Tn5 carries a streptomycin resistance determinant downstream from the kanamycin resistance gene

Transposon Mutagenesis of Azospirillum brasilense and Azospirillum lipoferum: Physical Analysis of Tn5 and Tn5-Mob Insertion Mutants

Tn5-induced insertion mutants were generated in Azospirillum brasilense Sp7 and A. lipoferum SpBr17 by mating with Escherichia coli strains carrying suicide plasmid vectors. The sources of Tn5 were the suicide plasmids pGS9 and pSUP2021. Kanamycin-resistant Azospirillum colonies appeared from crosses with E. coli at maximum frequencies of 10 per recipient cell. Transposon Tn5 also conferred streptomycin resistance on Azospirillum colonies as was observed earlier for Rhizobium sp. Eight Tn5-induced Km SmA. brasilense Sp7 mutants with reduced nitrogen-fixing capacity were isolated. The potential use of Tn5-Mob for labeling and mobilization of Azospirillum-indigenous plasmids was demonstrated by isolating Tn5-Mob insertions in the megaplasmids of A. brasilense Sp7.

Protection of photosynthesis against ultraviolet-B radiation by carotenoids in transformants of the cyanobacterium synechococcus PCC7942

The cyanobacterium Synechococcus PCC7942 was transformed with various carotenogenic genes, and the resulting transformants either accumulated higher amounts of beta-carotene and zeaxanthin or showed a shift in the carotenoid pattern toward the formation of zeaxanthin. These transformants were exposed to ultraviolet-B (UV-B) radiation, and the degradation of phycobilins, the inactivation of photosynthetic oxygen evolution, and the activity of photosystem II were determined. In the genetically modified cells, the influence on destruction of phycobilins was negligible. However, protection of photosynthetic reactions against UV-B damage was observed and was dependent on the carotenoid concentrations in the different transformants. Furthermore, it was shown that endogenous zeaxanthin is more effective than beta-carotene. Our results suggest that carotenoids exert their protective function as antioxidants to inactivate UV-B-induced radicals in the photosynthetic membrane.

Sequence and phylogenetic analysis of the Rhizobium leguminosarum biovar trifolii pyrE gene, overproduction, purification and characterization of orotate phosphoribosyltransferase

The pyrE gene of Rhizobium leguminosarum biovar trifolii (Rl) was subcloned and its sequence is presented. The nucleotide sequence analysis suggests that this gene is not regulated by transcriptional attenuation as seen for the pyrE and pyrB genes of Escherichia coli (Ec) and Salmonella typhimurium. The Rl pyrE gene was subcloned into Ec AT2538 pyrE60 where the Rl pyrE gene product, orotate phosphoribosyltransferase (OPRTase), was overproduced. Using Ec AT2538 pyrE60 overproducing Rl OPRTase, the enzyme was purified to homogeneity utilizing ammonium sulfate fractionation and affinity chromatography with an orotate monophosphate agarose matrix. The electrophoretically pure OPRTase was characterized and found to be a 24.7 +/- 0.3-kDa protein with a K(m) of 27.6 micromol l(-1). The deduced amino acid sequence for OPRTase was compared with OPRTases from other organisms and found to be most similar to that of Bacillus subtilis (Bs). The Rl OPRTase exhibits 37% identity and 46% similarity to the Bs OPRTase.

Bleomycin increases amikacin and streptomycin resistance in Escherichia coli harboring transposon Tn5

The antitumor antibiotic bleomycin acts as a transcriptional inducer of the neo-ble-str operon of the transposon Tn5, increasing the resistance level to streptomycin and amikacin in Tn5-containing Escherichia coli. The mechanism may involve a recA-independent induction mediated by DNA damage.

