Electrotransformation of Xanthomonas campestris by RF DNA of filamentous phage phi Lf

Functional characterization and transcriptional analysis of degQ of Xanthomonas campestris pathovar campestris

High-temperature-requirement protein A (HtrA) family proteins play important roles in controlling protein quality and are recognized as virulence factors in numerous animal and human bacterial pathogens. The role of HtrA family proteins in plant pathogens remains largely unexplored. Here, we investigated the HtrA family protein, DegQ, in the crucifer black rot pathogen Xanthomonas campestris pathovar campestris (Xcc). DegQ is essential for bacterial attachment and full virulence of Xcc. Moreover, the degQ mutant strain showed increased sensitivity to heat treatment and sodium dodecyl sulfate. Expressing the intact degQ gene in trans in the degQ mutant could reverse the observed phenotypic changes. In addition, we demonstrated that the DegQ protein exhibited chaperone-like activity. Transcriptional analysis displayed that degQ expression was induced under heat treatment. Our results contribute to understanding the function and expression of DegQ of Xcc for the first time and provide a novel perspective about HtrA family proteins in plant pathogen.

Expression and function of clpS and clpA in Xanthomonas campestris pv. campestris

ATP-dependent proteases (FtsH, Lon, and Clp family proteins) are ubiquitous in bacteria and play essential roles in numerous regulatory cell processes. Xanthomonas campestris pv. campestris is a Gram-negative pathogen that can cause black rot diseases in crucifers. The genome of X. campestris pv. campestris has several clp genes, namely, clpS, clpA, clpX, clpP, clpQ, and clpY. Among these genes, only clpX and clpP is known to be required for pathogenicity. Here, we focused on two uncharacterized clp genes (clpS and clpA) that encode the adaptor (ClpS) and ATPase subunit (ClpA) of the ClpAP protease complex. Transcriptional analysis revealed that the expression of clpS and clpA was growth phase-dependent and affected by the growth temperature. The inactivation of clpA, but not of clpS, resulted in susceptibility to high temperature and attenuated virulence in the host plant. The altered phenotypes of the clpA mutant could be complemented in trans. Site-directed mutagenesis revealed that K223 and K504 were the amino acid residues critical for ClpA function in heat tolerance. The protein expression profile shown by the clpA mutant in response to heat stress was different from that exhibited by the wild type. In summary, we characterized two clp genes (clpS and clpA) by examining their expression profiles and functions in different processes, including stress tolerance and pathogenicity. We demonstrated that clpS and clpA were expressed in a temperature-dependent manner and that clpA was required for the survival at high temperature and full virulence of X. campestris pv. campestris. This work represents the first time that clpS and clpA were characterized in Xanthomonas.

The lolB gene in Xanthomonas campestris pv. campestris is required for bacterial attachment, stress tolerance, and virulence

BACKGROUND

Xanthomonas campestris pv. campestris (Xcc) is a Gram-negative bacterium that can cause black rot disease in crucifers. The lipoprotein outer membrane localization (Lol) system is involved in the lipoprotein sorting to the outer membrane. Although Xcc has a set of annotated lol genes, there is still little known about the physiological role in this phytopathogen. In this study, we aimed to characterize the role of LolB of Xcc in bacterial attachment, stress tolerance, and virulence.

RESULTS

To characterize the role of LolB, lolB mutant was constructed and phenotypic evaluation was performed. The lolB mutant revealed reductions in bacterial attachment, extracellular enzyme production, and virulence. Mutation of lolB also resulted in reduced tolerance to a myriad of stresses, including heat and a range of membrane-perturbing agents. Trans-complementation of lolB mutant with intact lolB gene reverted these altered phenotypes to the wild-type levels. From subsequent reporter assay and reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis, the expression of genes that encode the major extracellular enzymes and the stress-related proteins was reduced after lolB mutation.

CONCLUSIONS

The results in this work contribute to the functional understanding of lolB in Xanthomonas for the first time, and provide new insights into the function of lolB in bacteria.

Functional Characterization and Transcriptional Analysis of clpP of Xanthomonas campestris pv. campestris

The caseinolytic protease (Clp) system is essential for survival under stress conditions and for virulence in several pathogenic bacteria. Xanthomonas campestris pv. campestris (Xcc) is a plant pathogen which causes black rot disease in crucifers. In this study, the Xcc clpP gene which is annotated to encode the proteolytic core of Clp was characterized. Mutation of clpP resulted in susceptibility to high temperature and puromycin stresses. Site-directed mutagenesis revealed that S105, H130, and D179 are critical amino acid residues for ClpP function in puromycin tolerance. Inactivation of clpP also revealed an attenuation of virulence on the host plant and a reduction in the production of extracellular cellulase, mannanase, pectinase, and protease. The affected phenotypes of the clpP mutant could be complemented to wild-type levels by the intact clpP gene. Transcriptional analysis revealed that expression of clpP is induced under heat shock condition.

Functional characterization and proteomic analysis of lolA in Xanthomonas campestris pv. campestris

Background

The gram-negative Xanthomonas campestris pv. campestris is the pathogenic bacterium that causes black rot disease in crucifers. The virulence determinants of this bacterium include extracellular enzymes, exopolysaccharides, and biofilm formation. Here, one transposon mutant of X. campestris pv. campestris strain 17 that affects biofilm formation was isolated, and subsequent analyses led to the identification of the lolA gene, which encodes an outer membrane lipoprotein chaperone.

Results

The lolA mutant exhibited significant reductions in bacterial attachment, extracellular enzyme production, virulence, and tolerance in the presence of myriad membrane-perturbing agents. These phenotypic changes of the mutant could be complemented to the wild-type level through the intact lolA gene. Proteomic analysis revealed that 109 proteins were differentially expressed after lolA mutation. These differentially expressed proteins were categorized in various functional groups and were mainly associated with the membrane component, were involved in transport, and contained receptor activity. Through reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis, deletion of lolA was determined to have caused significantly reduced expression of genes that encode the major extracellular enzymes, the biofilm-related proteins, and the virulence-related proteins. The RT-qPCR analysis also indicated that the expression of several genes that encode putative outer membrane lipoproteins and TonB-dependent receptors was reduced after lolA mutation.

Conclusions

This is the first report to define the lolA gene as a virulence factor and to contribute to the functional understanding of, and provide new information concerning, the role of lolA in Xanthomonas. Furthermore, the results of this study provide and extend new insights into the function of lolA in bacteria.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1387-9) contains supplementary material, which is available to authorized users.

