A plasmid-mediated cephalosporinase from Achromobacter species

Genetic Characterization of AmpC and Extended-Spectrum Beta-Lactamase Phenotypes in Escherichia coli and Salmonella From Alberta Broiler Chickens

Horizontal gene transfer is an important mechanism which facilitates bacterial populations in overcoming antimicrobial treatment. In this study, a total of 120 Escherichia coli and 62 Salmonella enterica subsp. enterica isolates were isolated from broiler chicken farms in Alberta. Fourteen serovars were identified among Salmonella isolates. Thirty one percent of E. coli isolates (37/120) were multiclass drug resistant (resistant to ≥ 3 drug classes), while only about 16% of Salmonella isolates (10/62) were multiclass drug resistant. Among those, eight E. coli isolates had an AmpC-type phenotype, and one Salmonella isolate had an extended-spectrum beta-lactamase (ESBL)-type beta-lactamase phenotype. We identified both AmpC-type (bla_CMY-2) and ESBL-type (bla_TEM) genes in both E. coli and Salmonella isolates. Plasmids from eight of nine E. coli and Salmonella isolates were transferred to recipient strain E. coli J53 through conjugation. Transferable plasmids in the eight E. coli and Salmonella isolates were also transferred into a lab-made sodium azide-resistant Salmonella recipient through conjugation. The class 1 integrase gene, int1, was detected on plasmids from two E. coli isolates. Further investigation of class 1 integron cassette regions revealed the presence of an aadA gene encoding streptomycin 3’’-adenylyltransferase, an aadA1a/aadA2 gene encoding aminoglycoside 3’’-O-adenyltransferase, and a putative adenylyltransferase gene. This study provides some insight into potential horizontal gene transfer events of antimicrobial resistance genes between E. coli and Salmonella in broiler chicken production.

Detection of expanded-spectrum β-lactamases in Gram-negative bacteria in the 21st century

Emerging β-lactamase-producing-bacteria (ESBL, AmpC and carbapenemases) have become a serious problem in our community due to their startling spread worldwide and their ability to cause infections which are difficult to treat. Diagnosis of these β-lactamases is of clinical and epidemiological interest. Over the past 10 years, several methods have been developed aiming to rapidly detect these emerging enzymes, thus preventing their rapid spread. In this review, we describe the range of screening and detection methods (phenotypic, molecular and other) for detecting these β-lactamases but also whole genome sequencing as a tool for detecting the genes encoding these enzymes.

Efficacy of cefepime and imipenem in experimental murine pneumonia caused by porin-deficient Klebsiella pneumoniae producing CMY-2 beta-Lactamase

Previous studies have shown decreased in vitro activity of zwitterionic cephalosporins and carbapenems against porin-deficient Klebsiella pneumoniae expressing a plasmid-mediated AmpC-type beta-lactamase (PACBL). The in vitro and in vivo activities of cefepime and imipenem were evaluated against the porin-deficient strain K. pneumoniae C2 and its CMY-2-producing derivative [K. pneumoniae C2(pMG248)]. The MICs (in micrograms/milliliter) of cefepime and imipenem against K. pneumoniae C2 were 0.125 and 0.25, respectively, while the corresponding values against K. pneumoniae C2(pMG248) were 8 and 16. Cefepime showed a greater inoculum effect than imipenem against both strains. Imipenem showed a significant postantibiotic effect (>2 h) against K. pneumoniae C2(pMG248) at 1x, 2x, 4x, 6x, and 8x MIC. The maximum concentrations of drug in serum of cefepime and imipenem in a pneumonia model using mice were 124.1 and 16.9 mug/ml, respectively. DeltaT/MIC for K. pneumoniae C2 and C2(pMG248) were 1.29 h and 0.34 h for imipenem and 2.96 h and 1.27 h for cefepime. Both imipenem (30 mg/kg of body weight every 3 h) and cefepime (60 mg/kg every 4 h), administered for 72 h, increased the survival rate (86.6% and 100%) compared with untreated control animals (26.6%, P < 0.003) infected with K. pneumoniae C2. For the CMY-2-producing strain, imipenem, but not cefepime, increased the survival rate compared to the controls (86.6% and 40% versus 40%, P < 0.01). Bacterial concentration of the lungs was significantly decreased by both antimicrobials. In conclusion, imipenem was more active in terms of survival than cefepime for the treatment of murine pneumonia caused by a porin-deficient K. pneumoniae expressing PACBL CMY-2.

