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L Sundström

Publications and source records attributed to L Sundström.

31 records · Page 2Linked to original sources
PubMed1993

A novel dihydrofolate reductase cassette inserted in an integron borne on a Tn21-like element.

In this study, a 498-bp dhfrXII gene coding for trimethoprim resistance was found inserted in a cassette-like manner in the recombinationally active locus, the integron, borne on a transposon Tn21-like element. The dhfrXII cassette is distinct from those cassettes earlier observed in integrons and was found here upstream of two similarly inserted cassettes. The second one carried the new unidentified orfF, which is 85% identical to the orfD cassette in R46. The third cassette contained the aadA2 gene mediating spectinomycin resistance. The plasmid carrying this Tn21-like element was originally isolated from a trimethoprim-resistant urinary tract pathogen, Escherichia coli, from Turku City Hospital, Turku, Finland. By colony hybridization and polymerase chain reaction, this group of three cassettes, including dhfrXII, was detected in four additional E. coli strains of similar origin and in four Shigella strains isolated in Finland but originating from Asia. The dihydrofolate reductase produced from dhfrXII showed an unusual drug resistance in that 50% of the enzymatic activity remained at a trimethoprim concentration of 1 mM.

Amino Acid Sequence↗
PubMed1993

Characterization of transposon Tn5086, carrying the site-specifically inserted gene dhfrVII mediating trimethoprim resistance.

Two different enteric plasmids of widely separate origins were observed to carry a new 15.3-kb trimethoprim resistance transposon, Tn5086, also mediating resistance to mercuric ions and to a low level of sulfonamide. The trimethoprim resistance gene characterized from Tn5086 was found to be distinct from those found earlier and was designated type VII. Molecular analysis demonstrated that Tn5086 is closely related to Tn21. The internal part of Tn21 and Tn5086, the element referred to as the integron, was found to be different. First, the integron of Tn5086 contains a 0.62-kb cassette formed by the trimethoprim resistance gene dhfrVII and its immediate surroundings instead of the 0.86-kb aadA1 cassette of Tn21. Second, the integron of Tn5086 lacks a 4.2-kb segment 3' of sulI in Tn21. The dhfrVII gene commences with a UUG codon but was otherwise seen to be markedly related to the cassette genes dhfrI, dhfrV, and dhfrVI. The four related dihydrofolate reductases of 157 amino acids encoded by these genes contain a glutamate instead of the aspartic acid residue found at position 27 of the active center of the chromosomal enzyme from Escherichia coli.

Amino Acid Sequence↗
PubMed1991

Analysis of genetic localization of the type I trimethoprim resistance gene from Escherichia coli isolated in Finland.

Among a collection of clinical Escherichia coli isolates, the type I dihydrofolate reductase (DHFR) mediating trimethoprim resistance was generally observed to be chromosomally determined. Only a minority of isolates carried the type I DHFR gene simultaneously on a plasmid. The majority of E. coli isolates studied also hybridized with a probe specific for the transposition gene tnsC of transposon Tn7; and in most of these isolates, Tn7 was found to be inserted into a preferred site in the E. coli chromosome. A minority of isolates that harbored the type I DHFR gene in the chromosome lacked a complete Tn7. Some of these harbored the type I DHFR gene inserted in a structure similar to that containing the gene for streptomycin resistance in Tn21. In the other isolates that were negative for a complete Tn7, the sequences upstream of the type I DHFR gene were demonstrated to be homologous to those flanking the type I DHFR gene in Tn7. This could indicate that the antibiotic resistance region of Tn7 may occur independently of this transposon. In two isolates, no sequences resembling Tn7 or Tn21 were found adjacent to the type I DHFR gene.

Bacterial Proteins↗
PubMed1991

Site-specific insertion of three structural gene cassettes in transposon Tn7.