WxcX is involved in bacterial attachment and virulence in Xanthomonas campestris pv. campestris

Xanthomonas campestris pv. campestris (Xcc) is the causative agent of black rot in crucifers. Here, one EZ-Tn5 transposon mutant of Xcc, altered in bacterial attachment, was isolated. Further analysis revealed that the transposon was inserted in the wxcX gene (encodes a hypothetical protein) of the transposon mutant. Sequence analysis revealed that WxcX is highly conserved in Xanthomonas, but none has been characterized. In this study, it was indicated that mutation of wxcX resulted in enhanced bacterial attachment, reduced virulence on the host cabbage, and increased sensitivity to sodium dodecyl sulfate. The affected phenotypes of the wxcX mutant could be complemented to wild-type levels by the intact wxcX gene. Site-directed mutagenesis revealed that E408 and E411 are critical amino acid residues for WxcX function in bacterial attachment. Taken together, our results demonstrate the roles of wxcX in attachment, virulence, and tolerance to sodium dodecyl sulfate in Xanthomonas for the first time.

Characterization of IS26-composite transposons and multidrug resistance in conjugative plasmids from Enterobacter cloacae

SHV-12 is the most widespread resistance determinant of Enterobacter cloacae in Taiwan; however, blaSHV-12 has rarely been mobilized. Six multidrug-resistant E. cloacae isolates were collected. After conjugal transfer, plasmid profiling and analysis of incompatibility groups was performed to characterize the genetic context of blaSHV-12 -containing fragments. The presence of mobile genetic elements was demonstrated by PCR, cloning, sequencing and bioinformatics analyses. Four different β-lactamase genes (blaTEM-1 , blaSHV-12 , blaCTX-M-3 and/or blaCTX-M-14 ) were observed in the conjugative plasmids belonging to the IncHI2 (n = 4), IncI1 or IncP incompatibility groups. The IS26-blaSHV-12 -IS26 locus was located in five different genetic environments. A novel structural organization of a class 1 integron with the aac(6')-IIc cassette truncated by IS26 was identified in one isolate. Thus, blaSHV-12 was obtained from different plasmids through IS26-mediated homologous recombination. IS26 plays a vital role in the distribution of mobile resistance elements between different plasmids found in multidrug-resistant E. cloacae isolates.

Characterization of genes encoding proteins containing HD-related output domain in Xanthomonas campestris pv. campestris

The Gram-negative plant pathogen Xanthomonas campestris pv. campestris (Xcc) is the causative agent of black rot in crucifers. The production of Xcc virulence factors is regulated by Clp and RpfF. HD-related output domain (HDOD) is a protein domain of unknown biochemical function. The genome of Xcc encodes three proteins (GsmR, HdpA, and HdpB) with an HDOD. The GsmR has been reported to play a role in the general stress response and cell motility and its expression is positively regulated by Clp. Here, the function and transcription of hdpA and hdpB were characterized. Mutation of hdpA resulted in enhanced bacterial attachment. In addition, the expression of hdpA was positively regulated by RpfF but not by Clp, subject to catabolite repression and affected by several stress conditions. However, mutational analysis and reporter assay showed that hdpB had no effect on the production of a range of virulence factors and its expression was independent of Clp and RpfF. The results shown here not only extend the previous work on RpfF regulation to show that it influences the expression of hdpA in Xcc, but also expand knowledge of the function of the HDOD containing proteins in bacteria.

Functional characterization and transcriptome analysis reveal multiple roles for prc in the pathogenicity of the black rot pathogen Xanthomonas campestris pv. campestris

Gram-negative phytopathogenic Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot in crucifers. The ability of Xcc to incite this disease in plants depends on a number of factors, including exopolysaccharides, extracellular enzymes and biofilm production. In this study, transposon mutagenesis led to identification of the prc gene, encoding a tail-specific protease, which plays a role in Xcc pathogenesis. Mutation of prc resulted in decreased virulence, extracellular protease production and bacterial attachment, with restoration to the levels of wild type by the intact prc gene. From subsequent quantitative RT-PCR analysis and reporter assay, the major extracellular protease gene prt1, biofilm-related gene galE encoding a UDP-galactose 4-epimerase and two putative adhesin genes (yapH and XC_4290 encoding autotransporter-like protein H and hemagglutinin, respectively) were found to be reduced in the prc mutant. Results of transcriptome profiling of Xcc wild type and prc mutant by RNA sequencing (RNA-Seq) showed that mutation of prc in Xcc leads to alteration in the transcriptional levels (more than twofold) of 91 genes. These differentially expressed genes were associated with a wide range of biological functions such as carbohydrate transport and metabolism, cell wall/membrane biogenesis, posttranslational modification, protein turnover and chaperones, inorganic ion transport and metabolism and signal transduction mechanisms. The results of this study facilitate the functional understanding of and provide new information about the regulatory role of prc.

Functional characterization and transcriptional analysis of galE gene encoding a UDP-galactose 4-epimerase in Xanthomonas campestris pv. campestris

The Gram-negative plant pathogen Xanthomonas campestris pv. campestris (Xcc) is the causative agent of black rot in crucifers, a disease that causes tremendous agricultural loss. In this study, the Xcc galE gene was characterized. Sequence and mutational analysis demonstrated that the Xcc galE encodes a UDP-galactose 4-epimerase (EC 5.1.3.2), which catalyzes the interconversion of UDP-galactose and UDP-glucose. Alanine substitution of the putative catalytic residues (Ser124, Tyr147, and Lys151) of GalE caused loss of epimerase activity. Further study showed that the Xcc galE mutant had reduced biofilm formation ability. Furthermore, reporter assays revealed that galE transcription exhibits a distinct expression profile under different culture conditions, is subject to catabolite repression, and is positively regulated by Clp and RpfF. In addition, the galE transcription initiation site was mapped. This is the first time that UDP-galactose 4-epimerase has been characterized in the crucifer pathogen Xcc.

