Systematic analysis of xanthomonads (Xanthomonas spp.) associated with pepper and tomato lesions

The taxonomy and evolutionary relationships among members of the genus Xanthomonas associated with tomato and pepper have been a matter of considerable controversy since their original description in 1921. These bacteria, which are a major affliction of tomato and pepper crops in warm and humid regions, were originally described as a single species, but subsequent research has shown the existence of at least two genetic groups differentiated by physiological, biochemical and pathological characteristics. This work synthesizes the findings from several approaches, including pathogenicity tests, enzymic activity, restriction fragment analysis of the entire genome, DNA-DNA hybridization and RNA sequence comparisons based on a 2097 base sequence comprising the 16S rRNA gene, the intergenic spacer located between the 16S and 23S rRNA genes and a small region of the 23S rRNA gene. Within the group of xanthomonads pathogenic on pepper and tomato four distinct phenotypic groups exist, of which three form distinct genomic species. These include Xanthomonas axonopodis pv. vesicatoria (A and C group), Xanthomonas vesicatoria (B group) and Xanthomonas gardneri (D group). On the basis of phenotypic and genotypic differences between A- and C-group strains, the C strains should be considered as a subspecies within Xanthomonas axonopodis pv. vesicatoria.

[Hydrolase from Xanthomonas rubrilineans for synthesis of cefalexin]

The use of peptide hydrolase (EC 3.4.13.1) from Xanthomonas rubrilineans for synthesis of the antibiotic cephalexin from 7-aminodesacetoxycephalosporanic acid was studied. The optimum conditions for production of cephalexin were determined, and the yield exceeded 80%. A method for monitoring the synthesis of this antibiotic synthesis by means of a conventional amino acid analyzer is proposed.

Mutants of Xanthomonas oryzae pv. oryzae deficient in general secretory pathway are virulence deficient and unable to secrete xylanase

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight, a serious disease of rice. A virulence- and xylanase-deficient mutant of Xoo was isolated following ethyl methane sulfonate (EMS) mutagenesis. A cosmid clone that restored virulence and xylanase secretion was obtained from a genomic library by functional complementation. Transposon mutagenesis and marker exchange studies revealed genes on the cloned DNA that were required for xylanase production and virulence. Sequence analysis with transposon-specific primers revealed that these genes were homologues of xps F and xps D, which encode components of a protein secretion system in Xanthomonas campestris pv. campestris. Enzyme assays showed xylanase accumulation in the periplasmic space and cytoplasm of the xps F mutant and the complementing clone restored transport to the extracellular space.

Characterization of an extracellular poly(3-hydroxy-5-phenylvalerate) depolymerase from Xanthomonas sp. JS02

A bacterium, JS02, capable of degrading an aromatic medium-chain-length polyhydroxyalkanoate (PHA(MCL)), poly(3-hydroxy-5-phenylvalerate) (PHPV), was isolated from wastewater-treatment sludge (Ju et al. 1998), and was identified as a Xanthomonas species. An extracellular PHPV depolymerase was purified from the concentrated culture broth of Xanthomonas sp. JS02 by using a chromatography series on Sephadex G-75, QAE-Sephadex A-50 and hydroxyapatite. The molecular mass of the purified enzyme was estimated to be 41.7 kDa. The purified enzyme could hydrolyse PHPV and p-nitrophenyl (PNP)-esters of fatty acids, but did not hydrolyse short-chain-length PHAs, though the culture supernatant could hydrolyse them. The optimum pH range was 8.0-9.0 and the optimum temperature was 60 degrees C for PNP-octanoate hydrolysis. The Km values for PNP-hexanoate and PNP-octanoate were 10.9 and 0.88 microM, respectively.

Construction and characterization of regulated L-arabinose-inducible broad host range expression vectors in Xanthomonas

Several versions of broad host range (BHR), L-arabinose-inducible expression vectors were constructed. These expression vectors were based on a high copy number BHR pBBR1MCS-4 replicon that could replicate in both enteric and non-enteric Gram-negative bacteria. Two versions of expression cassettes containing multiple cloning sites either with or without a ribosome binding site were placed under transcriptional control of the Escherichia coli BAD promoter and araC gene. Three versions of vectors containing ampicillin or kanamycin or tetracycline resistance genes as selectable markers were constructed. In all six new L-arabinose-inducible BHR expression vectors containing many unique cloning sites, selectable markers were made to facilitate cloning and expression of genes in various Gram-negative bacteria. A Tn9 chloramphenicol acetyl transferase (cat) gene was cloned into an expression vector, resulting in pBBad18Acat that was used to establish optimal expression conditions (addition of 0.02% L-arabinose to mid-exponential phase cells for at least 1 h) in a Xanthomonas campestris pv. phaseoli. Comparison of the Cat enzyme activities between uninduced and a 180-min L-arabinose-induced culture showed a greater than 150-fold increased Cat specific activity. In addition, L-arabinose induction of exponential phase cells harboring pBBad18Acat gave a higher amount of Cat than similarly treated stationary phase cells. The usefulness of the expression vector was also demonstrated in both enteric and non-enteric Gram-negative bacteria.

Characterization of the chromosomal aac(6')-Iz gene of Stenotrophomonas maltophilia

The aac(6')-Iz gene of Stenotrophomonas maltophilia BM2690 encoding an aminoglycoside 6'-N-acetyltransferase was characterized. The gene was identified as a coding sequence of 462 bp corresponding to a protein with a calculated mass of 16,506 Da, a value in good agreement with that of ca. 16,000 found by in vitro coupled transcription-translation. Analysis of the deduced amino acid sequence indicated that the protein was a member of the major subfamily of aminoglycoside 6'-N-acetyltransferases. The enzyme conferred resistance to amikacin but not to gentamicin, indicating that it was an AAC(6') of type I. The open reading frame upstream from the aac(6')-Iz gene was homologous to the fprA gene of Myxococcus xanthus (61% identity), which encodes a putative pyridoxine (pyridoxamine) 5'-phosphate oxidase. Pulsed-field gel electrophoresis of total DNA from BM2690 and S. maltophilia ATTC 13637 digested with XbaI, DraI, and SpeI followed by hybridization with rRNA and aac(6')-Iz-specific probes indicated that the gene was located in the chromosome. The aac(6')-Iz gene was detected by DNA-DNA hybridization in all 80 strains of S. maltophilia tested. The MICs of gentamicin against these strains of S. maltophilia were lower than those of amikacin, netilmicin, and tobramycin, indicating that production of AAC(6')-Iz contributes to aminoglycoside resistance in S. maltophilia.