Identification of a carbenicillin-hydrolyzing beta-lactamase in Alcaligenes denitrificans subsp. xylosoxydans

Eleven strains of Alcaligenes denitrificans subsp. xylosoxydans produced a beta-lactamase with a pI of 5.7 with kinetic data characteristic of a PSE-1-type enzyme. A CARB-type enzyme was identified by using an intragenic DNA probe of blaCARB. Hybridization of genomic DNA after XbaI restriction and pulsed-field electrophoresis suggested a chromosomal location for the gene.

Novel plasmid-mediated beta-lactamase (MIR-1) conferring resistance to oxyimino- and alpha-methoxy beta-lactams in clinical isolates of Klebsiella pneumoniae

Klebsiella pneumoniae isolates from 11 patients at the Miriam Hospital were identified as resistant to cefoxitin and ceftibuten as well as to aztreonam, cefotaxime, and ceftazidime. Resistance could be transferred by conjugation or transformation with plasmid DNA into Escherichia coli and was due to the production of a beta-lactamase with an isoelectric point of 8.4 named MIR-1. In E. coli, MIR-1 conferred resistance to aztreonam, cefotaxime, ceftazidime, ceftibuten, ceftriaxone, and such alpha-methoxy beta-lactams as cefmetazole, cefotetan, cefoxitin, and moxalactam. In vitro, MIR-1 hydrolyzed cephalothin and cephaloridine much more rapidly than it did penicillin G, ampicillin, or carbenicillin. Cefotaxime was hydrolyzed at 10% the rate of cephaloridine. Cefoxitin inactivation could only be detected by a microbiological test. The inhibition profile of MIR-1 was similar to that of chromosomally mediated class I beta-lactamases. Potassium clavulanate had little effect on cefoxitin or cefibuten resistance and was a poor inhibitor of MIR-1 activity. Cefoxitin or imipenem did not induce MIR-1. The gene determining MIR-1 was cloned on a 1.4-kb AccI-PstI fragment. Under stringent conditions, probes for TEM-1 and SHV-1 genes and the E. coli ampC gene failed to hybridize with the MIR-1 gene. However, a provisional sequence of 150 bp of the MIR-1 gene proved to be 90% identical to the sequence of ampC from Enterobacter cloacae but only 71% identical to that of E. coli, thus explaining the lack of hybridization to the E. coli ampC probe. Plasmid profiles of the 11 K. pneumoniae clinical isolates were not identical, but each contained a plasmid from 40 to 60 kb that hybridized with the cloned MIR-1 gene. Both transfer-proficient and transfer-deficient MIR-1 plasmids belonged to the N incompatibility group. Thus, the resistance of these K. pneumoniae strains was the result of plasmid acquisition of a class I beta-lactamase, a new resistance determinant that expands the kinds of beta-lactam resistance capable of spread by plasmid dissemination among clinical isolates.

Susceptibility of Alcaligenes denitrificans subspecies xylosoxydans to beta-lactam antibiotics

The susceptibility of 56 clinical isolates and two reference strains of Alcaligenes denitrificans subsp. xylosoxydans to beta-lactam agents was tested and related to beta-lactamase activity of the strains. The MICs of 12 beta-lactams determined by an agar dilution method showed that all the strains were sensitive to imipenem and moxalactam. Forty-one cloxacillin-sensitive beta-lactamase producing strains were highly susceptible to azlocillin, piperacillin and ticarcillin, and less susceptible to several cephalosporins (cefamandole, cefoperazone, ceftazidime). The 17 remaining beta-lactamase-producing strains, which were sensitive to clavulanic acid and to a lesser extent cloxacillin, had variable resistance to the penicillins tested and synergy was obtained when these penicillins were combined with clavulanic acid or tazobactam. The choice of agents for treatment of infections with this organism must take into account the susceptibility phenotype of clinical isolates.