GsmR, a response regulator with an HD-related output domain in Xanthomonas campestris, is positively controlled by Clp and is involved in the expression of genes responsible for flagellum synthesis

In prokaryotes, two-component signal transduction systems, consisting of a histidine kinase and a response regulator, play a critical role in regulating a range of cellular functions. A recent study suggests that XCC3315, a response regulator with a CheY-like receiver domain attached to an uncharacterized HD-related output domain (HDOD domain), plays a role in the general stress response of the Gram-negative bacterium Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot in cruciferous plants. Here, we demonstrated genetically that XCC3315, designated as gsmR (general stress and motility regulator), is involved in the expression of genes responsible for flagellum synthesis, including rpoN2, flhF, flhB, and fliC. Site-directed mutagenesis revealed that Glu9 and Arg100 in the receiver domain and Gly205, Asp263, His287, Trp298 and His311 in the HDOD are critical amino acids for GsmR function in cell motility regulation. The gsmR transcription initiation site was mapped. Promoter analysis and gel retardation assay revealed that the expression of gsmR is positively controlled by the global transcriptional regulator Clp in a direct manner, and is subject to catabolite repression. Our findings not only extend the previous work on Clp regulation to show that it influences the expression of gsmR in Xcc, but are also the first to characterize the expression of this response regulator gene in this phytopathogen. Furthermore, GsmR is the first HDOD-containing protein of bacteria in which key amino acids have been experimentally identified and characterized.

Contribution of Phe-7 to Tat-dependent export of β-lactamase in Xanthomonas campestris

Strains of Xanthomonas campestris pv. campestris isolated in Taiwan are commonly resistant to ampicillin owing to the constitutive expression of a chromosomally encoded β-lactamase that is secreted into the periplasm. In this study, we found that levels of β-lactamase vary among X. campestris pv. campestris strains, a difference that can be attributed to amino acid substitutions at least at positions 7 and 206, with the former having the major impact. Bioinformatic and PCR analyses indicated that X. campestris pv. campestris possesses tatABC genes and that the signal peptide of X. campestris pv. campestris pre-Bla contains the typical twin-arginine motif (N-R-R-Q-F-L at amino acid residues 3 to 8 in strain X. campestris pv. campestris strain 11), suggesting that Bla is secreted via the Tat pathway. To assess the importance of Phe(7) in the efficient export of X. campestris pv. campestris Bla, we prepared mutant constructs containing amino acid substitutions and monitored their expression by measuring enzyme activity and detecting Bla protein by Western blotting. The results indicate that replacement of Phe(7) with Leu severely inhibited Bla export whereas replacement with Pro almost abolished it. Although a change to Arg caused moderate inhibition of export, replacement with Tyr had no effect. These results suggest that for efficient export of Bla by X. campestris pv. campestris, the aromatic-aromatic interactions and stability of protein structure around the twin-arginine motif are important, since only proteins that can attain a folded state in the cytoplasm are competent for export via the Tat pathway.

Transcriptional analysis and functional characterization of XCC1294 gene encoding a GGDEF domain protein in Xanthomonas campestris pv. campestris

The nucleotide cyclic di-GMP is a second messenger in bacteria that regulates a range of cellular functions including the virulence of pathogens. GGDEF is a protein domain involved in the synthesis of cyclic di-GMP. The genome of the crucifer pathogen Xanthomonas campestris pv. campestris (Xcc) encodes 21 proteins with a GGDEF domain. Clp, a homolog of the model transcription factor Crp of Escherichia coli, is a global regulator in Xcc. The aim of this study is to identify genes encoding GGDEF domain proteins whose expression is regulated by Clp. Results of reporter assay and RT-PCR analysis suggested that Clp regulates the expression of a set of genes encoding proteins harboring GGDEF domain. The transcription initiation site of XCC1294, one of the Clp regulated gene encoding a GGDEF domain protein, was mapped. Promoter analysis and gel retardation assay indicated that the transcription of XCC1294 is positively and directly regulated by Clp. Furthermore, transcription of XCC1294 was subject to catabolite repression and affected by several stress conditions. We also showed that mutation of XCC1294 results in enhanced surface attachment. In addition, transcription of three putative adhesin genes (xadA, fhaC, and yapH) was increased in the XCC1294 mutant. Taken together, the data presented here indicate that Clp positively regulates expression of XCC1294, and that XCC1294 serves a regulator of bacterial attachment and regulates different adhesin genes expression.

Functional characterization of copA gene encoding multicopper oxidase in Xanthomonas campestris pv. campestris

The gram-negative plant pathogenic Xanthomonas campestris pv. campestris (Xcc) is the causative agent of black rot in crucifers, a disease causing tremendous loss in agriculture. Copper-containing bactericides have been widely used to control this disease for many years, possibly leading to the development of copper resistance in Xcc. Homologues of copper resistance genes copLAB are present in the Xcc genome, but none has been characterized. In this study, mutations in copL, copA, and copB decreased Xcc copper tolerance. Among them, the copA mutant displayed the most significant reduction. The copA mutant also resulted in a reduction in virulence on the host cabbage. Sequence and mutational analysis demonstrated that copA encodes a multicopper oxidase and that CopA is able to catalyze the oxidation of 2,6-dimethoxyphenol. Alanine substitutions in each of the putative copper binding residues (H538, H583, C584, and H585) of CopA caused a loss of function including copper tolerance and oxidase activity. Furthermore, reporter assays showed that copA transcription is inducible in the presence of copper, subject to catabolite repression, and repressed under conditions of high osmolarity, nitrogen starvation, or oxygen limitation. This is the first time that multicopper oxidase has been characterized in the crucifer pathogen Xcc.

XCC2731, a GGDEF domain protein in Xanthomonas campestris, is involved in bacterial attachment and is positively regulated by Clp

In Xanthomonas campestris pv. campestris (Xcc), which is the causative agent of black rot in crucifers, the virulence factor level is substantially decreased in the mutant deficient in RpfG, a phosphodiesterase that degrades the second messenger cyclic di-GMP. The rpfG mutant also grew in an aggregated state. It is indicated that expression of Pseudomonas GGDEF domain protein WspR (a diguanylate cyclase that synthesizes cyclic di-GMP) in wild-type Xcc can produce a phenocopy of the rpfG mutant. In this study, we showed that over-expression of GGDEF domain protein XCC2731 in wild-type Xcc caused (i) aggregation of cells, (ii) reduction in motility, and (iii) decrease in production of virulence factor extracellular enzymes and exopolysaccharides. Site-directed mutagenesis of the conserved G, G, and E residues of the GGDEF domain in XCC2731 abolished its function. The XCC2731 mutant has attenuated virulence. Furthermore, XCC2731 mutant was affected in surface attachment. Using the 5' RACE method, the XCC2731 transcription initiation site was mapped at nucleotide G, 15nt upstream of the XCC2731 start codon. Transcriptional fusion assay and gel retardation analysis indicated that Clp (cAMP receptor protein-like protein) positively regulates XCC2731 transcription in a direct manner. Reporter analysis also revealed that XCC2731 transcription is subject to catabolite repression, and reduced under conditions of oxygen limitation and high osmolarity. Our findings not only extend previous work on Clp regulation to show that it influences the expression of XCC2731 in Xcc, but also are the first to characterize the GGDEF domain protein gene expression in this phytopathogen.