Identification of catalytic residues of pepstatin-insensitive carboxyl proteinases from prokaryotes by site-directed mutagenesis

Pepstatin-insensitive carboxyl proteinases from Pseudomonas sp. (PCP) and Xanthomonas sp. (XCP) have no conserved catalytic residue sequences, -Asp*-Thr-Gly- (Asp is the catalytic residue) for aspartic proteinases. To identify the catalytic residues of PCP and XCP, we selected presumed catalytic residues based on their high sequence similarity, assuming that such significant sites as catalytic residues will be generally conserved. Several Ala mutants of Asp or Glu residues were constructed and analyzed. The D170A, E222A, and D328A mutants for PCP and XD79A, XD169A, and XD348A mutants for XCP were not converted to mature protein after activation, and no catalytic activity could be detected in these mutants. The specificity constants toward chromogenic substrate of the other PCP and XCP mutants, except for the D84A mutant of PCP, were similar to that of wild-type PCP or XCP. Coupled with the result of chemical modification (Ito, M., Narutaki, S., Uchida, K., and Oda, K. (1999) J. Biochem. (Tokyo) 125, 210-216), a pair of Asp residues (170 and 328) for PCP and a pair of Asp residues (169 and 348) for XCP were elucidated to be their catalytic residues, respectively. The Glu(222) residue in PCP or Asp(79) residue in XCP was excluded from the candidates as catalytic residues, since the corresponding mutant retained its original activity.

[Enzymatic synthesis of beta-lactam antibiotics. II. Aminocephalosporins]

Enzymatic synthesis of cephalexin and cefaclor with the use of immobilized aminocephalosporin synthetase from Xanthomonas sp. as a biocatalyst was studied. The employment of a mathematical model based on the acyl-enzyme mechanism of the biocatalyst action was shown possible for the quantitative description of the antibiotic syntheses. A relationship providing determination of the complex of the physico-chemical investigations required for the enzymatic synthesis design was suggested. Kinetic and thermodynamic parameters of the processes were evaluated and the ratios of the maximum conversion of the key amino acids and the initial concentrations of the substrate and nucleophile were calculated. The regions of the model fit to the experimental data within a wide range of the substrate and nucleophile concentrations were defined. The technology for the enzymatic synthesis of aminocephalosporins exemplified by cephalexin and cefaclor was designed.

Inhibition studies on the metallo-beta-lactamase L1 from Stenotrophomonas maltophilia

In an effort to identify a competitive inhibitor that can be used in future spectroscopic and crystallographic studies and to better understand the interaction of a mercaptoacetic acid-thiolester-containing compound with metallo-beta-lactamase L1 from Stenotrophomonas maltophilia, inhibition studies using two thiol-containing compounds were conducted. N-(2'-Mercaptoethyl)-2-phenylacetamide is a competitive inhibitor of L1 with a K(i) of 50 +/- 3 microM, and this compound is not a time-dependent inactivator of L1. N-Benzylacetyl-d-alanylthioacetic acid is a competitive inhibitor of L1 with a K(i) of 1.6 +/- 0.3 microM. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric studies revealed that 2 mol of mercaptoacetate covalently bind to L1 upon incubation of the enzyme with N-benzylacetyl-d-alanylthioacetic acid; however, this covalently modified enzyme has the same activity as wild-type L1. Last, inhibition studies were used to demonstrate that 4-morpholinoethanesulfonic acid does not inhibit L1, even at concentrations up to 300 mM. This work identifies two possible competitive inhibitors which can be used in future structural studies and further demonstrates inhibitory heterogeneity among the metallo-beta-lactamases.

Correlation between genotype and beta-lactamases of clinical and environmental strains of Stenotrophomonas maltophilia

Heterogeneity of beta-lactamase production by 17 clinical and nine environmental isolates of Stenotrophomonas maltophilia was investigated using MICs of six different beta-lactam antibiotics, isoelectric focusing (IEF) and pulsed field gel electrophoresis. There was no clear correlation between the results of IEF, genotype and MIC determination. Environmental isolates were more susceptible than clinical isolates; eight clinical and none of the environmental isolates expressed high-level resistance to meropenem. Only two isolates expressed high-level resistance to ceftazidime. These results indicate that further studies are required to elucidate the extent of genetic heterogeneity within the L1 and L2 beta-lactamase genes.

Kinetic mechanism of metallo-beta-lactamase L1 from Stenotrophomonas maltophilia

The reaction of nitrocefin with metallo-beta-lactamase L1 from Stenotrophomonas maltophilia was studied using rapid-scan and stopped-flow ultraviolet-visible (UV-vis) studies in an effort to discern the kinetic mechanism used by L1 to hydrolyze penicillins and cephalosporins. Rapid-scan and stopped-flow UV-vis studies of nitrocefin hydrolysis by L1 identified three species: (1) the substrate (nitrocefin) displayed an absorbance peak at 390 nm (epsilon = 11 500 M(-1) cm(-1)) that decreased during the reaction with a rate constant of 170 +/- 30 s(-1); (2) the product (hydrolyzed nitrocefin) displayed an absorbance peak at 485 nm (epsilon = 17 420 M(-1) cm(-1)) that increased during the reaction with rate constant of 40 +/- 1 s(-1); and (3) an intermediate displayed an absorbance peak at 665 nm (epsilon = 32 000 M(-1) cm(-1)) that increased initially with a rate constant of 190 +/- 3 s(-1) and then decreased with a rate constant of 38 +/- 2 s(-1). Single-turnover experiments demonstrated that there were no pre-steady-state bursts in the reaction of L1 with nitrocefin; moreover, the progress curves could be fit to a kinetic mechanism that includes the formation of a transient intermediate by using KINSIM and the rate constants given above. Progress curves from experiments conducted at different reaction conditions or with a different substrate could also be fit to the proposed kinetic mechanism. The evidence for the presence of an intermediate along with kinetic simulations supports a hydrolytic mechanism for L1 that involves an intermediate whose breakdown is rate-determining.

Subsite preferences of pepstatin-insensitive carboxyl proteinases from bacteria

Pseudomonas sp. 101 carboxyl proteinase (PCP) and Xanthomonas sp. T-22 carboxyl proteinase (XCP), the first and second unique carboxyl proteinases from prokaryotes to be isolated and characterized, are not inhibited by the classical carboxyl proteinase inhibitor pepstatin. In this study, we elucidated their subsite preferences by using a series of synthetic chromogenic substrates, Lys-Pro-Ile(P3)-Glu(P2)-Phe*Nph-Arg(P2')-Leu(P3') (Nph is p-nitrophenylalanine, Phe*Nph is the cleavage site) with systematic substitutions at the P3, P2, P2', and P3' positions. Among 45 substrates tested, the best substrate for PCP had a Leu replacement at the P2 position (kcat = 27.2 s-1, Km = 4.22 microM, kcat/Km = 6.43 microM-1.s-1), and that for XCP had an Ala replacement at the P3 position (kcat = 79.4 s-1, Km = 6.05 microM, kcat/Km = 13.1 microM-1. s-1). PCP and XCP preferred such charged amino acid residues as Glu, Asp, Arg, or Lys at the P2' position. This suggested that the S2' subsites of PCP and XCP are occupied by hydrophilic residues, similar to that of pepstatin-insensitive carboxyl proteinase from Bacillus coagulans J-4 [Shibata et al. (1998) J. Biochem. 124, 642-647]. In contrast, the S2' subsite of pepstatin-sensitive carboxyl proteinases (aspartic proteinases) is hydrophobic in nature. Thus, the hydophilic nature of the S2' subsite appears to be a distinguishing feature of pepstatin-insensitive carboxyl proteinases.