Evolutionary perspectives on multiresistance beta-lactamase transposons

A series of intragenic DNA probes, encoding the major part of the transposase resolvase and inverted repeats of transposons Tn3, Tn21, and Tn2501, were used in hybridization assays for homologous DNA sequences in 18 transposons studied. The tnpA and tnpR probes detected extensive homology with Tn3-like and Tn21-like elements for 11 transposons. This high degree of homology was confirmed with the 38- and 48-base-pair inverted-repeat oligonucleotide probes of Tn3, Tn21, and Tn2501. The Southern-type gel hybridization experiments localized the tnpA-homologous sequences on the physical DNA maps constructed. The genetic and physical maps of the transposons were compared, as were their nucleic acid sequence homologies. These comparisons suggested a subfamily of mobile elements distinct from but related to the Tn21 group. Based on these results, an evolutionary model is proposed and a pedigree is presented for the genesis of multiresistance beta-lactamase transposons.

Resistance to third generation cephalosporins: the current situation

Newer beta-lactam antibiotics, notably the third generation cephalosporins (3 GC) have been designed for providing high intrinsic potency against a large variety of microorganisms. Bacterial resistance can occur however, and nowadays, clinicians are concerned by novel situations where even most recently developed compounds can be ineffective. A first situation is generated by bacteria which produce great amounts of chromosomal cephalosporinase. The resistance emerges during therapy, in hospital isolates which are classified as susceptible with conventional susceptibility testing. The prevalence of 3 GC resistance among these gram-negative rods with inducible beta-lactamase seems to increase in some institutions but the significance of susceptibility testing in this regard is doubtful. It is probably more important to note that the prevalence of gram-negative rods with inducible beta-lactamases remains stable. A second problem arose with the abrupt development of plasmid mediated beta-lactamases markedly active against 3 GC. This resistance is underestimated because some strains fall into susceptibility range of 3 GC as determined by MICs or inhibition zone sizes. These extended spectrum enzymes are now distributed over four continents and represent a growing threat.

SAR-2: identification of a novel plasmid-encoded beta-lactamase from India

A novel beta-lactamase has been identified in an Escherichia coli strain isolated in South India. The beta-lactamase gene was carried on a plasmid (pUK734) along with resistance determinants to sulphonamides and tetracycline. The novel enzyme has a pI of 8.3 and an Mr of 36,000. The enzyme has a broad-spectrum of activity against both penicillins and cephalosporins. It is also active against oxacillin and methicillin.

Novel plasmid-mediated beta-lactamase from Escherichia coli that inactivates oxyimino-cephalosporins

A highly cephem-resistant Escherichia coli strain, FP1546, isolated from the fecal flora of laboratory dogs previously administered beta-lactam antibiotics was found to produce a beta-lactamase, FEC-1, of 48-kilodalton size and pI 8.2. FEC-1 hydrolyzed cefuroxime, cefotaxime, cefmenoxime, and ceftriaxone, as well as the enzymatically less-stable antibiotics cephaloridine, cefotiam, and cefpiramide. Of the oxyimino-cephalosporins, ceftizoxime was fairly stable to FEC-1. FEC-1 differed notably from chromosomal E. coli cephalosporinase, especially in its broad-spectrum substrate profile and its high inhibition by clavulanic acid, sulbactam, and imipenem. A conjugation study revealed that FEC-1 was encoded by a 74-megadalton plasmid, pFCX1. This may be the first instance of a plasmid-mediated oxyimino-cephalosporinase from E. coli.

In vitro susceptibility of Alcaligenes denitrificans subsp. xylosoxidans to 24 antimicrobial agents

The in vitro susceptibilities of 37 clinical isolates of Alcaligenes denitrificans subsp. xylosoxidans to 24 antimicrobial agents were determined. Imipenem was the only drug with consistent activity (MIC for 90% of isolates, 2 micrograms/ml). Piperacillin, ticarcillin-clavulanic acid, ceftazidime, and co-trimoxazole were active against most strains. All the isolates were resistant to ampicillin, cefazolin, cefuroxime, cefamandole, cefotetan, ceftriaxone, cefotaxime, aztreonam, amdinocillin, and temocillin. Most isolates were resistant to the aminoglycosides tested, including amikacin. Lack of activity was also observed for all new 4-quinolone antimicrobial agents.