Activation of a cryptic streptomycin-resistance gene in the Bacteroides erm transposon, Tn4551

Bacteroides compound transposons encoding erm resistance are highly homologous but previous studies have shown some divergence of Tn4551. Results presented here describe a novel Tn4551 streptomycin-resistance gene, aadS, that was phenotypically silent in wild-type Bacteroides. However, aadS expression could be activated by a trans-acting chromosomal mutation. The aadS-encoded peptide displayed significant homology to Gram-positive streptomycin-dependent adenyltransferases, and enzymatic analysis confirmed the production of this activity. Examination of the nucleotide sequence showed that 200 bp upstream of aadS, the DNA base composition changed abruptly from 31% G+C to 48% G+C. These two regions were demarcated by a DNA sequence with homology to the recombination hot spots reported for Tn21 and the Bacteroides ermFU gene and to sequences at the ends of the chromosomal Bacteroides conjugal element, XBU4422.

Procedure for Obtaining Efficient Root Nodulation of a Pea Cultivar by a Desired Rhizobium Strain and Preempting Nodulation by Other Strains

The specificity between the sym-2 gene bred into certain cultivars of pea ( Pisum sativum L.) and the nodX gene, present only rarely in isolates of Rhizobium leguminosarum , can be exploited to preempt competition or nodulation blocking by a Rhizobium strain indigenous to a soil environment. The principle is to isolate an R. leguminosarum strain prevalent in a locale, convert it into a strain that will nodulate a desirable pea cultivar carrying sym-2 by establishing nodX in it, and then use the resulting Rhizobium strain with the pea cultivar carrying sym-2 . To accomplish this, we first constructed a transposon Tn 5 derivative called Tn 5nodX and an efficient delivery vehicle that is suicidal in R. leguminosarum . We tested the potential utility of the system in greenhouse experiments. The results are encouraging enough to warrant extensive experiments under field conditions.

New derivatives of transposon Tn5 suitable for mobilization of replicons, generation of operon fusions and induction of genes in gram-negative bacteria

Three types of new variants of the broad-host-range transposon Tn5 are described. (i) Tn5-mob derivatives with the new selective resistance (R) markers GmR, SpR and TcR facilitate the efficient mobilization of replicons within a wide range of Gram-negative bacteria. (ii) Promoter probe transposons carry the promoterless reporter genes lacZ, nptII, or luc, and NmR, GmR or TcR as selective markers. These transposons can be used to generate transcriptional fusions upon insertion, thus facilitating accurate determinations of gene expression. (iii) Tn5-P-out derivatives carry the npt- or tac-promoter reading out from the transposon, and TcR, NmR or GmR genes. These variants allow the constitutive expression of downstream genes. The new Tn5 variants are available on mobilizable Escherichia coli vectors suitable as suicidal carriers for transposon mutagenesis of non-E. coli recipients and some on a phage lambda mutant to be used for transposon mutagenesis in E. coli.

Improved expression of streptomycin resistance in plants due to a deletion in the streptomycin phosphotransferase coding sequence

Previous studies have shown that a chimeric streptomycin phosphotransferase (SPT) gene can function as a dominant marker for plant cell transformation. The SPT marker previously described by Jones and co-workers has a limited value since it conferred a useful level of resistance only to a fraction (10%) of Nicotiana plumbaginifolia transgenic lines. Expression of resistance was species specific: no such resistant transformants were found in N. tabacum. In this paper we describe an improved SPT construct that utilizes a mutant Tn5 SPT gene. The mutant gene, SPT*, encodes a protein with a two amino acid deletion close to its COOH-terminus. In N. tabacum cell culture the efficiency of transformation with the improved streptomycin resistance marker was comparable to kanamycin resistance. When the chimeric SPT* gene was introduced linked to a kanamycin resistance gene, streptomycin resistance was expressed in most of the transgenic N. tabacum lines.