Transposon Tn7 has been known to carry genes for resistance to trimethoprim and spectinomycin. A poorly expressed streptothricin resistance gene, identical to the sat gene found in transposons Tn1825 and Tn1826, was localized between the two mentioned genes in Tn7. The surroundings of all three resistance genes indicated site-specific insertion of genetic cassettes.

Amino Acid Sequence↗
PubMed1990

Increase of trimethoprim resistance among Shigella species, 1975-1988: analysis of resistance mechanisms.

Trimethoprim (TMP) resistance among Shigella species isolated from Finnish travelers increased from 3.0% in 1975-1982 to 42.0%-43.8% in 1987-1988. Of the 317 TMP-resistant Shigella isolates identified during 1975-1988, 175 (55%) collected in 1985-1987 and in 1988 were tested further. Almost all (98%) were highly resistant to TMP, suggesting a plasmid-mediated origin. The type I dihydrofolate reductase (DHFR) gene was detected in 85% of the isolates studied. Twenty-three percent of the type I DHFR-positive isolates failed to hybridize with a probe detecting only Tn7-derived sequences, suggesting that the type I DHFR gene may occur independently of transposon Tn7. Four of the five Shigella species isolated from travelers to Sri Lanka hybridized with the probe for type V DHFR gene, implying a local distribution of the type V DHFR gene. The type II and type III DHFR genes were not found among the isolates studied. Only 12% of the TMP-resistant Shigella isolates failed to hybridize with any of the DHFR gene probes used.

DNA Probes↗
PubMed1990

Trimethoprim resistance in Escherichia coli isolates from a geriatric unit.

The frequency of trimethoprim resistance among Escherichia coli isolates from urine samples collected at Turku City Hospital, Turku, Finland, remained at 40% during 1984 to 1988. The proportion of highly resistant (MIC, greater than or equal to 1,024 micrograms/ml) isolates increased, however, and most of these harbored the type I dihydrofolate reductase gene. Only a few isolates possessed type II or VII genes.

Aged↗
PubMed1990

The dhfrI trimethoprim resistance gene of Tn7 can be found at specific sites in other genetic surroundings.

The dhfrI gene, mediating high-level trimethoprim resistance, was earlier found only on Tn7. Evidence is given here for an alternative location of this gene at a site identical to sites observed earlier for dhfrII on plasmid R388, dhfrV on pLMO20, and aadA on Tn21. All these genes and dhfrI are precisely inserted as discrete GTTA-flanked elements at distinct loci in very conserved surrounding sequences. One of these dhfrI insertions was observed to occur in association with a similarly inserted aadA nucleotidyltransferase gene, which mediates streptomycin and spectinomycin resistance. Close to the insertion site, there is an open reading frame translating into a 337-amino-acid peptide which shows striking similarities to recombinases of the integrase family, sulI, the sulfonamide resistance gene, is very often found close to the insertion point forming a genetic surrounding, originally observed as a part of Tn21-like transposons. The alleged integration mechanism thus provides a recombination pathway for the genetic linkage of sulfonamide and other antibiotic resistance genes, including the most frequently encountered gene for trimethoprim resistance, dhfrI. Furthermore, the newly observed location of dhfrI could shed light on the evolution of the antibiotic resistance region of Tn7, which could be able to take up genes by the same mechanism as that of Tn21-like transposons.

Base Sequence↗
PubMed1988

Site-specific recombination promotes linkage between trimethoprim- and sulfonamide resistance genes. Sequence characterization of dhfrV and sulI and a recombination active locus of Tn21.