Diversity of plasmids in Achromobacter xylosoxidans isolates responsible for a seemingly common-source nosocomial outbreak

Achromobacter xylosoxidans, an uncommon yet highly resistant opportunistic pathogen, was isolated from nine hospitalized patients during an 8-month period. It had been isolated from only seven patients with either nonfatal infection or colonization from 1981 to 1984. From June 1985 to January 1986, A. xylosoxidans was isolated 18 times from seven different sites (sputum, 7 times; urine, 4 times; blood, 3 times; and lung, pleural fluid, wound tissue, and tracheal aspirate, 1 time each). Four patients died, including the three with bacteremia. All but two patients had nosocomial infections and either were on the same ward or were cared for by the same staff members. Eleven A. xylosoxidans strains yielded eight distinct plasmids (8, 21, 23, 26, 38, 50, 51, and 64 megadaltons). Whole-cell peptide patterns of 10 of these strains were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Isolates from the same patient contained the same plasmids and had identical peptide patterns but differed from other strains in both parameters. Plasmids were absent from the two community-acquired isolates. Although nosocomial strains showed similar antibiotic resistance patterns (only moxalactam and ticarcillin-clavulanic acid were uniformly active) and cross-contamination was strongly suggested epidemiologically, results of plasmid and peptide analyses did not support the possibility of a single-strain outbreak.

Identification of an inducible penicillinase of the lithoautotrophic hydrogen-oxidizing bacterium Alcaligenes eutrophus

The growth of Alcaligenes eutrophus in the presence of benzylpenicillin under heterotrophic and autotrophic conditions was studied. The drug induced a penicillinase in the cells, which can be readily released and extracted from the cells after a lysozyme and EDTA treatment in the course of spheroplast formation. The isoelectric point of the enzyme is 8.1 and the molar mass was estimated to be nearly 25 kg/mol. Phenoxypenicillin is hydrolyzed in the presence of the enzyme at a higher relative rate than benzylpenicillin, ampicillin, amoxycillin and azlocillin. The cephalosporins tested, i.e. cephalosporin C, cefalexin, cefotaxime and 7-aminocephalosporanic acid, were hydrolyzed at a substantially lower relative rate than the penicillins, indicating that the enzyme is a penicillinase.

Molecular cloning and DNA homology of plasmid-mediated beta-lactamase genes

Molecular cloning of DNA fragments between 1.5 and 8 kb from BamHI, EcoRI, HindIII, SalI, or Sau3A digests permitted the isolation of structural genes coding for TEM-1, ROB-1, OXA-1, OXA-3, OXA-4, OXA-5, PSE-1, PSE-2, PSE-3, PSE-4, CARB-3, CARB-4, AER-1, and LCR-1 beta-lactamases. Ampicillin-resistant clones were selected and it was confirmed that they contained the respective beta-lactamase genes by isoelectric focusing. Detailed physical maps of 14 different recombinant plasmids were constructed using 8 restriction endonucleases. Plasmid deletions and lacZ fusions were used to localize the beta-lactamase structural genes. DNA probes were constructed for the TEM-1, ROB-1, OXA-1, and PSE-1 genes. Under conditions of high stringency, hybridization was observed between the genes for TEM-1 and TEM-2 or TLE-1, OXA-1 and OXA-4, and PSE-1 and PSE-4 or CARB-3, while the ROB-1 gene probe showed no cross-hybridization. Such bla gene probes should facilitate studies of beta-lactamase molecular epidemiology.

Mechanisms of resistance to cephalosporin antibiotics

Cephalosporins, like other beta-lactams, bind to the bacterial penicillin-binding proteins (PBPs). These correspond to the D-ala-D-ala trans-, carboxy- and endo-peptidases responsible for catalysing the cross-linking of newly formed peptidoglycan. Resistance arises when the PBPs-and particularly the transpeptidases-are modified, or when they are protected by beta-lactamases or 'permeability barriers'. Target-mediated cephalosporin resistance can involve either reduced affinity of an existing PBP component, or the acquisition of a supplementary beta-lactam-insensitive PBP. beta-lactamases are produced widely by bacteria and may be determined by chromosomal or plasmid DNA. The chromosomal beta-lactamases are species-specific, but can be classified into a few broad groups. The plasmid-mediated enzymes cross interspecific and intergeneric boundaries. The level of beta-lactamase-mediated resistance relates to the amount of enzyme produced with or without induction; to the location of the enzyme (extracellular for Gram-positive organisms and periplasmic in Gram-negative ones); and to the kinetics of the enzyme's activity. In Gram-positive organisms the PBPs are located on the outer aspect of the cytoplasmic membrane and so shielding by permeability barriers is minimal. In Gram-negative cells, however, the PBPs are protected by the outer membrane, which most beta-lactams cross by diffusion through aqueous pores composed of 'porin' proteins. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to drug entry. Such relationships are less clear for Pseudomonas aeruginosa, where the cell may contain additional barriers between the outer membrane and the PBPs. Although elevated cephalosporin resistance often is attributed to a single factor (PBP-modification, beta-lactamase action or impermeability) an organism's response to a drug often reflects the interplay of several factors. Mathematical models can be proposed to describe this interplay.