Transcriptional analysis of pmeA gene encoding a pectin methylesterase in Xanthomonas campestris pv. campestris

Exopolysaccharides and several extracellular enzymes of Xanthomonas campestris pv. campestris (Xcc), the causative agent of black rot in crucifers, are virulence determinants. It is known that Clp (cAMP receptor protein-like protein) and RpfF (an enoyl-CoA hydratase homologue required for the synthesis of the diffusible signal factor, DSF) regulate production of these factors. In this study, plate assay revealed that Xcc possesses pectin methylesterase activity and that its expression is controlled by Clp and RpfF. Mutational analysis has demonstrated that pmeA encodes a pectin methylesterase. Using the 5' RACE method, the pmeA transcription initiation site was mapped. Transcriptional fusion assays showed that pmeA transcription is positively regulated by Clp and RpfF, subject to catabolite repression which is independent of Clp or RpfF, and repressed under conditions of high osmolarity or oxygen limitation. This study not only extends previous work on Clp and RpfF regulation by showing that they both influence the expression of pmeA in Xcc, but also, for the first time, characterizes pectin methylesterase gene expression in Xanthomonas.

Characterization of Xanthomonas campestris pv. campestris heat shock protein A (HspA), which possesses an intrinsic ability to reactivate inactivated proteins

hspA encodes a small heat shock protein (sHSP) in Xanthomonas campestris pv. campestris, the causative agent of black rot in cruciferous plants. In this study, two-dimensional gel electrophoresis, promoter activity assays, and Northern hybridization results revealed that HspA expression was induced by heat shock but not by other stresses, although low-level expression was detectable by reverse transcription-polymerase chain reaction (RT-PCR) under normal culture conditions. An hspA mutant exhibited reduced tolerance to heat, especially in the presence of MgSO4, but no change in pathogenicity. Results of size-exclusion chromatography indicated that purified HspA(his), containing six C-terminal histidine residues, formed two different size classes of oligomeric complexes--410 and 820 kDa. In contrast, HspA(ter), the unmodified protein translated from the original hspA gene, formed only the 820-kDa complex. These results suggest that the C-terminus of HspA is important for oligomerization. Both HspA820(his) and HspA410(his) were able to partially protect luciferase against heat-induced aggregation. Unlike other reported sHSPs that commonly capture denaturing proteins in refoldable states until refolded by adenosine triphosphate-dependent chaperone systems, HspA(his) alone was capable of reactivating heat-inactivated EcoRI. Thus, Xanthomonas campestris pv. campestris HspA has potential application as a protective agent during the storage of proteins.

Mutation of the gene encoding a major outer-membrane protein in Xanthomonas campestris pv. campestris causes pleiotropic effects, including loss of pathogenicity

Xanthomonas campestris pv. campestris (Xcc) is the phytopathogen that causes black rot in crucifers. The xanthan polysaccharide and extracellular enzymes produced by this organism are virulence factors, the expression of which is upregulated by Clp (CRP-like protein) and DSF (diffusible signal factor), which is synthesized by RpfF. It is also known that biofilm formation/dispersal, regulated by the effect of controlled synthesis of DSF on cell-cell signalling, is required for virulence. Furthermore, a deficiency in DSF causes cell aggregation with concomitant production of a gum-like substance that can be dispersed by addition of DSF or digested by exogenous endo-beta-1,4-mannanase expressed by Xcc. In this study, Western blotting of proteins from a mopB mutant (XcMopB) showed Xcc MopB to be the major outer-membrane protein (OMP); Xcc MopB shared over 97 % identity with homologues from other members of Xanthomonas. Similarly to the rpfF mutant, XcMopB formed aggregates with simultaneous production of a gummy substance, but these aggregates could not be dispersed by DSF or endo-beta-1,4-mannanase, indicating that different mechanisms were involved in aggregation. In addition, XcMopB showed surface deformation, altered OMP composition, impaired xanthan production, increased sensitivity to stressful conditions including SDS, elevated temperature and changes in pH, reduced adhesion and motility and defects in pathogenesis. The finding that the major OMP is required for pathogenicity is unprecedented in phytopathogenic bacteria.

Transcriptional regulation and molecular characterization of the manA gene encoding the biofilm dispersing enzyme mannan endo-1,4-beta-mannosidase in Xanthomonas campestris

Exopolysaccharide and several extracellular enzymes of Xanthomonas campestris pv. campestris (Xcc), the causative agent of black rot in crucifers, are important virulence determinants. It is known that Clp (cAMP receptor protein-like protein) and RpfF (an enoyl-CoA hydratase homologue required for the synthesis of diffusible signal factor, DSF) regulate the production of these determinants. Addition of DSF or Xcc extracellular protein containing partially purified mannanase (EC 3.2.1.78, encoded by manA) can disperse the cell aggregates formed by rpfF mutant. In this study, nucleotide G 64 nt upstream of the manA translation start codon was determined as the transcription initiation site by the 5' RACE technique. Transcriptional fusion assays showed that manA transcription is positively regulated by Clp and RpfF and induced by locust bean gum. The manA coding region was cloned and expressed in E. coli as recombinant ManA (rManA). The rManA was purified by affinity chromatography, and its biochemical properties were characterized. The rManA had a pH optimum at 7.0 (0.1 M Hepes) and a temperature optimum at about 37 degrees C. Sequence and mutational analyses demonstrated that Xcc manA encodes the major mannanase, a member of family 5 of glycosyl hydrolases. This study not only extends previous work on Clp and RpfF regulation by showing that they both influence the expression of manA in Xcc, but it also for the first time characterizes Xanthomonas mannanase at the protein level.