The crystal structure of the L1 metallo-beta-lactamase from Stenotrophomonas maltophilia at 1.7 A resolution

The structure of the L1 metallo-beta-lactamase from the opportunistic pathogen Stenotrophomonas maltophilia has been determined at 1.7 A resolution by the multiwavelength anomalous dispersion (MAD) approach exploiting both the intrinsic binuclear zinc centre and incorporated selenomethionine residues. L1 is unique amongst all known beta-lactamases in that it exists as a tetramer. The protein exhibits the alphabeta/betaalpha fold found only in the metallo-beta-lactamases and displays several unique features not previously observed in these enzymes. These include a disulphide bridge and two substantially elongated loops connected to the active site of the enzyme. Two closely spaced zinc ions are bound at the active site with tetrahedral (Zn1) and trigonal bipyramidal (Zn2) co-ordination, respectively; these are bridged by a water molecule which we propose acts as the nucleophile in the hydrolytic reaction. Ligation of the second zinc ion involves both residues and geometry which have not been previously observed in the metallo-beta-lactamases. Simulated binding of the substrates ampicillin, ceftazidime and imipenem suggests that the substrate is able to bind to the enzyme in a variety of different conformations whose common features are direct interactions of the beta-lactam carbonyl oxygen and nitrogen with the zinc ions and of the beta-lactam carboxylate with Ser187. We describe a catalytic mechanism whose principal features are a nucleophilic attack of the bridging water on the beta-lactam carbonyl carbon, electrostatic stabilisation of a negatively charged tetrahedral transition state and protonation of the beta-lactam nitrogen by a second water molecule co-ordinated by Zn2. Further, we propose that direct metal:substrate interactions provide a substantial contribution to substrate binding and that this may explain the lack of specificity which is a feature of this class of enzyme.

[Biosynthesis of leucine aminopeptidase by Xanthomonas rubrilineans 67 in culture with various concentrations of nitrogen containing compounds]

The dynamics of consumption of amine and ammonium nitrogen and glucose in the process of Xanthomonas rubrilineans 67 growth and biosynthesis of leucine aminopeptidase was studied. It was shown that the rate of leucine, alanine and glycine consumption as a source of amine nitrogen out of 16 amino acids was the highest during the fermentation. Addition of these three amino acids or their mixtures to the medium at definite stages of the fermentation process increased the leucine aminopeptidase biosynthesis by 50 to 100 per cent. Ammonium nitrogen was not used by X.rubrilineans 67. The consumption of glucose during the fermentation was even: by the 24th hour of the process the medium contained about 10 per cent of the glucose initial concentration. The optimal temperature for the culture growth and leucine aminopeptidase biosynthesis was determined. It was shown to be 28 degrees C. Higher aeration increased the culture productivity.

XspI, a new type II restriction endonuclease from a Xanthomonas species

A new type II restriction endonuclease, XspI, was purified 11-fold in seven steps to homogeneity from Xanthomonas sp. strain YK1 grown aerobically in Luria broth. The final specific activity of the purified enzyme was 3890 micrograms of lambda DNA digested per h per mg of protein. The molecular weight of the native enzyme was determined to be 54,000. Sodium dodecyl sulfate-gel electrophoresis revealed two identical subunits of molecular weight 29,000. The purified enzyme was most active at 37 degrees C in the presence of 100 mM KCl and 5 mM MgCl2 under pH range from 7.5 to 8.0. The enzyme was stable at least for 1 h at 37 degrees C, but was inactivated after 20 min at 65 degrees C. XspI recognized the tetranucleotide sequence, 5'-CTAG-3', and cleaved it between C and T, like its isoschizomers MaeI and BfaI. The ability of the source organism to grown aerobically, together with the heat inactivation of the enzyme, confers practical advantages upon XspI over its known isoschizomers.

Distribution of Xanthomonas oryzae pv. oryzae DNA modification systems in Asia

The presence or absence of two DNA modification systems, XorI and XorII, in 195 strains of Xanthomonas oryzae pv. oryzae collected from different major rice-growing countries of Asia was assessed. All four possible phenotypes (XorI+ XorII+, XorI+ XorII-, XorI- XorII+ and XorI- XorII-) were detected in the population at a ratio of approximately 1:2:2:2. The XorI+ XorII+ and XorI- XorII+ phenotypes were observed predominantly in strains from southeast Asia (Philippines, Malaysia, and Indonesia), whereas strains with the phenotypes XorI- XorII- and XorI+ XorII- were distributed in south Asia (India and Nepal) and northeast Asia (China, Korea, and Japan), respectively. Based on the prevalence and geographic distribution of the XorI and XorII systems, we suggest that the XorI modification system originated in northeast Asia and was later introduced to southeast Asia, while the XorII system originated in southeast Asia and moved to northeast Asia and south Asia. Genomic DNA from all tested strains of X. oryzae pv. oryzae that were resistant to digestion by endonuclease XorII or its isoschizomer PvuI also hybridized with a 7.0-kb clone that contained the XorII modification system, whereas strains that were digested by XorII or PvuI lacked DNA that hybridized with the clone. Size polymorphisms were observed in fragments that hybridized with the 7.0-kb clone. However, a single hybridization pattern generally was found in XorII+ strains within a country, indicating clonal maintenance of the XorII methyl-transferase gene locus. The locus was monomorphic for X. oryzae pv. oryzae strains from the Philippines and all strains from Indonesia and Korea.

Disassociation of sigma subunit from RNA polymerase of Xanthomonas oryzae pv. oryzae by phage Xp10 infection

The sigma subunit of Xanthomonas oryzae pv. oryzae is disassociated from host RNA polymerase after phage Xp10 infection. To clarify the possible mechanism for this observation, sigma subunit was purified and an antiserum against sigma subunit was prepared. Immunoprecipitation of RNA polymerase by the anti-core RNA polymerase antiserum, followed by immunoblotting with anti-sigma subunit antibody, revealed that sigma subunit was lost from RNA polymerase within 10 minutes after Xp10 infection. Loss of sigma subunit was not observed under other stress conditions including heat and cold stress, starvation and growth to stationary phase. Two-dimensional immunoblotting analysis did not reveal any covalent modification of either sigma subunit or RNA polymerase after Xp10 infection. These results suggest that separation of th subunit from RNA polymerase may be due to competition with other binding factors.