A broad-host-range vector system for cloning and translational lacZ fusion analysis

A broad-host-range vector system for studying translational fusions was constructed. The region that retains the origin of replication, nic, mob, and rep genes of the broad-host-range plasmid RSF1010 was isolated as either an HincII or a PstI-PvuII restriction fragment. These restriction fragments were ligated to tetracycline, kanamycin, or streptomycin/spectinomycin resistance genes to generate plasmids pUI501, pUI511, pUI504, and pUI506. A functional lacZ gene lacking downstream lac operon sequences together with the lac promoter was constructed from plasmids pMC1871 and pUC18. This lacZ gene was inserted into pUI501 and pUI511 to generate plasmids pUI502, pUI503, pUI512, and pUI513. An oligodeoxynucleotide sequence that carries three unique blunt-end restriction sites was synthesized, annealed, and ligated in frame to the amino-terminal end of the lacZ gene in each of these plasmids. This multiple cloning sequence will allow translational fusions to the lacZ gene in all three reading frames. The stability of these plasmids and the expression of the lacZ gene in both Escherichia coli and Rhodobacter sphaeroides were studied.

A clinical isolate of transposon Tn5 expressing streptomycin resistance in Escherichia coli

The central region of transposon Tn5 carries three antibiotic resistance markers: neo, ble, and str. The str gene codes for a phosphotransferase that inactivates streptomycin. This activity is phenotypically expressed in several gram-negative bacteria but not in Escherichia coli. We identified a Tn5 variant in E. coli clinical isolates that express streptomycin resistance. This transposon carries a 6-base-pair deletion within the str gene, near the 3' end. The same kind of mutation had been previously obtained experimentally from Tn5.

Enumeration of Tn5 mutant bacteria in soil by using a most- probable-number-DNA hybridization procedure and antibiotic resistance

Investigations were made into the utility of DNA hybridization in conjunction with a microdilution most-probable-number procedure for the enumeration of Rhizobium spp. and Pseudomonas putida in soil. Isolates of Rhizobium spp. and P. putida carrying the transposon Tn5 were added to sterile and nonsterile Burbank sandy loam soil and enumerated over time. Soil populations of rhizobia were enumerated by colony hybridization, most-probable-number-DNA hybridization procedure, plate counts, plant infectivity most probable number, and fluorescent antibody counts. Population values compared well for all methods at 5 and 30 days after the addition of cells, although the fluorescent antibody method tended to overestimate the viable population. In nonsterile soil, most-probable-number-DNA hybridization procedure enumerated as few as 10 P. putida Tn5 cells g of soil-1 and 100 R. leguminosarum bv. phaseoli Tn5 cells g of soil-1 and should have utility for following the fate of genetically engineered microorganisms released to the environment. Among the Kmr isolates containing Tn5, approximately 5% gave a dark, more intense autoradiograph when probed with 32P-labeled pGS9 DNA, which facilitated their detection in soil. Hybridization with a pCU101 probe (pGS9 without Tn5) indicated that donor plasmid sequences were being maintained in the bacterial chromosome. Transposon-associated antibiotic resistance was also utilized as a phenotypic marker. Tn5 vector-integrate mutants were successfully enumerated at low populations (10 to 100 cells g of soil-1) in soil by both phenotypic (Kmr) and genotypic (DNA probe) analysis. However, determination of the stability of Tn5 or Tn5 and vector sequences in the bacteria is necessary.

Isolation and characterization of symbiotic mutants of bradyrhizobium sp. (Arachis) strain NC92: mutants with host-specific defects in nodulation and nitrogen fixation

Random transposon Tn5 mutagenesis of Bradyrhizobium sp. (Arachis) strain NC92, a member of the cowpea cross-inoculation group, was carried out, and kanamycin-resistant transconjugants were tested for their symbiotic phenotype on three host plants: groundnut, siratro, and pigeonpea. Two nodulation (Nod- phenotype) mutants were isolated. One is unable to nodulate all three hosts and appears to contain an insertion in one of the common nodulation genes (nodABCD); the other is a host-specific nodulation mutant that fails to nodulate pigeonpea, elicits uninvaded nodules on siratro, and elicits normal, nitrogen-fixing nodules on groundnut. In addition, nine mutants defective in nitrogen fixation (Fix- phenotype) were isolated. Three fail to supply symbiotically fixed nitrogen to all three host plants. Surprisingly, nodules elicited by one of these mutants exhibit high levels of acetylene reduction activity, demonstrating the presence of the enzyme nitrogenase. Three more mutants have partially effective phenotypes (Fix +/-) in symbiosis with all three host plants. The remaining three mutants fail to supply fixed nitrogen to one of the host plants tested while remaining partially or fully effective on the other two hosts; two of these mutants are Fix- in pigeonpea and Fix +/- on groundnut and on siratro, whereas the other one is Fix- on groundnut but Fix+ on siratro and on pigeonpea. These latter mutants also retain significant nodule acetylene reduction activity, even in the ineffective symbioses. Such bacterial host-specific fixation (Hsf) mutants have not previously been reported.