Clp and RpfF up-regulate transcription of pelA1 gene encoding the major pectate lyase in Xanthomonas campestris pv. campestris

Exopolysaccharide and several extracellular enzymes of Xanthomonas campestris pv. campestris (Xcc), the causative agent of black rot in crucifers, are virulence determinants. In this study, two Xcc annotated extracellular pectate lyase genes, pelA1 and pelA2, belonging to family 1 of the polysaccharide lyase, were characterized. Sequence and mutational analyses have demonstrated that pelA1 encodes the major pectate lyase, whereas pelA2 is not transcribed. Using the 5' RACE method, the pelA1 transcription initiation site was mapped at nucleotide G, 103 nt upstream of the pelA1 start codon. Promoter analysis demonstrated that polygalacturonic acid and CaCl(2) induce the expression of pelA1. Transcriptional fusion assays also indicated that Clp (cAMP receptor protein-like protein) and RpfF (an enoyl-CoA hydratase homologue that is required for the synthesis of cis-11-methyl-2-dodecenoic acid, a low molecular weight diffusible signal factor, DSF) positively regulate pelA1 transcription. Gel retardation assays showed that Clp exerts a positive control over expression of pelA1 by direct binding to the upstream Clp-binding site. In conclusion, the present research demonstrated that pelA1 codes for the major pectate lyase in Xcc strain Xc17 and that its expression is up-regulated by Clp and RpfF. This is the first study to characterize pectate lyase gene expression in Xcc.

Identification of a hypothetical protein of plant pathogenic Xanthomonas campestris as a novel beta-galactosidase

Xc17L, a lactose-utilizing mutant of Xanthomonas campestris pv. campestris previously isolated by mutagenesis with nitrous acid, displays a level of beta-galactosidase 3.5-fold higher than that in the parental Xc17. In this study, the gene encoding the enzyme displaying a higher specific activity in Xc17L was inactivated by mini-Tn5 transposition. Sequencing revealed that the product (579 aa, 63.5 kDa) of this gene, designated galD, was previously annotated to encode a hypothetical protein on the genome. Mutation of the gene by marker exchange, complementation test and Western blot analysis together confirmed that galD is indeed the gene involved in beta-galactosidase elevation in Xc17L. With only the N-terminal region possessing similarity to the known beta-galactosidases and partially conserved consensus motif, GalD is recognized as a member of the glycosyl hydrolase family 35. Insertion with GmOmega, which causes polar effects, into the upstream genes followed by Western blotting showed that galD is cotranscribed with the upstream genes and expressed constitutively. Mutation in galD causes no significant changes including pathogenicity in the bacterium.

Identification of a novel cephalosporinase (DHA-3) in Klebsiella pneumoniae isolated in Taiwan

A strain of Klebsiella pneumoniae resistant to cefoxitin and oxyimino-cephalosporins, but susceptible to cefepime, was isolated from an adult patient hospitalised in Taichung, Taiwan. Isoelectric focusing revealed three beta-lactamases with isoelectric points of 5.4, 8.2 and 7.9, respectively. Following PCR with plasmid DNA templates and gene sequencing, these enzymes were shown to correspond to TEM-1, SHV-5 and a novel DHA-1-like enzyme (designated DHA-3). The bla genes for TEM-1 and SHV-5 were transferable, but the bla(DHA-3) gene was non-self-transferable in conjugation experiments. All three bla genes were successfully introduced by electrotransformation into an Escherichia coli recipient (DH5alpha), resulting in a similar resistance profile to that observed in the original donor strain. Other K. pneumoniae strains producing DHA-1-like enzymes have been identified previously in Taiwan, and this report suggests that DHA-type beta-lactamases are continuing to emerge in this country.

Deduction of upstream sequences of Xanthomonas campestris flagellar genes responding to transcription activation by FleQ

Xanthomonas campestris pv. campestris (Xcc), a close relative to Pseudomonas aeruginosa, is the pathogen causing black rot in cruciferous plants. In P. aeruginosa, FleQ serves as a cognate activator of sigma54 in transcription from several sigma54-dependent promoters of flagellar genes. These P. aeruginosa promoters have been analyzed for FleQ-binding sequences; however, no consensus was deduced. Xcc, although lacks fleSR, has a fleQ homologue residing among over 40 contiguously clustered flagellar genes. A fleQ mutant, Xc17fleQ, constructed by insertional mutation is deficient in FleQ protein, non-flagellated, and immobile. Transcriptional fusion assays on six putative sigma54-dependent promoters of the flagellar genes, fliE, fliQ, fliL, flgG, flgB, and flhF, indicated that each of them is also FleQ dependent. Each of these promoters has a sequence with weak consensus to 5'-gaaacCCgccgCcgctTt-3', immediately upstream of the predicted sigma54-binding site, with an imperfect inverted repeat containing a GC-rich center flanked by several A and T at 5'- and 3'-ends, respectively. Replacing this region in fliE promoter with a HindIII recognition sequence abolished the transcription, indicating that this region responds to transcription activation by FleQ.

Clp upregulates transcription of engA gene encoding a virulence factor in Xanthomonas campestris by direct binding to the upstream tandem Clp sites

In Xanthomonas campestris, the causative agent of black rot in crucifers, the endoglucanase level is greatly decreased in the mutant deficient in Clp, a homologue of cyclic AMP receptor protein (CRP). It is established that Clp has the same DNA binding specificity as CRP at positions 5, 6, and 7 (GTG motif) of the DNA half site. In this study, the engA transcription initiation site was determined by the 5' RACE method, and two consensus Clp-binding sites, site I and site II centered at -69.5 and -42.5, respectively, were located. Transcriptional fusion assays indicated that Clp greatly activates engA transcription. Site-directed mutagenesis indicated that position 5 of GTG motif in site II is essential for both DNA-protein complex formation in electrophoretic mobility shift assays and engA transcription in vivo. In addition, mutation at position 5 of site I drastically reduces the promoter activity, indicating that binding of Clp to site I exerts a synergistic effect on the transcription activation by site II. engA appears to be the first X. campestris gene known to be activated by Clp via a direct binding to the promoter.

Expression of heat-shock genesgroESLinXanthomonas campestrisis upregulated by CLP in an indirect manner

CLP is a homologue of cyclic AMP-receptor protein in Xanthomonas campestris. In this study, proteomic analysis and Western blotting showed that the clp mutant (TC820) of X. campestris synthesizes less GroESL proteins than the parental P20H. The groESL upstream regions, nt -583 to -32 (552 bp) and nt -178 to -29 (150 bp) relative to the groESL initiation codon, were cloned for transcriptional fusion assays. The 150-bp region, bearing putative sigma24- and sigma32-binding sites and the CIRCE element all known to regulate groESL operon, expressed the same levels of beta-galactosidase (300 U/ml) in both strains, indicating that CLP is not involved in the expression from this region. At early exponential phase, the 552-bp region displayed extremely high levels of promoter activity, 11,000 U/ml in P20H versus 5000 U/ml in TC820. The enzyme levels were about 2000 U/ml at stationary phase in both strains, indicating high levels of expression when cells cease growing. These results suggest that the sequence responding to CLP regulation resides between nt -178 and -583. However, since this region has no CLP-binding site and showed no binding to CLP in gel retardation assay, CLP is likely acting indirectly. This communication appears to be the first description of the positive regulation of a bacterial heat-shock operon by a CRP homologue.