Molecular heterogeneity of the L-1 metallo-beta-lactamase family from Stenotrophomonas maltophilia

We have determined the nucleotide sequence of the blaS gene encoding the carbapenem-hydrolyzing L-1 beta-lactamase from Stenotrophomonas maltophilia GN12873. Analysis of the DNA and deduced amino acid sequences identified a product of 290 amino acids. Comparisons of the L-1 amino acid sequence with those of other zinc beta-lactamases showed 88.6% identity with the L-1 enzyme from S. maltophilia IID1275 and less than 20% identity with other class B metalloenzymes.

Overexpression, purification, and characterization of the cloned metallo-beta-lactamase L1 from Stenotrophomonas maltophilia

The metallo-beta-lactamase L1 from Stenotrophomonas maltophilia was cloned, overexpressed, and characterized by spectrometric and biochemical techniques. Results of metal analyses were consistent with the cloned enzyme having 2 mol of tightly bound Zn(II) per monomer. Gel filtration chromatography demonstrated that the cloned enzyme exists as a tightly held tetramer with a molecular mass of ca. 115 kDa, and matrix-assisted laser desorption ionization and time-of-flight mass spectrometry indicated a monomeric molecular mass of 28.8 kDa. Steady-state kinetic studies with a number of diverse penicillin and cephalosporin antibiotics demonstrated that L1 effectively hydrolyzes all tested compounds, with k(cat)/Km values ranging between 0.002 and 5.5 microM(-1) s(-1). These characteristics of the recombinant enzyme are contrasted to those previously reported for metallo-beta-lactamases isolated directly from S. maltophilia.

[Clinical significance of the determination of ESBL (extended- spectrum beta-lactamase) production by gram-negative nosocomial bacteria]

BACKGROUND

Data on the incidence of multiresistant nosocomial bacterial strains of Xanthomonas (Stenotrophomonas) maltophilia, producing in addition to metallo-beta-lactamases also new types of broad spectrum beta-lactamases (ESBL) called for analyses of the production of different types of enzymes.

METHODS AND RESULTS

Nosocomial multiresistant strains of Xanthomonas (Stenotrophomonas) maltophilia isolated from patients in 1996 were tested for the production of ESBL by a double diffusion disc test. The production of different types of ESBL was revealed, in particular production of ESBL specific for inactivation of aztreonam which is reversible by the action of clavulanate. The test of ESBL production, proved suitable also for orientation as regards selection of a suitable antibiotic in the treatment of infections caused by multiresistant or naturally resistant bacteria, possibly also the possible synergic action of antibiotics may be detected. The effect of mutual action of different antibiotics with a different inhibitory action of beta-lactamase inhibitors is illustrated when the test of ESBL production is used, where the effect of aztreonam on the inhibition of growth of ofloxacin resistant colonies was proved.

CONCLUSIONS

The double diffusion test of production of new beta-lactamases (EBL) in multiresistant Gram-negative nosocomial strains made it possible to detect the possible synergic action of antibiotics in combination with the action of beta-lactamase inhibitors.

Sequence analysis and enzyme kinetics of the L2 serine beta-lactamase from Stenotrophomonas maltophilia

The L2 serine active-site beta-lactamase from Stenotrophomonas maltophilia has been classified as a clavulanic acid-sensitive cephalosporinase. The gene encoding this enzyme from S. maltophilia 1275 IID has been cloned on a 3.3-kb fragment into pK18 under the control of a Ptac promoter to generate recombinant plasmid pUB5840; when expressed in Escherichia coli, this gene confers resistance to cephalosporins and penicillins. Sequence analysis has revealed an open reading frame (ORF) of 909 bp with a GC content of 71.6%, comparable to that of the L1 metallo-beta-lactamase gene (68.4%) from the same bacterium. The ORF encodes an unmodified protein of 303 amino acids with a predicted molecular mass of 31.5 kDa, accommodating a putative leader peptide of 27 amino acids. Comparison of the amino acid sequence with those of other beta-lactamases showed it to be most closely related (54% identity) to the BLA-A beta-lactamase from Yersinia enterocolitica. Sequence identity is most obvious near the STXK active-site motif and the SDN loop motif common to all serine active-site penicillinases. Sequences outside the conserved regions display low homology with comparable regions of other class A penicillinases. Kinetics of the enzyme from the cloned gene demonstrated an increase in activity with cefotaxime but markedly less activity with imipenem than previously reported. Hence, the S. maltophilia L2 beta-lactamase is an inducible Ambler class A beta-lactamase which would account for the sensitivity to clavulanic acid.

A novel Arthrobacter beta-galactosidase with homology to eucaryotic beta-galactosidases

An Arthrobacter beta-galactosidase has homology with the lysosomal acid beta-galactosidases from humans and mice and with a Xanthomonas manihotis enzyme. Phylogenetic analysis of the deduced amino acid sequence showed an unusual pattern, with this procaryotic enzyme clustering within the animal clade. The gene encodes a subunit of 52 kDa, and the enzyme appears to be active as a dimer. The enzyme hydrolyzed substrates with either a beta-1,4 or a beta-1,3 linkage.

Inhibition of metallo-beta-lactamases by a series of mercaptoacetic acid thiol ester derivatives

A series of mercaptoacetic acid thiol esters have been identified as metallo-beta-lactamase inhibitors. Electrospray mass spectrometry (ESMS) has shown that irreversible inhibition of the Bacillus cereus II metallo-beta-lactamase by SB214751, SB214752, and SB213079 was concomitant with a 90-Da increase in mass of the enzyme. Tryptic digestion of the B. cereus II inhibited with SB214751 illustrated that the peptide fragment, containing the only cysteine of the enzyme, had undergone a mass increment of 90 Da. It was further demonstrated that B. cereus II hydrolyzed this type of compound across the thiol ester bond to yield mercaptoacetic acid. Mercaptoacetic acid is the only molecular fragment common to SB214751, SB214752, and SB213079, and free mercaptoacetic acid does not bind covalently to B. cereus II. Therefore, it is concluded that these compounds inhibit B. cereus II by the mechanism-based delivery of mercaptoacetic acid, forming a disulfide linkage with the active sites cysteine (predicted mass shift = +90 Da) under the aerobic conditions of the assay. The different thiol esters examined had a broad range of potencies against the metallo-beta-lactamases tested. For example SB214751, SB214752, and SB213079 all had 50% inhibitory concentrations of < 10 and > 1,000 microM for the Stenotrophomonas maltophilia L-1 and Bacteroides fragilis CfiA enzymes, respectively. SB216968 was particularly active against the Aeromonas hydrophila CphA metallo-beta-lactamase and was found to be an uncompetitive inhibitor of this enzyme (Ki = 3.9 microM), whereas it exhibited irreversible inhibition of the L-1 enzyme. These observations with this series of compounds have revealed subtle differences between the active sites of different metallo-beta-lactamases. Finally, a novel application for isothermal titration calorimetry for assessing the zinc chelating activity of candidate inhibitors is also presented.