A new gene for trimethoprim resistance, dhfrV, found in several plasmid isolates with different characteristics, was sequenced and found to correspond to a peptide of 157 amino acids showing 75% similarity with the previously characterized, drug resistant dihydrofolate reductase of type I. The sequenced surroundings of dhfrV in plasmid pLMO20, were found to be almost identical with genetic areas surrounding resistance genes in transposon Tn21 and in R plasmid R388. The trimethoprim resistance genes of pLMO20 and R388 and the spectinomycin resistance gene of Tn21 could be regarded as having been inserted, by recombination, into an evolutionary older structure containing the sulfonamide resistance gene, sulI. The latter gene was sequenced and found to correspond to a peptide of 279 amino acids and with a molecular weight of 30,126 daltons. The inserted genes were found to be governed by a promoter situated in the highly conserved structure and also controlling expression of sulI. The insertion points of the different resistance genes were precisely defined, and at the 3' ends of the inserted genes inverted repeats allowing the formation of stem and loop structures were found. Similar structures were found at the 3' ends of the antibiotic resistance genes in Tn7, which could indicate similar recombination mechanisms to be effective in the evolutionary construction of all these different resistance elements.

Amino Acid Sequence↗
PubMed1987

Novel type of plasmid-borne resistance to trimethoprim.

A novel trait for transferable resistance to high concentrations of trimethoprim was found to dominate among enterobacteria collected from different parts of Sri Lanka. Drug resistance was a result of the production of dihydrofolate reductase with a decreased sensitivity to antifolates. By characterization of the partially purified enzyme and by restriction enzyme digestion analysis, the newly found gene was shown to be distinct from the earlier known plasmid-borne resistance genes which express dihydrofolate reductases of types I, II, and III. Cloning of fragments containing the resistance gene and further restriction enzyme digestion analysis showed that this gene was inserted very close to a sulfonamide resistance gene. Evolution of trimethoprim resistance in Sri Lanka thus seems to have taken a different route from that taken in the industrialized world, where transposon Tn7 seems to dominate. The close combination of the new trimethoprim resistance gene with sulfonamide resistance on the plasmids studied would effect an efficient spread of these genes, since trimethoprim has most often been used in combination with a sulfonamide.

Chromatography, Ion Exchange↗
PubMed1986

New observations regarding evolution of trimethoprim resistance.

A clinically isolated strain of Escherichia coli, resistant to more than 1000 mg/l of trimethoprim, expressed chromosomal dihydrofolate reductase to a level 200-fold higher than that of drug sensitive E. coli K-12 strains, and this high cellular enzyme activity was found to increase further when the cells were cultured in the presence of trimethoprim. The induced increase in enzyme activity was dependent on the drug concentration. The increase was six-fold at 100 mg/l of trimethoprim. The aberrantly regulated dihydrofolate reductase gene mediating trimethoprim resistance could be transduced into E. coli K-12 or moved by recombination into an F' factor and then transferred into trans position in relation to the corresponding chromosomal gene. In either of these positions, the synthesis of dihydrofolate reductase could be induced to increase by adding trimethoprim to the culture medium. The observed induction was dependent on protein synthesis, since it could be abolished by chloramphenicol. No other folic acid analogue was found to induce increased expression of the dihydrofolate reductase gene. Also thymine starvation had no effect. Two further clinical isolates of E. coli, highly resistant to trimethoprim, were shown to produce drug resistant, plasmid-mediated dihydrofolate reductases, which were distinct from the earlier known enzyme types I and II.

Bacteria↗
PubMed1985

Cultured human alveolar macrophages from smokers with lung cancer: resolution of factors that stimulate fibroblast proliferation, production of collagenase, or prostaglandin E2.

Marked connective tissue remodelling involves both destruction and repair in inflammatory lung diseases. Throughout the remodelling event, it was reasoned that alveolar macrophages may release substances similar to those produced by blood monocyte-macrophages that affect fibroblast functions, ie, the interleukin 1 family of monokines (or cytokines). We have examined human alveolar macrophage cultures obtained after bronchoalveolar lavage of freshly excised lungs from heavy smokers with bronchial carcinoma. Crude culture media contained fibroblast proliferative activity and collagenase- and PGE2- production-stimulating activity. The main peak of these biological activities was located around approximately 18 kilodaltons (kD) on gel filtration chromatography. Resolution of this peak by high performance liquid chromatography showed the presence of three distinct peaks, with quantitative and qualitative differences in biological activities. This suggests the presence of heterogeneous factors.

Cell Division↗