Constitutive expression of a chromosomal class A (BJM group 2) beta-lactamase in Xanthomonas campestris

Sequencing of the upstream region of the beta-lactamase gene from Xanthomonas campestris pv. campestris 11 (bla(XCC-1)) revealed the cognate ampR1 gene (289 amino acids, 31 kDa). It runs divergently from bla(XCC-1) with a 100-bp intergenic region (IG) containing partially overlapped promoters with structural features typical of the bla-ampR IG. The deduced AmpR1 protein shows significant identity in amino acid sequence and conserved motifs with AmpR proteins of other species, e.g., of Pseudomonas aeruginosa (58.2% amino acid identity). Results of insertional mutation, complementation tests, and beta-lactamase assays suggested that expression of bla(XCC-1) was constitutive and dependent on AmpR1. Four bla genes and two ampR genes are present in the fully sequenced X. campestris pv. campestris ATCC 33913 genome, with XCC3039 and XCC3040 considered the analogues of bla(XCC-1) and ampR1, respectively. An ampR1 homologue was detected by Southern hybridization in the ampicillin-resistant Xanthomonas strains, which appear to express beta-lactamase constitutively. Although the significance remains to be studied, constitutive expression of beta-lactamase by a widespread bacterial genus raises environmental concerns regarding the dissemination of resistance genes.

Flagellin gene fliC of Xanthomonas campestris is upregulated by transcription factor Clp

Clp, a homologue of cyclic AMP receptor protein (CRP), of Xanthomonas campestris regulates the expression of many genes. In this study, proteomic analysis showed the amounts of several extracellular proteins in a clp mutant to be reduced, including the flagellin encoded by fliC. Transcriptional fusion assay showed that activity of fliC promoter is reduced by 2.3-fold in clp mutant compared to the wild-type, coincident with the protein levels. The clp mutant is slightly reduced in motility; however, electron microscopy showed no significant change in the monotrichous flagellation. A fleQ homologue with conserved Clp-binding site in the upstream region is present in the fully sequenced X. campestris genome, suggesting that regulation of the flagellar genes is similar to that of Pseudomonas aeruginosa in involving Vfr, the CRP homologue, and FleQ in a cascade manner except that Vfr downregulates fleQ. Concomitant loss of flagellum and motility in fliC mutant and absence of a second homologue in the genome sequence suggest that X. campestris possesses a single flagellin gene, fliC. In addition, mutation of this gene does not affect virulence.

Involvement of tonB-exbBD1D2 operon in infection of Xanthomonas campestris phage phi L7

phi L7 is a lytic bacteriophage infecting Xanthomonas campestris pv. campestris, a Gram-negative bacterium producing xanthan gum and causing black rot in crucifers. A mutant resistant to phi L7 was isolated by Tn5 mutagenesis. Sequence analysis indicated that the gene responsible for the mutation is tonB encoding an inner membrane protein previously shown to be required for iron uptake and pathogenesis. This gene is clustered with three other genes, tonB-exbB-exbD1-exbD2. Results of insertional mutations, DNA and protein sequence analyses, phage sensitivity tests, transfection tests, complementation tests, and phage adsorption assays together with the cellular location of the proteins indicate that TonB, ExbB, and ExbD1 are essential for penetration of phage phi L7. The genome organization, structural features of the tonB-exb region, and transcriptional analyses including Northern hybridization, reporter assays, and primer extension together indicate that the four genes are organized into an operon.

Transcription of Xanthomonas campestris prt1 gene encoding protease 1 increases during stationary phase and requires global transcription factor Clp

Xanthomonas campestris pv. campestris produces three proteases, Prt1, Prt2, and Prt3, the first two of which are involved in pathogenicity. In this study, nucleotide A 84 nt upstream of the prt1 start codon, which is 8 nt downstream of the -10 sequence, was determined as the transcription start site by the 5(') RACE (rapid amplification of cDNA ends) method. Using Pprt1-lacZ transcriptional fusion constructs for assays, several interesting characteristics of prt1 promoter were revealed. The expression is inducible by LB medium or casein proteins and involves the global transcription factor Clp (cyclic AMP receptor protein-like protein). The region containing bp -392 to -80 relative to the prt1 translation initiation codon is required for maximal expression, in which bp -392 to -207 responds to the Clp-mediated regulation and the induction. In presence of inducers and the clp wild-type background, the levels of expression continue to increase following cell growth until 30 h after the cultures entering stationary phase. Since prt1 promoter shows no response to stressful conditions and neither growth nor cell viability is affected by prt1 mutation, Prt1 appears to be a secondary metabolite of X. campestris pv. campestris.

oriC region and replication termination site, dif, of the Xanthomonas campestris pv. campestris 17 chromosome

A 13-kb DNA fragment containing oriC and the flanking genes thdF, orf900, yidC, rnpA, rpmH, oriC, dnaA, dnaN, recF, and gyrB was cloned from the gram-negative plant pathogen Xanthomonas campestris pv. campestris 17. These genes are conserved in order with other eubacterial oriC genes and code for proteins that share high degrees of identity with their homologues, except for orf900, which has a homologue only in Xylella fastidiosa. The dnaA/dnaN intergenic region (273 bp) identified to be the minimal oriC region responsible for autonomous replication has 10 pure AT clusters of four to seven bases and only three consensus DnaA boxes. These findings are in disagreement with the notion that typical oriCs contain four or more DnaA boxes located upstream of the dnaA gene. The X. campestris pv. campestris 17 attB site required for site-specific integration of cloned fragments from filamentous phage phiLf replicative form DNA was identified to be a dif site on the basis of similarities in nucleotide sequence and function with the Escherichia coli dif site required for chromosome dimer resolution and whose deletion causes filamentation of the cells. The oriC and dif sites were located at 12:00 and 6:00, respectively, on the circular X. campestris pv. campestris 17 chromosome map, similar to the locations found for E. coli sites. Computer searches revealed the presence of both the dif site and XerC/XerD recombinase homologues in 16 of the 42 fully sequenced eubacterial genomes, but eight of the dif sites are located far away from the 6:00 point instead of being placed opposite the cognate oriC. The differences in the relative position suggest that mechanisms different from that of E. coli may participate in the control of chromosome replication.