Regulation of the oxidative stress protective enzymes, catalase and superoxide dismutase in Xanthomonas--a review

Xanthomonas showed atypical regulation of catalase (Kat) and superoxide dismutase with respect to growth phase and response to various inducers. The highest levels of both enzymes were detected during early log phase of growth and declined as growth continued. This was in contrast to resistance levels to superoxides, H2O2 and organic peroxides, which reached maximum levels during stationary phase. Xanthomonas catalase was induced over six fold by superoxide generators and methyl methane sulfonate but weakly by H2O2. The regulation pattern of these enzymes could be important during plant/microbe interactions. To facilitate elucidation of Xanthomonas kat gene regulation, highly conserved regions of monofuctional Kat amino acid sequences were used to synthesize oligodeoxyribonucleotide primers for use in PCR reactions with Xanthomonas genomic DNA as templates. The Xanthomonas-specific PCR kat probe was used to isolate a functional kat from Xanthomonas campestris pv. phaseoli.

Substrate specificities of pepstatin-insensitive carboxyl proteinases from gram-negative bacteria

Pseudomonas carboxyl proteinase (PCP), isolated from Pseudomonas sp. 101, and Xanthomonas carboxyl proteinase (XCP), isolated from Xanthomonas sp. T-22, are the first and second examples of unique carboxyl proteinases [EC 3.4.23.33] which are insensitive to aspartic proteinase inhibitors, such as pepstatin, diazoacetyl-DL-norleucine methylester, and 1,2-epoxy-3(p-nitrophenoxy)propane. The substrate specificities of PCP and XCP were studied using a series of synthetic chromogenic peptide substrates with the general structure, P5-P4-P3-P2-Phe-Nph-P2'-P3' (P5, P4, P3, P2, P2', P3': a variety of amino acids, Nph is p-nitro-L-phenylalanine, and the Phe-Nph bond is cleaved). PCP and XCP were shown to hydrolyze a synthetic substrate, Lys-Pro-Ala-Leu-Phe-Nph-Arg-Leu, most effectively among 28 substrates. The kinetic parameters of this peptide for PCP were Km = 6.3 microM, Kcat = 51.4 s-1, and kcat/Km = 8.16 microM-1.s-1. The kinetic parameters for XCP were Km = 3.6 microM, kcat = 52.2 s-1, and kcat/Km = 14.5 microM-1.s-1. PCP showed a stricter substrate specificity than XCP. That is, the specificity constant (kcat/Km) of each substrate for PCP was in general < 0.5 microM-1.s-1, but was drastically improved by the replacement of Lys by Leu at the P2 position. On the other hand, XCP showed a less stringent substrate specificity, with most of the peptides exhibiting reasonable kcat/Km values (> 1.0 microM-1.s-1). Thus it was found that the substrate specificities of PCP and XCP differ considerably, in spite of the high similarity in their primary structures. In addition, tyrostatin was found to be a competitive inhibitor for XCP, with a Ki value of 2.1 nM, as well as for PCP (Ki = 2.6 nM).

The XmnI restriction-modification system: cloning, expression, sequence organization and similarity between the R and M genes

The xmnIRM genes from Xanthomonas manihotis 7AS1 have been cloned and expressed in Escherichia coli. The nucleotide (nt) sequences of both genes were determined. The XmnI methyltransferase (MTase)-encoding gene is 1861 bp in length and codes for 620 amino acids (aa) (68660 Da). The restriction endonuclease (ENase)-encoding gene is 959 bp long and therefore codes for a 319-aa protein (35275 Da). The two genes are aligned tail to tail and they overlap at their respective stop codons About 4 x 10(4) units/g wet cell paste of R.XmnI was obtained following IPTG induction in a suitable E. coli host. The xmnIR gene is expressed from the T7 promoter. M.XmnI probably modifies the first A in the sequence, GAA(N)4TTC. The xmnIR and M genes contain regions of conserved similarity and probably evolved from a common ancestor. M.XmnI is loosely related to M.EcoRI. The XmnI R-M system and the type-I R-M systems probably derived from a common ancestor.

Cloning and expression of an isovaleryl pepstatin-insensitive carboxyl proteinase gene from Xanthomonas sp. T-22

Xanthomonas carboxyl proteinase (XCP), isolated from Xanthomonas sp. T-22, is the second example of the unique carboxyl proteinases [EC 3.4.23.33] which are insensitive to the classical aspartic proteinase inhibitor. The gene coding for XCP was cloned, sequenced, and expressed in Escherichia coli. The XCP gene contains an open reading frame of 2,481 base pairs encoding a protein of 827 amino acid residues with a M(r) of 83,677. The XCP was synthesized as a large precursor consisting of three regions: NH2-terminal prepro (N-Prepro) (237 amino acid residues); mature XCP (398 a.a.residues); and COOH-terminal pro (C-Pro) (192 a.a. residues). The N-Prepro and mature XCP regions had no sequence similarity to any other proteins reported so far, except the carboxyl proteinase from Pseudomonas sp. 101 [Oda, K., Takahashi, T., Tokuda, Y., Shibano, Y., and Takahashi, S. (1994) J. Biol. Chem. 269, 26518-26524]. The C-Pro region showed high similarity to COOH-terminal regions of other microbial proteinase precursors. E. coli carrying a plasmid containing the cloned wild-type XCP gene produced an 84-kDa protein. This protein was processed into a mature, active form under acidic conditions. This process was completely blocked by tyrostatin, an XCP-specific inhibitor from Kitasatosporia sp. 55, indicating an autocatalytic processing. The purified recombinant XCP had the same characteristics as authentic XCP except for the NH2-terminal amino acid sequence. When the mutant XCP gene truncated in the C-Pro region was expressed in E. coli, an expected 64-kDa protein was detected in the cells, and also processed into the 42-kDa active form under the acidic conditions. Thus, the C-Pro region was not essential for the formation of active mature XCP.