A new symbiotic cluster on the pSym megaplasmid of Rhizobium meliloti 2011 carries a functional fix gene repeat and a nod locus

A 290-kilobase (kb) region of the Rhizobium meliloti 2011 pSym megaplasmid, which contains nodulation genes (nod) as well as genes involved in nitrogen fixation (nif and fix), was shown to carry at least six sequences repeated elsewhere in the genome. One of these reiterated sequences, about 5 kb in size, had previously been identified as part of a cluster of fix genes located 220 kb downstream of the nifHDK promoter. Deletion of the reiterated part of this fix cluster does not alter the symbiotic phenotype. Deletion of the second copy of this reiterated sequence, which maps on pSym 40 kb upstream of the nifHDK promoter, also has no effect. Deletion of both of these copies however leads to a Fix- phenotype, indicating that both sequences carry functionally reiterated fix gene(s). The fix copy 40 kb upstream of nifHDK is part of a symbiotic cluster which also carries a nod locus, the deletion of which produces a marked delay in nodulation.

Transposon-induced symbiotic mutants of Bradyrhizobium japonicum: isolation of two gene regions essential for nodulation

Two strains of the soybean endosymbiont Bradyrhizobium japonicum, USDA 110 and 61 A101 C, were mutagenized with transposon Tn5. After plant infection tests of a total of 6,926 kanamycin and streptomycin resistant transconjugants, 25 mutants were identified that are defective in nodule formation (Nod-) or nitrogen fixation (Fix-). Seven Nod- mutants were isolated from strain USDA110 and from strain 61 A101 C, 4 Nod- mutants and 14 Fix- mutants were identified. Subsequent auxotrophic tests on these symbiotically defective mutants identified 4 His- Nod- mutants of USDA110. Genomic Southern analysis of the 25 mutants revealed that each of them carried a single copy of Tn5 integrated in the genome. Three 61 A101 C Fix- mutants were found to have vector DNA co-integrated along with Tn5 in the genome. Two independent DNA regions flanking Tn5 were cloned from the three non-auxotrophic Nod- mutants and one His-Nod- mutant of USDA110. Homogenotization of the cloned fragments into wild-type strain USDA110 and subsequent nodulation assay of the resulting homogenotes confirmed that the Tn5 insertion was responsible for the Nod- phenotype. Partial EcoR1 restriction enzyme maps around the Tn5 insertion sites were generated. Hybridization of these cloned regions to the previously cloned nod regions of R. meliloti and nif and nod regions of B. japonicum USDA110 showed no homology, suggesting that these regions represent new symbiotic clusters of B. japonicum.

A series of Tn5 variants with various drug-resistance markers and suicide vector for transposon mutagenesis

A series of variants of transposon Tn5 were constructed by replacement of the 2.7-kb central segment which encodes kanamycin resistance with various other resistance-coding genes: tetracycline, chloramphenicol, gentamicin, trimethoprim, streptomycin or ampicillin. A thermosensitive replication mutant of the broad-host-range transmissible plasmid R388 was also constructed for use as a suicide vector for the delivery of transposable elements.