Peritonitis caused by Alcaligenes denitrificans subsp. xylosoxydans: case report and review of the literature

We report the third human case of peritonitis caused by Alcaligenes denitrificans subsp. xylosoxydans and review the English literature regarding community-acquired and nonsocomial infection and colonization that results from this bacterium. The biochemical and genetic characteristics supporting the pathogenic potential of A. denitrificans subsp. xylosoxydans are reviewed, and the antimicrobial susceptibility profile of the organism is summarized.

Contribution of beta-lactamases to bacterial resistance and mechanisms to inhibit beta-lactamases

Resistance of bacteria to beta-lactam antibiotics has become a serious problem in the past several decades. Virtually all Staphylococcus aureus, and many Hemophilus influenzae, Branhamella catarrhalis, Neisseria gonorrhoeae, Enterobacteriaceae, and Bacteroides species possess beta-lactamases that hydrolyze penicillins and cephalosporins. The most common plasmid-mediated beta-lactamase is the TEM enzyme (Richmond-Sykes type IIIa), which is present in Hemophilus, Neisseria, and Enterobacteriaceae. One technique to overcome bacterial resistance has been the development of beta-lactamase inhibitors. Clavulanic acid is a beta-lactamase inhibitor that inhibits the beta-lactamases of S. aureus, Hemophilus, Neisseria, Branhamella, Eschericia coli, Klebsiella, and Bacteroides. Clavulanate acts as a "suicide" inhibitor, forming a stable enzyme complex that binds to serine at the active site of the enzyme. Clavulanate readily crosses the outer cell wall of most Enterobacteriaceae to interact with beta-lactamases in the periplasmic space. Clavulanate does not inhibit beta-lactamases such as the Richmond-Sykes type I enzymes found in Pseudomonas aeruginosa, Enterobacter, and Citrobacter species, which are inducible enzymes that function primarily as cephalosporinases.

Five novel plasmid-determined beta-lactamases

Five novel plasmid-determined beta-lactamases named TLE-1, OXA-4, OXA-5, OXA-6, and OXA-7 were detected in ampicillin-resistant isolates of Escherichia coli and carbenicillin-resistant strains of Pseudomonas aeruginosa. TLE-1 resembled TEM-1 in substrate profile and reactions with inhibitors but differed in isoelectric point (5.55) and enzyme banding pattern on flat-bed electrofocusing.OXA-4, OXA-5, OXA-6, and OXA-7 hydrolyzed oxacillin, methicillin, and cloxacillin readily but differed from OXA-1, OXA-2, and OXA-3 in substrate profiles, inhibitor reactions, and isoelectric points (7.5 to 7.8).OXA-4 and OXA-6 were unusual for members of the OXA group in their sensitivity to inhibition by cloxacillin. OXA-5 and OXA-7 had isoelectric points close to that of SHV-1, emphasizing the need in beta-lactamase classification for studies in addition to isoelectric focusing. These five new enzymes bring the number of plasmid-determined beta-lactamases known in gram-negative organisms to more than 20. The evolution of such enzymatic diversity remains to be explored.

Genetic and biochemical properties of AER-1, a novel carbenicillin-hydrolyzing beta-lactamase from Aeromonas hydrophila

A novel carbenicillin-hydrolyzing beta-lactamase has been discovered in a blood isolate of Aeromonas hydrophila. The enzyme resembles plasmid-determined carbenicillinases in substrate profile but differs in isoelectric point (pI 5.9) and molecular weight (22,000) and has been termed AER-1. No evidence for a plasmid location could be obtained in A. hydrophila, but the AER-1 gene and resistance to chloramphenicol, streptomycin, and sulfonamide could be transferred by mobilization with IncP plasmids to Escherichia coli, where the gene cluster inserted at a unique chromosomal site. The linked resistances are similar to those found on multiresistance beta-lactamase transposons, but since insertion of the A. hydrophila gene cluster was site specific and recA+ dependent, the cluster is not a functional transposon.

