hydrolyzes ATP. These systems are members of the superfamily of ABC transporters. We performed a molecular phylogenetic analysis of the sequences of 70 hydrophobic membrane proteins of these transport systems in order to investigate their evolutionary history. Proteins were grouped into 8 clusters. Within each cluster, protein sequences displayed significant similarities, suggesting that they derive from a common ancestor. Most clusters contained proteins from systems transporting analogous substrates such as monosaccharides, oligopeptides, or hydrophobic amino acids, but this was not a general rule. Proteins from diverse bacteria are found within each cluster, suggesting that the ancestors of current clusters were present before the divergence of bacterial groups. The phylogenetic trees computed for hydrophobic membrane proteins of these permeases are similar to those described for the periplasmic substrate-binding proteins. This result suggests that the genetic regions encoding binding protein-dependent permeases evolved as whole units. Based on the results of the classification of the proteins and on the reconstructed phylogenetic trees, we propose an evolutionary scheme for periplasmic permeases. According to this model, it is probable that these transport systems derive from an ancestral system having only 1 hydrophobic membrane protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Opine-regulated promoters and LysR-type regulators in the nopaline (noc) and octopine (occ) catabolic regions of Ti plasmids of Agrobacterium tumefaciens

Essential steps in the uptake and catabolism of the plant tumor metabolites nopaline and octopine in Agrobacterium spp. are performed by proteins encoded in the nopaline catabolic (noc) and octopine catabolic (occ) regions of Ti plasmids. We investigated the opine activation of the genes by using (i) promoter studies of Agrobacterium spp. and (ii) analysis of the promoter interaction with the regulatory proteins NocR (noc) and OccR (occ). The noc region contained two nopaline-induced promoters (Pi1[noc] and Pi2[noc]) and one autogenously regulated promoter (Pr [control of NocR expression]). Pi2 and Pr overlapped and were divergently oriented (Pi2 [noc]). DNA binding studies and DNase I footprints indicated that NocR bound specifically to single binding sites in Pi1[noc] and Pi2/Pr[noc] and that Pi2 and Pr were regulated from the same binding site. The binding was independent of the inducer nopaline, and nopaline caused small changes in the footprint. The promoters in the noc and occ regions shared sequence motif and contained the sequence T-N11-A, which is characteristic for LysR-type-regulated promoters. The occ region contained one octopine-induced and one autogenously regulated promoter (Pi/Pr[occ]) in the same arrangement as Pi2/Pr[noc] in the noc region. Promoter deletions indicated that sequences flanking the OccR binding site determined the extent of induction, although they did not bind OccR. The promoter bound OccR in the absence and presence of octopine. The opine caused a change in the mobility of the DNA-protein complex with the complete promoter. The resected fragments did not reveal this opine-induced shift, and it was also not detectable with the DNA-NocR complexes with the two promoters of the noc region.

Characterization of a nopaline-induced gene for a 40 kDa protein in the nopaline catabolic (noc) region of Ti plasmid pTiC58

The nopaline catabolic region (noc) in Ti plasmid pTiC58 codes for functions in the utilization of nopaline. The enzymes are identified, except for a 40 kDa protein (40k gene). The sequence reveals a polypeptide with no homology to other known proteins, except for similarities in two out of three motifs characteristic for D-isomer specific 2-hydroxyacid dehydrogenases.

Genetic analysis of the agrocinopine catabolic region of Agrobacterium tumefaciens Ti plasmid pTiC58, which encodes genes required for opine and agrocin 84 transport

The acc region, subcloned from pTiC58 of classical nopaline and agrocinopine A and B Agrobacterium tumefaciens C58, allowed agrobacteria to grow using agrocinopine B as the sole source of carbon and energy. acc is approximately 6 kb in size. It consists of at least five genes, accA through accE, as defined by complementation analysis using subcloned fragments and transposon insertion mutations of acc carried on different plasmids within the same cell. All five regions are required for agrocin 84 sensitivity, and at least four are required for agrocinopine and agrocin 84 uptake. The complementation results are consistent with the hypothesis that each of the five regions is separately transcribed. Maxicell experiments showed that the first of these genes, accA, encodes a 60-kDa protein. Analysis of osmotic shock fractions showed this protein to be located in the periplasm. The DNA sequence of the accA region revealed an open reading frame encoding a predicted polypeptide of 59,147 Da. The amino acid sequence encoded by this open reading frame is similar to the periplasmic binding proteins OppA and DppA of Escherichia coli and Salmonella typhimurium and OppA of Bacillus subtilis.

Structural, functional, and evolutionary relationships among extracellular solute-binding receptors of bacteria

Extracellular solute-binding proteins of bacteria serve as chemoreceptors, recognition constituents of transport systems, and initiators of signal transduction pathways. Over 50 sequenced periplasmic solute-binding proteins of gram-negative bacteria and homologous extracytoplasmic lipoproteins of gram-positive bacteria have been analyzed for sequence similarities, and their degrees of relatedness have been determined. Some of these proteins are homologous to cytoplasmic transcriptional regulatory proteins of bacteria; however, with the sole exception of the vitamin B12-binding protein of Escherichia coli, which is homologous to human glutathione peroxidase, they are not demonstrably homologous to any of the several thousand sequenced eukaryotic proteins. Most of these proteins fall into eight distinct clusters as follows. Cluster 1 solute-binding proteins are specific for malto-oligosaccharides, multiple oligosaccharides, glycerol 3-phosphate, and iron. Cluster 2 proteins are specific for galactose, ribose, arabinose, and multiple monosaccharides, and they are homologous to a number of transcriptional regulatory proteins including the lactose, galactose, and fructose repressors of E. coli. Cluster 3 proteins are specific for histidine, lysine-arginine-ornithine, glutamine, octopine, nopaline, and basic amino acids. Cluster 4 proteins are specific for leucine and leucine-isoleucine-valine, and they are homologous to the aliphatic amidase transcriptional repressor, AmiC, of Pseudomonas aeruginosa. Cluster 5 proteins are specific for dipeptides and oligopeptides as well as nickel. Cluster 6 proteins are specific for sulfate, thiosulfate, and possibly phosphate. Cluster 7 proteins are specific for dicarboxylates and tricarboxylates, but these two proteins exhibit insufficient sequence similarity to establish homology. Finally, cluster 8 proteins are specific for iron complexes and possibly vitamin B12. Members of each cluster of binding proteins exhibit greater sequence conservation in their N-terminal domains than in their C-terminal domains. Signature sequences for these eight protein families are presented. The results reveal that binding proteins specific for the same solute from different bacteria are generally more closely related to each other than are binding proteins specific for different solutes from the same organism, although exceptions exist. They also suggest that a requirement for high-affinity solute binding imposes severe structural constraints on a protein. The occurrence of two distinct classes of bacterial cytoplasmic repressor proteins which are homologous to two different clusters of periplasmic binding proteins suggests that the gene-splicing events which allowed functional conversion of these proteins with retention of domain structure have occurred repeatedly during evolutionary history.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of site-directed mutations in conserved domains of MalK, a bacterial member of the ATP-binding cassette (ABC) family [corrected]

Site-directed mutagenesis was used to change four amino acid residues (Q82, P152, L179, H192) in the MalK subunit of S. typhimurium maltose transport system which are highly conserved among members of the ATP-binding cassette (ABC) family. Replacement of H192 caused complete failure to complement the transport defect of a malK strain whereas changes of the other residues resulted in reduced or wild-type activity. The purified mutant proteins exhibited ATPase activity comparable to wild-type MalK.