At least two nodD genes are necessary for efficient nodulation of alfalfa by Rhizobium meliloti

A Rhizobium meliloti DNA region (nodD1) involved in the regulation of other early nodulation genes has been delimited by directed Tn5 mutagenesis and its nucleotide sequence has been determined. The sequence data indicate a large open reading frame with opposite polarity to nodA, -B and -C, coding for a protein of 308 (or 311) amino acid residues. Tn5 insertion within the gene caused a delay in nodulation of Medicago sativa from four to seven days. Hybridization of nodD1 to total DNA of Rhizobium meliloti revealed two additional nodD sequences (nodD2 and nodD3) and both were localized on the megaplasmid pRme41b in the vicinity of the other nod genes. Genetic and DNA hybridization data, combined with nucleotide sequencing showed that nodD2 is a functional gene, while requirement of nodD3 for efficient nodulation of M. sativa could not be detected under our experimental conditions. The nodD2 gene product consists of 310 amino acid residues and shares 86.4% homology with the nodD1 protein. Single nodD2 mutants had the same nodulation phenotype as the nodD1 mutants, while a double nodD1-nodD2 mutant exhibited a more severe delay in nodulation. These results indicate that at least two functional copies of the regulatory gene nodD are necessary for the optimal expression of nodulation genes in R. meliloti.

A cell-free system from Rhizobium meliloti to study the specific expression of nodulation genes

An in vitro transcription-translation system was developed using cell-free extracts from the symbiotic nitrogen-fixing bacterium Rhizobium meliloti strain 41. Conditions for preparation of the 30,000 X g supernatant extract and for measurement of protein-synthesizing activity were determined and compared to the activity of an Escherichia coli cell-free system. Genes expressed in the free-living or in the symbiotic state were studied. The product of a recA-like gene (41-kDa protein) was synthesized both in R. meliloti and E. coli extracts, although less efficiently in the heterologous system. In agreement with earlier results obtained in E. coli minicells, three proteins (44, 28.5 and 23 kDa) were synthesized from a cloned 3.3 X 10(3)-base DNA region carrying genes for nodulation (nod). However, differences in the transcription-translation of nod and host specificity (hsn) genes were observed when protein expression was compared in R. meliloti and E. coli cell-free extracts, and the possible explanations of these findings are discussed.

Expression of Tn5-encoded streptomycin resistance in E. coli

Four Tn5 mutations able to express streptomycin resistance in E. coli were obtained independently. These mutations (called Tn5) were localized and sequenced. All of them consist of a 6 bp deletion in the str gene near the 3' end. The mutation affects a region peculiar for its repetition of an identical 6 bp sequence. The mutation does not affect the level of transcription of the kan, ble, str operon of Tn5, neither does it increase the level of translation of str. The mutation seems to interfere with a post-translational event.

Genetic manipulations in Rhizobium meliloti utilizing two new transposon Tn5 derivatives

Two derivatives of the prokaryotic transposon Tn5 were constructed in vitro. In Tn5-233, the central area of Tn5, which carries resistance to kanamycin/neomycin, bleomycin and streptomycin, is replaced by a fragment carrying resistance to the aminocyclitol antibiotics gentamycin/kanamycin and streptomycin/spectinomycin. In Tn5-235, the Escherichia coli beta-galactosidase gene is inserted within the streptomycin resistance gene of Tn5, and constitutively expressed from a Tn5 promoter. Both constructs transpose with about the same frequency as Tn5 in Escherichia coli and Rhizobium meliloti. When a Tn5-derivative is introduced into an R. meliloti strain which already contains a different Tn5-derivative, in situ transposon replacement is obtained at high frequency, presumably by a pair of crossovers between the IS50 sequences at the ends of the incoming and resident transposons. In this way we converted a previously isolated recA::Tn5 mutant into the corresponding recA::Tn5-233 strain, which can now be used as a genetic background in the study of complementation of other Tn5-induced mutations. We also replaced the drug markers of several Tn5-induced exo mutants, which we were then able to map relative to each other by transduction with phage phi M12. In a strain carrying Tn5-235 located near Tn5-233, we were able to isolate deletions of the intervening markers, presumably resulting from general recombination between the two transposons, by screening for loss of the Lac+ phenotype. Unlike Tn5 itself, resident Tn5-233 does not appear to suppress transposition of another incoming Tn5-derivative.