A reappraisal of the diversity and class distribution of aspartate transcarbamoylases in gram-negative bacteria

Recently, the subunit composition of class A aspartate transcarbamoylases (ATCases) in fluorescent pseudomonads has been clarified. We present evidence that distribution of this type of ATCase may be more widespread than at first suspected. Bacterial ATCases exist in three forms: class A (molecular mass approximately 450-500 kDA); class B, typified by Escherichia coli ATCase (approximately 300 kDa); and class C, typified by Bacillus subtilis ATCase (approximately 100 kDa). Using gradient gel electrophoresis with activity-staining to scan bacterial sonicates, we report the existence of six more class ATCases. We have purified one of these, Acinetobacter calcoaceticus ATACase, and found its subunit composition to be similar to that of the pseudomonad ATCases. Two of these ATCases come from bacteria outside the gamma-subgroup of the Proteobacteria, one from the alpha-subgroup and one from Deinococcus radiophilus, a species phylogenetically remote from the Proteobacteria. Unexpectedly, three bacterial species, closely related to the fluorescent pseudomonads and acinetobacters, have ATCases of 100 kDa (class C). One of these, Stenotrophomonas (formerly Xanthomonas) maltophilia has been purified and found to be a homotrimer of 35 kDa polypeptide chains. We believe this is the first time that class C ATCases have been reported in Gram-negative bacteria. A distinctive cluster in the gamma-3 subgroup of the Proteobacteria is formed by the enteric bacteria and their relatives. So far only class B ATCases have been reported in this group. The evolutionary implications of these findings are discussed.

Heterologous growth phase- and temperature-dependent expression and H2O2 toxicity protection of a superoxide-inducible monofunctional catalase gene from Xanthomonas oryzae pv. oryzae

Catalase is an important protective enzyme against H2O2 toxicity. Here, we report the characterization of a Xanthomonas oryzae pv. oryzae catalase gene (katX). The gene was localized and its nucleotide sequence was determined. The gene codes for a 77-kDa polypeptide. The deduced katX amino acid sequence shares regions of high identity with other monofunctional catalases in a range of organisms from bacteria to eukaryotes. The transcriptional regulation of katX was atypical of bacterial monofunctional kat genes. Northern (RNA) analysis showed that katX transcription was highly induced by treatments with low concentrations of menadione, a superoxide generator, and methyl methanesulfonate, a mutagen. It was only weakly induced by H2O2. Unlike in other bacteria, a high level of catalase in Xanthomonas spp. provided protection from the growth-inhibitory and killing effects of H2O2 but not from those of organic peroxides and superoxide generators. Unexpectedly, heterologous expression of katX in Escherichia coli was both growth phase and temperature dependent. Catalase activity in E. coli kat mutants harboring katX on an expression vector was detectable only when the cells entered the stationary phase of growth and at 28 degrees C. The patterns of transcription regulation, heterologous expression, and physiological function of katX are different from previously studied bacterial kat genes.

[Lytic action of lysoamidase from Xanthomonas sp. correlates with the presence of the target ribitol teichoic acids in the cell wall of gram-positive bacteria]

Lysoamidase, a bacteriolytic complex from the culture liquid of Xanthomonas sp., hydrolyzed the cells walls of Staphylococcus aureus, Streptomyces chrysomallus, and Streptomyces azureus, which contain ribitol teichoic acids in addition to peptidoglycan. The cell walls of Streptomyces roseoflavus, Glycomyces harhinensis, and Nocardiopsis dassonvillei, containing glycerol teichoic acids, were not hydrolyzed by lysoamidase. The extent of the hydrolysis of 20-h Str. chrysomallus cells and cell walls, containing 40% ribitol teichoic acids, was considerably higher than that of 40-h cells and cell walls, containing 15% teichoic acids. Homogeneous bacteriolytic enzymes of the lysoamidase complex (muramidase and two bacteriolytic peptidases) most efficiently hydrolyzed S. aureus and Str. chrysomallus cell walls, characterized by the highest content of ribitol teichoic acids, and did not hydrolyze purified peptidoglycan.

Purification and characterization of a newly screened microbial peptide amidase

A microbial peptide amidase was found in a limited screening and purified about 500-fold from Stenotrophomonas maltophilia. The native enzyme has a molecular mass of 38 kDa (gel filtration). The sequence of the first 16 amino acids was determined by Edman degradation. The isoelectric point was found to be around 5.8. The peptide amidase exhibited a pH optimum of 6.0 and a temperature optimum of about 39-45 degrees C. The enzyme is stable in 50 mM TRIS/HCl, pH 7.5, at 30 degrees C, and the residual activity was found to be above 90% after 1 week of incubation. The biocatalyst is not inhibited by potential inhibitors like Hg2+, EDTA, D-cycloserine or dithiothreitol and only weakly influenced by inhibitors of serine proteases. The peptide amidase deamidates selectively C-terminal amide groups in peptide amides without hydrolysing internal peptide bonds or amide functions in the side-chain of glutamine or asparagine. Unprotected amino acid amides are not hydrolysed. The enzyme is stereoselective with regard to L-enantiomers in the C-terminal position.

Prolidase from Xanthomonas maltophilia: purification and characterization of the enzyme

Prolidase (iminodipeptidase, EC 3.4.13.9) was purified from an extract of Xanthomonas maltophilia, by ammonium sulfate fractionation and sequential chromatographies on DEAE-Toyopearl, Toyopearl HW65C, FPLC-Hiload Superdex 200 pg, and FPLC-Hitrap Q columns, which an activity recovery of 2.3%. The enzyme was the most active at pH 7.5 with Leu-Pro as substrate. It was stable between pH 6.0 and 8.5 for 60 min at 37 degrees C and retained half of activity after 60 min at 37 degrees C. The isoelectric point of the enzyme was 3.7. Its molecular weight was estimated to be 100,000 by gel filtration on FPLC-Hiload Superdex 200 and 51,000 by SDS-PAGE, suggesting that it is a dimer. It hydrolyzed dipeptides only if proline is located at the carboxyl terminal position. The enzyme was inhibited by PCMB and o-phenanthroline, and was activated by Mn2+.

A novel beta-galactosidase gene isolated from the bacterium Xanthomonas manihotis exhibits strong homology to several eukaryotic beta-galactosidases

The gene encoding a beta-galactosidase from Xanthomonas manihotis was cloned into Escherichia coli. The gene resides on a 2.4 kb DNA fragment which was isolated from a partial Sau3A library in the cloning vector pUC19 using 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal) as the selection. The enzyme produced by the clone has a specificity for beta 1-3- > beta 1-4-linked galactose. The nucleotide sequence of the gene was determined. The deduced protein sequence contained 597 amino acids yielding a monomeric molecular mass of 66 kDa. The cloned beta-galactosidase showed no similarity to any known prokaryotic beta-galactosidase. However, extensive similarity was observed with eukaryotic beta-galactosidases from animals, plants and fungi. The strongest similarity was with the beta-galactosidases found in the human and mouse lysosomes (42 and 41% identity, respectively). Alignment of the X.manihotis and eukaryotic beta-galactosidase sequences revealed seven highly conserved domains common to each protein. Additionally, Domain 1 in X.manihotis showed similarity to regions within catalytic domains from seven xylanases and cellulases belonging to family 10 of glucosyl hydrolases. A region spanning Domain 2 showed similarity to the catalytic domain of endo beta 1-3 glucanases from tobacco and barley.