Rapid suppression of alpha 1-fetoprotein gene transcription by dexamethasone in developing rat liver

The administration of glucocorticosteroid hormones to newborn rats interrupts selectively (and reversibly, if the hormone is withdrawn) the hepatic production of alpha 1-fetoprotein (AFP). This results from a decreased concentration of AFP mRNA in the liver [Bélanger, L., Frain, M., Baril, P., Gingras, M.C., Bartkowiak, J., & Sala-Trepat, J.M. (1981) Biochemistry 20, 6665]. We have delineated further the mechanism and time course of this hormonal action in 4-day-old rats treated with dexamethasone (DEX). DNA from a recombinant plasmid containing a 578-bp insert of rat AFP cDNA was used to develop a cell-free nuclear run-off system and directly assess AFP gene transcription activity. Five minutes after DEX injection, AFP gene transcription activity is unchanged, but after 30 min, it drops to 25% that of the control; this correlates with the time required for translocation of DEX receptors to the nucleus. Dose-response curves also show that the degree of AFP gene suppression is closely correlated with the amount of DEX receptor translocated to the nucleus. The nuclear concentration of AFP mRNA, monitored by dot-blot hybridization, decreases to undetectable levels within 48 h, whereas that of albumin mRNA increases slightly, which indicates the selectivity of DEX action. These results show that DEX suppresses AFP gene expression at the transcriptional level and suggest a direct negative action of DEX-receptor complexes on the AFP chromatin transcription unit.

Rapid method for screening large numbers of Escherichia coli colonies for production of plasmid-mediated beta-lactamases

A rapid, simple assay for screening large numbers of Escherichia coli colonies for production of certain plasmid-mediated beta-lactamases (including TEM-1, TEM-2, HMS-1, SHV-1, OXA-1, PSE-1, PSE-4, and CEP-2) is described. The technique, a modification of the method of Slack et al. for detection of beta-lactamase in limited numbers of Haemophilus influenzae clinical isolates (Lancet ii:906, 1977), uses filter paper impregnated with benzylpenicillin and a pH indicator dye (bromocresol purple) that changes color in the presence of beta-lactamase activity. The test paper is briefly applied to an agar surface containing bacterial colonies which are subsequently scored individually on the paper by color: yellow indicates the presence of beta-lactamase, dark green its absence, and variegation (yellow and dark green) a mixed population. Concordance of the results of this assay with those of replica plating for antibiotic resistance was over 99%. Hundreds of colonies per plate can be scored quickly and remain viable for further evaluation. The assay appears to be useful for studies of the stability of plasmids encoding beta-lactamases and in cloning with vectors such as pBR322 in which insertion of DNA fragments can be detected by inactivation of the beta-lactamase gene. Whenever the assay is to be used, results should always be confirmed initially by another method, such as replica plating, because the test paper assay does not detect three beta-lactamases (OXA-2, OXA-3, and PSE-2) and also would miss intrinsic penicillin resistance.

Mapping of the plasmid (pLQ3) from Achromobacter and cloning of its cephalosporinase gene in Escherichia coli

We have constructed a physical map of the plasmid pLQ3 which was originally isolated from Achromobacter and which codes for a beta-lactamase. The enzyme specified by pLQ3 is expressed in Escherichia coli and is unusual in that it is a cephalosporinase, an enzyme usually coded for by chromosome. Plasmid pLQ3 is 12.4 kb in length and has a unique Bam HI site and two BglII sites. From a BamHI + BglII double digest of pLQ3, we have constructed a "shortened" plasmid, pLQ10, in which a 2.96-kb fragment is deleted. We have constructed a clone, pLQ22, in which a 3.27-kb fragment of pLQ3, carrying the beta-lactamase gene, is inserted into the BamHI site of pACYC184. By "comparative mapping" of single and multiple digests of each of these plasmids, we have been able to locate the cleavage sites for PstI, which makes seven cuts in pLQ3.