Molecular cloning and analysis of Staphylococcus aureus chromosomal aminoglycoside resistance genes

Most of the aminoglycoside resistant Staphylococcus aureus strains isolated in France are resistant to all the antibiotics belonging to this family. Two aminoglycoside-modifying enzymes were detected in the wild-type strains studied: an APH3'III and an AAC6'-APH2". These strains also carry two types of streptomycin resistance: high-level resistance due to chromosomal mutation(s) affecting ribosome affinity and low-level resistance, the mechanism of which was not characterized. All the aminoglycoside resistance genes were located on the chromosome. DNA fragments of 1.5 and 1.95 kb carrying the aphA and aacA genes, respectively, were isolated, by cloning, from the cellular DNA of a clinical isolate. When these genes were introduced into Escherichia coli and Bacillus subtilis strains, the enzymes synthesized were indistinguishable from those produced by the S. aureus strains. When the cellular DNAs of wild-type and resistant strains were hybridized with the cloned fragments, sequences homologous to the fragment carrying the aphA gene were found to be located at the same chromosomal site, while those hybridizing with the fragment carrying the aacA gene were at different chromosomal sites.

Identification of a Tn5 determinant conferring resistance to phleomycins, bleomycins, and tallysomycins

Tn5 conferred resistance to the related antibiotics, phleomycins, bleomycins, and tallysomycins in Escherichia coli and Salmonella typhimurium. For pure phleomycins the level of resistance was influenced by the structure of the terminal basic group. Deletion derivatives of a pBR322::Tn5 plasmid were used to show that the phleomycin resistance determinant is located between the previously identified neomycin and streptomycin resistance determinants. The pattern of expression of phleomycin and neomycin resistance in the deletion derivatives suggests that the phleomycin resistance gene is transcribed from the same promoter, PL, which is essential for expression of neomycin and streptomycin resistance. The location of the phleomycin resistance determinant correlates with the location of an open reading frame in the Tn5 sequence, which codes for a polypeptide of 126 amino acids.

Vector insertion mutagenesis of Rhizobium sp. strain ORS571: direct cloning of mutagenized DNA sequences

When the limited-host-range plasmid pVP2021 carrying Tn5 was mobilized into Rhizobium sp. strain ORS571 and stable acquisition of Tn5 was selected, ORS571 plasmid-genome cointegrates were exclusively obtained; direct Tn5 transposition was never observed. In every case, genomic cointegrates exhibited an additional (third) IS50 element that bordered VP2021 DNA sequences but maintained a single Tn5 element. Genomic cointegrates containing IS50 triplications were stable; neither phenotypic reversion nor resolution was detectable. Auxotrophic mutant strains (vector insertion mutants) were identified at expected frequencies among derivatives carrying ostensibly random genomic pVP2021 insertions; N2 fixation (Nif)-defective vector insertion mutants were observed among these derivatives at a frequency of 10(-3). The presence of integrated pVP2021 in ORS571 nif::VP2021 mutant genomes enabled VP2021 to constitute an endogenous cloning vector. After EcoRI or KpnI digestions, genomic nif::pVP2021 DNA sequences contiguous with integrated pVP2021 were directly cloned as new replicons without addition of an exogenous vector. Recombinant plasmids derived from two such nif::pVP2021 mutants hybridized to previously analyzed ORS571 Nif DNA sequences. Recombinant plasmid DNA and ORS571 Nif region DNA were found to be colinear; pVP2021 insertions could be accurately mapped. pVP2021 insertion-mutagenesis thus allows the direct cloning of ORS571 gene sequences for which mutant phenotypes can be selected or screened.