Mutation in the Xanthomonas campestris xanA gene required for synthesis of xanthan and lipopolysaccharide drastically reduces the efficiency of bacteriophage (phi)L7 adsorption

(Phi)L7 is a lytic phage infecting the gram-negative Xanthomonas campestis pv. campestris, a plant pathogen. To study phage-host interaction, a (phi)L7-resistant mutant was isolated from strain Xc17 by mini-Tn5 transposition and designated CH7LR. CH7LR could not plate (phi)L7 in double-layered assay and formed turbid clearing zones when the cell lawn was dropped with a high titer of (phi)L7. Sequence analysis showed that the mutated gene is xanA coding for phosphoglucomutase/phosphomannomutase, required for the synthesis of lipopolysaccharide and exopolysaccharide (xanthan). The involvement of xanA was confirmed by isolating another mutant with interrupted xanA and complementing with the cloned wild-type gene. Nonmucoid mutants are still sensitive to (phi)L7, indicating that xanthan is not involved in (phi)L7 adsorption. Since the mutants still exhibited low efficiencies of phage adsorption, we predict, by analogy with the cases in other bacteriophages of gram-negative bacteria, that other outer membrane components such as a protein are required for the formation of a complex receptor.

UDP-glucose dehydrogenase gene of Xanthomonas campestris is required for virulence

Xanthomonas campestris pv. campestris (Xc) is the casual agent of black rot in crucifers. The Xc gene (udgH) coding for UDP-glucose dehydrogenase, an enzyme catalyzing the conversion of UDP-glucose to UDP-glucuronic acid, was previously shown to be required for the biosynthesis of xanthan gum, a substance necessary for the bacterium to cause disease. In this study, the transcription start site of the udgH was determined and the promoter activity monitored by the xylE reporter system indicated that expression of the udgH increases following cell growth and that the udgH gene may possess a second promoter that is responsive to stationary-phase change retaining high levels of expression. Results of Southern hybridization suggest that the udgH gene may be ubiquitous in Xanthomonas, coincident with the notion that members of this genus are capable of xanthan gum biosynthesis. Mutation of the udgH gene in Xc and X. campestris pv. vesicatoria, the casual agent of leaf spot in pepper and tomato, was found to cause a loss of virulence.

Characterization of the IncW cryptic plasmid pXV2 from Xanthomonas campestris pv. vesicatoria

The gram-negative plant pathogen Xanthomonas campestris pv. vesicatoria strain Xv2 harbors an indigenous, cryptic plasmid pXV2 of 14.6 kb. This plasmid can only be maintained in Xanthomonas and is incapable of self-transmission. However, incompatibility testing classified it in IncW, a group containing the smallest number of naturally occurring, broad-host-range, conjugative plasmids. A pXV2 derivative containing only a 5.5-kb PstI fragment is stably maintained. Deletion of a 3.0-kb region from the PstI fragment causes a loss of plasmid stability. Nucleotide sequencing of the 2. 1-kb region essential for autonomous replication revealed a repA gene and a downstream noncoding region containing four iterons, two 17- and two 19-nt direct repeats, and an AT-rich region lying between the two sets of iterons. The sequence of the deduced RepA and the iterons shows homology to the RepA (39% identity) and the iterons, respectively, of the IncW plasmid pSa. Maxicell expression of the repA gene produced a protein of 35 kDa, a size similar to that deduced from the nucleotide sequence. Trans-complementation test confirmed that the repA gene and the iterons are indeed the essential elements for pXV2 replication.

Identification of a novel beta-lactamase produced by Xanthomonas campestris, a phytopathogenic bacterium

The Xanthomonas campestris pv. campestris 11 chromosome encodes a periplasmic beta-lactamase of 30 kDa. Gene replacement and complementation confirmed the presence of this enzyme. Its deduced amino acid sequence shows identity and conserved domains between it and Stenotrophomonas maltophilia L2 and other Ambler class A/Bush group 2 beta-lactamases. Southern hybridization detected a single homologous fragment in each of 12 other Xanthomonas strains, indicating that the presence of a beta-lactamase gene is common among xanthomonads.

The adsorption protein genes of Xanthomonas campestris filamentous phages determining host specificity

Gene III (gIII) of phiLf, a filamentous phage specifically infecting Xanthomonas campestris pv. campestris, was previously shown to encode a virion-associated protein (pIII) required for phage adsorption. In this study, the transcription start site for the gene and the N-terminal sequence of the protein were determined, resulting in the revision of the translation initiation site from the one previously predicted for this gene. For comparative study, the gIII of phiXv, a filamentous phage specifically infecting X. campestris pv. vesicatoria, was cloned and sequenced. The deduced amino acid sequences of these two pIIIs exhibit a high degree of identity in their C-terminal halves and possess the structural features typical of the adsorption proteins of filamentous phages: a signal sequence in the N terminus, a long glycine-rich region near the center, and a hydrophobic membrane anchorage domain in the C terminus. The regions between gIII and the upstream gVIII, 128 nucleotides in both phages, are larger than those of other filamentous phages. A hybrid phage of phiXv, consisting of the phiLf pIII and all the other components derived from phiXv, was able to infect X. campestris pv. campestris but not X. campestris pv. vesicatoria, indicating that gIII is the gene specifying host specificity and demonstrating the interchangeability of the pIIIs.

Chromosome map of Xanthomonas campestris pv. campestris 17 with locations of genes involved in xanthan gum synthesis and yellow pigmentation

No plasmid was detected in Xanthomonas campestris pv. campestris 17, a strain of the causative agent of black rot in cruciferous plants isolated in Taiwan. Its chromosome was cut by PacI, PmeI, and SwaI into five, two, and six fragments, respectively, and a size of 4.8 Mb was estimated by summing the fragment lengths in these digests. Based on the data obtained from partial digestion and Southern hybridization using probes common to pairs of the overlapping fragments or prepared from linking fragments, a circular physical map bearing the PacI, PmeI, and SwaI sites was constructed for the X. campestris pv. campestris 17 chromosome. Locations of eight eps loci involved in exopolysaccharide (xanthan gum) synthesis, two rrn operons each possessing an unique I-CeuI site, one pig cluster required for yellow pigmentation, and nine auxotrophic markers were determined, using mutants isolated by mutagenesis with Tn5(pfm)CmKm. This transposon contains a polylinker with sites for several rare-cutting restriction endonucleases located between the chloramphenicol resistance and kanamycin resistance (Kmr) genes, which upon insertion introduced additional sites into the chromosome. The recA and tdh genes, with known sequences, were mapped by tagging with the polylinker-Kmr segment from Tn5(pfm)CmKm. This is the first map for X. campestris and would be useful for genetic studies of this and related Xanthomonas species.