Mathematical corrections for bacterial loss in pharmacodynamic in vitro dilution models

In vitro dilution models are used to simulate in vivo drug concentration-time profiles and thus to study the effects of various antibiotic concentrations on the bacteria investigated. The major disadvantage of these models is permanent dilution of the bacterial culture, which falsifies the resulting kill curves. Known equations, which usually correct bacterial loss by simple first-order kinetics, do not take into account special test conditions, such as variable elimination rate constants, exceptionally long periods of investigation, or formation of biofilms. In the present investigation, we examined the validity of these equations with regard to the test conditions mentioned. We simulated the concentration-time curves resulting from continuous infusion of 1,000 mg of meropenem with steady-state levels of 2.5, 5.0, and 7.5 micrograms/ml in an in vitro dilution model. The resulting kill curves were compared with the kill curves obtained from incubation of bacteria in an undiluted system with meropenem at constant concentrations corresponding to the above-mentioned steady-state levels. Comparison of the matching kill curves showed that the common corrections, which do not consider the formation of biofilms in the compartments, partly overestimated the effect of bacterial dilution. We defined a factor, f, as an extension to the known equations which compensates for the effect of biofilms. Another extension was developed to allow the investigation of variable elimination rate constants. With the help of these extended mathematical corrections, we were able to fit the kill curves resulting from the in vitro dilution model exactly to the kill curves given by an undiluted system.

Clinical isolate of a Xanthomonas maltophilia strain producing L-1-deficient and L-2-inducible beta-lactamases

Xanthomonas maltophilia produces two inducible beta-lactamases, L-1 and L-2, and resists the antimicrobial activity of beta-lactam antibiotics including carbapenems. L-1 has carbapenemase activity and L-2 is a cephalosporinase. It has been suggested that these beta-lactamases share regulatory components. We isolated a recent clinical X. maltophilia strain susceptible to carbapenems and resistant to almost all the other beta-lactam antibiotics tested. beta-Lactamase induction with cefotaxime showed that the clinical isolate had low-level expression of L-1 beta-lactamase but remained inducible for L-2 enzyme. The possible relationship of this enzyme to carbapenem sensitivity is considered.

Prolylcarboxypeptidase (angiotensinase C): purification and characterization of the enzyme from Xanthomanas maltophilia

Prolylcarboxypeptidase (Angiotensinase C, EC 3.4.16.2) was purified to homogeneity from cell free extracts of Xanthomonas maltophilia by ammonium sulfate fractionation and sequential chromatographies on DEAE-Toyopearl, Sephadex G-150, FPLC-Hiload Superdex 200 pg, and FPLC-Hitrap SP columns, with an activity recovery of 15%. The molecular weight of the enzyme was found to be 330,000 by gel filtration and 83,000 by SDS-PAGE, suggesting a tetrameric form for the native enzyme. It had an optimum pH of 8.5 and stability between pH 8.0 and 11.0. The isoelectric point of the enzyme was 6.6. The enzyme hydrolyzed Pro-X bonds when proline was in the penultimate position from the carboxyl terminal. The enzyme was strongly inhibited by diisopropylfluorophosphate (DFP), while phenylmethylsulfonyl fluoride (PMSF), p-chloromercuribenzoic acid (PCMB), iodoacetamide, and metal chelators had no effect.

Purification and characterization of novel glycosidases from the bacterial genus Xanthomonas

Enzymatic analysis of oligosaccharides using exoglycosidases has become a powerful tool for determining the sequence and structure of sugar chains. The principal limitation to these methods has been the lack of highly purified and well-characterized enzymes. Using fluorescently labelled carbohydrate substrates and TLC, we have developed a method to identify glycosidases with novel specificities. This screening method led to the discovery that bacteria of the genus Xanthomonas are a rich source of exoglycosidases. From Xanthomonas manihotis, eight novel exoglycosidases have been isolated and characterized. A novel beta-N-acetylglucosaminidase has been purified that, unlike those previously described, will cleave N-acetylglucosamine without cleaving N-acetylgalactosamine residues. A novel beta-galactosidase has been isolated that preferentially hydrolyses beta(1-->3) galactosyl linkages. Three alpha-mannosidases have been isolated that serve as useful reagents in the analysis of high-mannose oligosaccharide structures: alpha 1-3,6 mannosidase, alpha 1-6 mannosidase and alpha 1-2,3 mannosidase. An alpha 1-3,6 galactosidase has been purified that does not hydrolyse terminal alpha 1-4 galactose residues. Two fucosidases, alpha 1-3,4 fucosidase and alpha 1-2 fucosidase, are similar to enzymes purified from other sources. Together, these glycosidases provide powerful reagents for determining the sequence of complex carbohydrates. Equally important is their usefulness in selectively removing specific sugar residues and thereby creating novel carbohydrates for analysing the biological roles of oligosaccharides.

The role of lipopolysaccharide anionic binding sites in aminoglycoside uptake in Stenotrophomonas (Xanthomonas) maltophilia

Aminoglycoside resistance was investigated in six clinical isolates of Stenotrophomonas (Xanthomonas) maltophilia by studying the uptake kinetics and by using a radiochemical method to detect aminoglycoside modifying enzymes. Furthermore, the lipopolysaccharides (LPS) were extracted and characterized by SDS-PAGE and chemical analysis. Dansyl-polymyxin displacement experiments confirmed the availability of anionic binding sites. Growing cells of the isolates bound dansyl-polymyxin but were not lysed.

A low-copy number plasmid mediating beta-lactamase production by Xanthomonas maltophilia

To delineate the mechanisms contributing to the high level of antimicrobial resistance often demonstrated by Xanthomonas maltophilia, plasmid DNA was isolated from 5 clinical isolates and analyzed. Purified plasmid DNA from a single isolate contained a 6.5 kb plasmid (pXM222) and a 5.6 kb plasmid, (pTHB). Transformation of pTHB into E. coli HB101 resulted in the expression of resistance to all penicillins tested and cefazolin.