Rhizobium meliloti carries two megaplasmids

In Rhizobium meliloti strain 41 the existence of a second megaplasmid (pRme41c) with a molecular weight similar to the sym megaplasmid pRme41b was demonstrated. Derivatives of the wild-type strain carrying pRme41b or pRme41c tagged with Tn5 allowed the examination of the transfer ability of both megaplasmids. The introduction of megaplasmids into the wild-type R. meliloti was not detected, probably because of the action of plasmid genes coding for entry exclusion of the same type of plasmid. However, transmissibility of both megaplasmids was observed in matings with Nod- or Fix- pRme41b deletion mutant recipients and with Agrobacterium tumefaciens at frequencies of 10(-6) - 10(-8). Introduction of the megaplasmids into the R. meliloti recipients resulted in the loss of the same plasmid. On the other hand, pRme41b and pRme41c were compatible. From the extent of deletions in various Nod- and Fix- mutants a DNA region carrying genes probably involved in "surface exclusion" on pRme41b was located. This DNA region is about 50 kb distant from the nod genes and exhibits strong homology with a DNA segment of pRme41c. Symbiotic genes on pRme41c were not identified.

Completion of the nucleotide sequence of the central region of Tn5 confirms the presence of three resistance genes

The DNA sequence of the region located downstream from the kanamycin resistance gene of Tn5 up to the right inverted repeat IS50R has been determined. This completes the determination of the sequence of Tn5 which is 5818 bp long. The 2.7 Kb central region contains three resistance genes: the kanamycin-neomycin resistance gene, a gene coding for resistance to CL990 an antimitotic-antibiotic compound of the bleomycin family and a third gene that confers streptomycin resistance in some bacterial species but is cryptic in E. coli. A Tn5* mutant able to express streptomycin resistance in E. coli was isolated. With this mutant, it was demonstrated that in E. coli the expression of the three resistance genes is coordinated in a single operon.

Transposon insertion mutagenesis of Pseudomonas aeruginosa with a Tn5 derivative: application to physical mapping of the arc gene cluster

For insertional mutagenesis of Pseudomonas aeruginosa, a derivative of the kanamycin-resistance (KmR) transposon Tn5 was constructed (Tn5-751) that carried the trimethoprim-resistance (TpR) determinant from plasmid R751 as an additional marker. Double selection for KmR and TpR avoided the isolation of spontaneous aminoglycoside-resistant mutants which occur at high frequencies in P. aeruginosa. As a delivery system for the recombinant transposon, plasmid pME305, a derivative of the broad-host-range plasma RP1, proved effective; pME305 is temperature-sensitive at 43 degrees C for maintenance in Escherichia coli and P. aeruginosa and deleted for IS21 and the KmR and primase genes. In matings with an E. coli donor carrying pME9(= pME305::Tn5-751), transposon insertion mutants of P. aeruginosa PAO were recovered at approx. 5 X 10(-7)/donor at 43 degrees C. Among Tn5-751 insertional mutants 0.9% were auxotrophs. A thr::Tn5-751 mutation near the recA-like locus rec-102 is useful for the construction of recombination-deficient strains. Several arc::Tn5-751 mutants could be isolated that were defective in anaerobic utilization of arginine as an energy source. From three of these mutants the arc gene region was cloned into an E. coli vector plasmid. Since Tn5-751 has a single EcoRI site between the TpR and KmR genes, EcoRI-generated fragments carrying either resistance determinant plus adjacent chromosomal DNA could be selected separately in E. coli. Thus, a restriction map of the arc region was constructed and verified by hybridization experiments. The arc genes were tightly clustered, confirming earlier genetic evidence.

The transposon Tn5 carries a bleomycin-resistance determinant

Transposon Tn5 carries a determinant for resistance to bleomycin (Bm). Deletion mapping and cloning experiments have shown that this determinant, gene ble, is located between the determinant for kanamycin (Km) and neomycin resistance (gene neo) and the determinant for streptomycin resistance (gene str). Genes neo, ble, and str belong to an operon controlled by the common promoter. The Mr of the ble product, as determined by polyacrylamide gel electrophoresis, is 12000 to 13000.