Purification and expression of the gene III protein from filamentous phage phi Lf

The gene III protein (pIII) from phi Lf, a filamentous phage of Xanthomonas campestris pv.campestris, was purified by gel filtration with FPLC. The gIII coding region was amplified by PCR, which was then cloned into pUC18 and expressed in Escherichia coli. The size of both pIII, purified from phage particle and expressed in E. coli, is similar to the value deduced from the nucleotide sequence as shown by Western blot analysis. This is different from the case in Ff phages (f1, fd, and M13), in which the size of pIII observed in SDS-polyacrylamide gel electrophoresis is substantially larger than the deduced value. Upon infection of X. c. pv. vesicatoria carrying cloned phi Lf gIII with phi Xv, a filamentous phage of pv. vesicatoria, the progeny particles in supernatant were able to infect both pv. campestris carrying cloned phi Lf gIII and pv. vesicatoria, indicating that a mixture of authentic phi Xv and chimeric phage consisting of phi Xv DNA and phi Lf pIII was produced. These results suggest pIII to be the adsorption protein required for host recognition.

Transcriptional analysis of the threonine dehydrogenase gene of Xanthomonas campestris

The nucleotide sequence has previously been determined for the Xanthomonas campestris pv. campestris gene coding for threonine dehydrogenase (tdh). Flanking this gene are the upstream region possessing promoter activity and the downstream perfect inverted repeat having potential to form a stem-loop structure which resembles a transcription terminator. In addition, Northern blot analysis suggested the transcript of this gene to be monocistronic. In the present study, the essential region for promoter activity was narrowed down to a stretch of 57 bp which still retained 84% of the promoter activity. The first nucleotide to be transcribed is the guanosine at 30 nt upstream from the proposed tdh start codon. The putative terminator exhibited transcriptional termination activity bidirectionally in both Escherichia coli and X. campestris. These observations indicate that the transcriptional structure of X. campestris tdh is different from that of E. coli where tdh and kbl are organized into the tdh operon. Furthermore, the expression of tdh in X. campestris is repressed by leucine, a situation different from that in E. coli where leucine induces the expression of tdh operon.

Sequence and copy number of the Xanthomonas campestris pv. campestris gene encoding 16S rRNA

A 6.7-kb Sau3A1 fragment containing ribosomal RNA genes was cloned from the chromosome of Xanthomonas campestris pv. campestris strain 17 by a PCR-based strategy. Nucleotide sequence was determined for the 16S rRNA gene (1,544 nt). This gene has a G+C content of 54.9% which is similar to the 16S rRNA genes of Escherichia coli and Pseudomonas aeruginosa but different from the value reported for the whole X. campestris chromosome (64%). Sequence alignment revealed that AGGAGG is consensus for ribosome binding, with the internal GGAG to be paired most frequently with the anti-Shine-Dalgarno sequence. This consensus sequence was found in the regions upstream from the initiation codon of 98 Xanthomonas genes among 116 aligned, but not in the remaining genes. This suggests that about 16% of the Xanthomonas genes do not possess typical ribosome binding sites and another mechanism may be required for recognition of correct translation initiation sites. Two rrn operons were detected in Xc17 chromosome by pulsed-field gel electrophoresis and Southern hybridization.

Molecular characterization of the gene coding for threonine dehydrogenase in Xanthomonas campestris

The Xanthomonas campestris pv. campestris 17 gene tdh, which codes for the threonine dehydrogenase (TDH), was cloned and sequenced. The deduced gene product, a polypeptide consisting of 340 amino acids (Mr = 37,048), has 63.5% identity to the E. coli TDH in amino acid sequence and shares residue conservation with the alcohol/polyol dehydrogenases from different organisms. TDH activity was not detectable in the tdh mutant constructed by gene replacement; however, the enzyme activity in the mutant complemented in trans by a plasmid containing the complete tdh sequence was increased by 15 folds over Xc17. Northern blot analysis detected an mRNA with a size similar to that of the Xc17 tdh coding region, suggesting that the tdh gene-containing transcript may be monocistronic.

The Xanthomonas campestris gumD gene required for synthesis of xanthan gum is involved in normal pigmentation and virulence in causing black rot

A cloned 4.1-kb EcoRI fragment from Xanthomonas campestris pv. campestris was previously shown to complement the non-mucoid mutant P22 and increase xanthan gum production after being transformed into the wild-type strain Xc17. The gene responsible for these effects was identified, sequenced, and shown to be the gumD gene which has previously been proposed to encode glucose transferase activity, an enzyme required for adding the first glucose residue to the isoprenoid glycosyl carrier lipid during xanthan synthesis. A gumD mutant, isolated from Xc17 by gene replacement, was shown to possess altered pigment xanthomonadin profiles and exhibit reduced virulence in causing black rot in broccoli. This study appears to be the first to demonstrate that interruption of a gene required for xanthan synthesis can lead to reduced virulence of X. campestris.

Sequence analysis of the small cryptic Xanthomonas campestris pv. vesicatoria plasmid pXV64 encoding a Rep protein similar to gene II protein of phage 12-2

The complete nucleotide sequence (1,851 bp) was determined for the Xanthomonas campestris pv. vesicatoria plasmid pXV64. Sequence analysis revealed an intergenic region (IG) of 355 bp and two oppositely running open reading frames, ORF1 and ORF2, encoding polypeptides of 39 and 16 kDa, respectively. While the function of ORF2 is not known, ORF1 is suggested to be the gene encoding Rep protein based on (i) similarity in amino acid sequence to that of the gene II protein (gIIP) of filamentous phage 12-2, (ii) presence of a sequence in the ori-containing region which is similar to the sequence around the Rep nicking site in some rolling circle-replicating replicons, and (iii) ability to support replication in trans of the region containing pXV64 ori (392 bp) which is located within the region including IG and a short stretch in the N-terminus of ORF2.