Biochemical properties of inducible beta-lactamases produced from Xanthomonas maltophilia

Four different beta-lactamases have been found in several strains of Xanthomonas maltophilia isolated from blood cultures during 1984 to 1991 at the Edinburgh Royal Infirmary. One was a metallo-beta-lactamase with predominantly penicillinase activity and an isoelectric point of 6.8. Its molecular size as determined by gel filtration was 96 kDa but was only 26 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), suggesting a tetramer of four equal subunits. The enzyme hydrolyzed all classes of beta-lactams except the monobactam aztreonam. This enzyme was not inhibited by potassium clavulanate or BRL 42715 but was inhibited by p-chloromercuribenzoate, mercuric chloride, and EDTA. The beta-lactamase was unstable in 50 mM sodium phosphate buffer (pH 8.0) but stable in 50 mM Tris HCl (pH 8.0). The other beta-lactamases focused as a series of different isoelectric points, ranging from pI 5.2 to 6.6. Together, these enzymes exhibited a broad spectrum of activity, hydrolyzing most classes of beta-lactams but not imipenem or aztreonam. Their molecular size was 48 kDa by Sephadex gel filtration and 24 kDa by SDS-PAGE, indicating that they were enzymes consisting of two equal subunits. They were inhibited by p-chloromercuribenzoate, mercuric chloride, potassium clavulanate, and BRL 42715 but not EDTA. This study demonstrated that X. maltophilia produces more than just the L1 and L2 beta-lactamases.

Rapid identification of metallo- and serine beta-lactamases

Simple methods to detect, identify, and differentiate metallo- and serine beta-lactamases were developed and used to differentiate enzymes produced by 17 clinical isolates of Xanthomonas maltophilia. All isolates exhibited beta-lactamase activity, and in 16 strains this was induced by imipenem. All but one isolate hydrolyzed imipenem (and meropenem), and in all cases this activity was inhibited by 1 mM EDTA. The metallo- and serine beta-lactamases in the cell extracts were distinguished on isoelectric focusing (IEF) gels by using the following procedures. (i) Cell lysates were preincubated with 83 mM EDTA prior to IEF and subsequent visualization with nitrocefin, and (ii) after IEF, the gels were overlaid with either 1 mM zinc sulfate or 100 microM BRL 42715 before staining with nitrocefin. Bands of beta-lactamase activity which were removed by BRL 42715 but unaffected by EDTA or zinc sulfate were categorized as serine beta-lactamases. Bands which were unaffected by BRL 42715 but inhibited by EDTA or enhanced by zinc sulfate were classified as metallo-beta-lactamases. By using this approach, seven metallo-beta-lactamases were differentiated with pI values of 4.8 (two strains), 5.5 (four strains), 5.7 (one strain), 6.0 (one strain), 6.4 (four strains), 6.6 (one strain), and 6.8 (three strains). The metallo-beta-lactamase band with a pI of 6.4 aligned with the recently characterized metallo-beta-lactamase from X. maltophilia 511. Heterogeneity was also observed for the serine beta-lactamases: 14 isolates elaborated serine beta-lactamase activity which focused with major bands with at least eight different pIs. The remaining three strains produced serine beta-lactamases which focused with five distinct bands with pIs of 6.4, 6.2, 5.7, 5.5, and 5.2. We conclude that X. maltophilia produces many types of metallo- and serine beta-lactamases distinguishable by these new methods and that the previously reported L-1 and L-2 enzymes are not solely representative of the beta-lactamases produced by this species.

Interrelated approach to optimization of biosynthesis and chemical isolation of biologically active substances: the production of penicillinamidase by Escherichia coli and peptidohydrolase by Xanthomonas sp

The solution to the problem of optimizing conditions for the isolation of biologically active substances (BAS) from microbial cells should be based on investigations of the structural and functional characteristics of cultures. Models of two bacterial cultures, of Escherichia coli and a Xanthomonas species, producing enzymes the localization of which differ, is described. The isolation of membrane-bound penicillinamidase from E. coli was optimal in the 'preautolysis' period, when the components of the cytoplasm autolysed but the membranes remained intact. In contrast, the isolation of the cytoplasmic enzyme peptidohydrolase from Xanthomonas sp. was optimal during the period when the cell membranes markedly changed. Thus the physiological state of the cultures and the localization of the BAS within the cells are important determinants for optimization of the isolation process. It follows that all stages of a technological process for the production of BAS, i.e. biosynthesis, chemical isolation, etc., should be interrelated for a successful outcome.

Synthesis of beta-lactam antibiotics containing alpha-aminophenylacetyl group in the acyl moiety catalyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase

D-(-)-Phenylglycyl-beta-lactamide amidohydrolase was isolated from Xanthomonas sp., purified, and characterized. A characteristic feature of the enzyme is its high specificity for substrates containing an alpha-aminophenylacetic group in the acyl moiety. Cephalexin and D-C-(-)-phenylglycine methyl ester (MEPG), being nonspecific penicillin acylase (EC 3.5.1.11) substrates, have the highest values of bimolecular constant kcat/Km (2.8 x 10(5) and 2.0 x 10(5) M-1 x s-1, respectively) in the case of amidohydrolase. On the contrary, benzylpenicillin is not hydrolyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase. The other peculiarity of the enzyme is its affinity for the charged forms of substrates. Using the amidohydrolase, it was found that the values of delta Go'pH7.0 for hydrolysis of the amide bond in cephalexin and ampicillin are -3.3 and -2.3 kJ mol-1, respectively. They are less by a minimum of 2.7 kJ mol-1 than those for other beta-lactam antibiotics. Detailed thermodynamic and kinetic studies of the synthesis of cephalexin from MEPG and 7-aminodeacetoxycephalosporanic acid (7-ADCA) catalyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase were undertaken. A kinetic scheme is proposed which describes well the experimental curves. The value of conversion of "nucleus" was found to be 76% when the synthesis was carried out from a 31.5 mM solution of 7-ADCA and an 88.5 mM solution of MEPG at pH 6.2 (optimum conditions). A 75% conversion of 7-aminocephalosporanic acid (7-ACA) was achieved in the synthesis of cephaloglycine catalyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase.

Structure and evolution of the XcyI restriction-modification system

The XcyI restriction-modification system from Xanthomonas cyanopsidis recognizes the sequence, CCCGGG. The XcyI endonuclease and methylase genes have been cloned and sequenced and were found to be aligned in a head to tail orientation with the methylase preceding and overlapping the endonuclease by one base pair. The nucleotide sequence codes for an N4 cytosine methyltransferase with a predicted molecular weight of 33,500 and an endonuclease comprised of 333 codons and a molecular weight of 36,600. Sequence comparisons revealed significant similarity between the XcyI, CfrI and SmaI methylisomers. In contrast, no similarity was detected between the primary structures of the XcyI and SmaI endonucleases. The XcyI restriction-modification system is highly homologous to the XmaI genes, although the DNA sequences flanking the genes rapidly diverge. The sequence of the XcyI endonuclease contains two motifs which have recently been identified as essential to the activity of the EcoRV endonuclease.