The relationship between base composition and codon usage in bacterial genes and its use for the simple and reliable identification of protein-coding sequences

Bacterial genes that code for proteins appear to possess a codon usage characteristic of their overall base composition. This results in different but predictable non-random distributions of nucleotides within codons, permitting the recognition of protein-coding sequences in a wide range of bacterial species. The nature of this distribution depends on the base composition of the coding sequence. The position-specific differences are especially conspicuous in genes of extreme G + C content, allowing the particularly reliable prediction of the reading frame and coding strand of experimentally determined DNA sequences. This finding has been exploited to identify the coding sequence of the viomycin phosphotransferase (vph) gene of Streptomyces vinaceus. An easily applied computer program ("Frame") has been written to carry out and display such analyses.

Characterization of transposon Tn5-facilitated donor strains and development of a chromosomal linkage map for Agrobacterium tumefaciens

We have further characterized the transposon Tn5-facilitated chromosomal gene transfer system developed for Agrobacterium tumefaciens 15955. Using a strain whose chromosome contained Tn5, we compared the chromosome-mobilizing ability of plasmid pDP37 (containing Tn5) with that of its parent plasmid R68.45. For R68.45, we observed nonpolar transmission from multiple origins. For pDP37 we found polarized transmission from a single origin near ilv. When we examined the transmission gradients of a number of pDP37-containing donor strains each differing at the site of the chromosomal insertion we found just two classes. One set of donors transmitted markers with a gradient of Ilv+ greater than Rifr greater than His+ greater than Met+, whereas the second set transferred His+ greater than Rifr greater than Ilv+ greater than Met+. Using representatives from each transmission class of donor strains, we conducted matings to measure the degree of linkage between pairs of adjacent donor markers. From this information we developed a map of the A. tumefaciens 15955 chromosome. Attempts to isolate R-prime plasmids or Hfr-like donors were unsuccessful.

Transposon Tn5 encodes streptomycin resistance in nonenteric bacteria

Strains of Caulobacter crescentus, Pseudomonas putida, Acinetobacter calcoaceticus, Rhizobium meliloti, and Rhodopseudomonas sphaeroides carrying the kanamycin resistance-encoding transposon Tn5 were 15 to 500 times more resistant to streptomycin than transposon-free strains. The streptomycin resistance determinant, which is separable from the kanamycin resistance determinant of Tn5, was not expressed in Escherichia coli or Klebsiella aerogenes.

General transduction in Rhizobium meliloti

General transduction by phage phi M12 in Rhizobium meliloti SU47 and its derivatives is described. Cotransduction and selection for Tn5 insertions which are closely linked to specific loci were demonstrated. A derivative of SU47 carrying the recA::Tn5 allele of R. meliloti 102F34 could be transduced for plasmid R68.45 but not for chromosomally located alleles. Phage phi M12 is morphologically similar to Escherichia coli phage T4, and restriction endonuclease analysis indicated that the phage DNA was ca. 160 kilobases in size.

Transposon Tn5 specifies streptomycin resistance in Rhizobium spp

Transposon Tn5 conferred streptomycin resistance on different strains of Rhizobium meliloti, Rhizobium leguminosarum, and Rhizobium trifolii but not on Escherichia coli. A gene (str) specifying this phenotype has been identified and localized on the physical and genetic map of Tn5. It is transcribed from the promoter of neo, the gene that encodes neomycin phosphotransferase. The str gene is downstream from neo in a single transcriptional unit, as revealed by molecular cloning of different segments of Tn5 and by cloning of the neo-str region of Tn5 downstream from a lac promoter. Fusion of the SalI-generated rightward segment of Tn5 (devoid of neo) to a part of a tetracycline resistance gene, tet, in a plasmid or downstream from a lac promoter in a plasmid resulted in significant levels of streptomycin resistance in an R. meliloti host, suggesting that the str gene product can function independent of neomycin phosphotransferase. A natural isolate of R. meliloti that does not express Tn5-associated streptomycin resistance has been identified. We have used the str of Tn5 as a genetic marker in Rhizobium spp.