Transformation of Azotobacter vinelandii with plasmids RP4 (IncP-1 group) and RSF1010 (IncQ group)

Azotobacter vinelandii: the source of 100 years of discoveries and many more to come

Azotobacter vinelandii has been studied for over 100 years since its discovery as an aerobic nitrogen-fixing organism. This species has proved useful for the study of many different biological systems, including enzyme kinetics and the genetic code. It has been especially useful in working out the structures and mechanisms of different nitrogenase enzymes, how they can function in oxic environments and the interactions of nitrogen fixation with other aspects of metabolism. Interest in studying A. vinelandii has waned in recent decades, but this bacterium still possesses great potential for new discoveries in many fields and commercial applications. The species is of interest for research because of its genetic pliability and natural competence. Its features of particular interest to industry are its ability to produce multiple valuable polymers - bioplastic and alginate in particular; its nitrogen-fixing prowess, which could reduce the need for synthetic fertilizer in agriculture and industrial fermentations, via coculture; its production of potentially useful enzymes and metabolic pathways; and even its biofuel production abilities. This review summarizes the history and potential for future research using this versatile microbe.

Tn10 mutagenesis in Azotobacter vinelandii

The tetracycline-resistant transposon Tn10 and its "high-hopper" derivative Tn10HH104 were introduced into the Azotobacter vinelandii genome using suicide conjugative plasmids derived from pRK2013. Several types of mutants induced by either of these elements are described. Nif- mutants (deficient in nitrogen fixation) were easily isolated, whereas the isolation of other mutant types (auxotrophs, sugar non-users) required special selection conditions. The characterization of the mutations as transposon insertions was often complicated and sometimes required a combination of genetic and physical tests. A common source of complication, the existence of double inserts, was found among the mutants induced by Tn10HH104 but not among those induced by Tn10. Both the high-hopper and the wild-type element proved to undergo secondary transpositions, albeit at different frequencies. Another type of complication, the existence of heterozygotes, occurred because of the high level of redundancy of the A. vinelandii genome.

Stable electrotransformation of symbiont candidate diazotrophic bacterium with plasmids carrying selectable and screenable marker genes

Nitrogen-fixing symbioses had been established between the originally asymbiotic soil bacterium Azotobacter vinelandii CCM289 and different lower and higher plant species. Better characterization and further development of such artificial systems require a reliable genetic transformation method for the introduction of marker genes into symbiont candidates. The performance of electroporation was evaluated using pJB3 (4.8 kb), pBI121 (12.8 kb) and pFAJ31.2 (24 kb) plasmid DNAs containing selectable (Ap, Km, Tc) and screenable (gusA, lacZ) marker genes. The adapted methods for the preparation of transformation-competent azotobacters and their electroporation (18 kV/cm electric field strength, 5 ms time constant, 0 degree C) provided up to 6.8 x 10(5) transformants per microgram plasmid DNA, which is about 10(3) times the transformation efficiency achieved in control experiments. No electrotransformants were obtained with the 24-kb pFAJ31.2. The size of plasmid DNA did not significantly affect the efficiency of transformation. Transformants were able to grow at antibiotic concentrations that were 100-200 times greater than the lowest amounts that completely inhibited the growth of wild-type bacteria. A constitutive expression of gusA gene was observed in transformants with the CaMV 35S promoter-gusA fusion containing pBI121, while lacZ expression was not detected under the control of the lac promoter in pJB3 transformants. Electroporated plasmids were reisolated from transformants in their original form, while non-transformed bacteria did not contain indigenous plasmids. PCR amplification and Southern DNA blot hybridization showed the integration of plasmid DNA into the host genome as well. Transformants retained their nitrogen-fixing ability and had normal morphological and growth characteristics. Experimental findings proved the stable maintenance of plasmid DNA in azotobacters, making possible the routine transformation and detection of these symbiont candidates.

Electroporation and expression of the broad host-range plasmid pRK2501 in Azotobacter vinelandii

Azotobacter vinelandii cells were transformed via high-voltage electroporation, with the broad host-range plasmid pRK2501. The number of transformants was dependent on the applied voltage, capacitance, and recovery procedure after electroporation. For example, Log, 4.44 transformants microgram-1 DNA were recovered in the A. vinelandii cell suspension electroporated at 1500 V and 25 microF capacitance (time constant 29.0 ms) and recovered on LB agar amended with 0.5 microgram/ml-1 kanamycin (pRK2501 encodes for both kanamycin and tetracycline resistance). Electroporation at 2500 V and capacitance settings of 25 and 3 microF did not produce any transformants. Cell survival was also poor at high voltages. A. vinelandii transformants were not recovered on N-free agar medium. In addition, no viable cells were recovered on N-free agar after electroporation at 2500 V, 25 microF; 2500 V, 3 microF; and 1500 V, 25 microF. Electroporation may be a useful method to genetically transform Azotobacter species for use in physiological and/or genetic studies.

Structure and regulation of the AMP nucleosidase gene (amn) from Escherichia coli

The gene for AMP nucleosidase from Escherichia coli (amn) has been sequenced and characterized. The gene codes for a transcript of 1.7 +/- 0.2 kb, and the open reading frame corresponds to a protein of 483 amino acids (Mr = 53848). Amino acid sequences from tryptic peptides of AMP nucleosidase, N-terminal amino acid analysis, and the amino acid composition confirm the gene assignment and the open reading frame of amn. Primer extension studies determined the 5'-end of the amn transcript. The 5'-regulatory region contains overlapping sequences with similarity to the consensus sequences for binding cAMP receptor protein and inorganic phosphate repressor protein. Addition of exogenous cAMP to E. coli deficient in adenylate cyclase resulted in a 3-fold increase in AMP nucleosidase activity. Growth of E. coli on limiting phosphate resulted in an 8-fold increase in the production of AMP nucleosidase. The amn gene was expressed in AMP nucleosidase deficient strains of Azotobacter vinelandii and E. coli. A pUC-amn construct is described that causes approximately 20% of the total protein in E. coli to be produced as AMP nucleosidase. Comparison of the amino acid sequence for AMP nucleosidase with that for yeast AMP deaminase indicates a region in which six of eight amino acids are identical but no other overall homology. The amino acid sequence showed poor agreement with consensus sequences for adenylate binding sites even though the enzyme is known to have a catalytic site for AMP and regulatory sites for MgATP and phosphate.

Transformation of Azotobacter vinelandii with plasmid DNA

Azotobacter vinelandii cells can be transformed at high frequencies with the broad-host-range plasmids pRK2501, RSF1010, and pGSS15, using a modification of the procedure developed by Page and von Tigerstrom (J. Bacteriol. 139:1058-1061, 1979) for chromosomal DNA-mediated transformation. The frequency of transformation per microgram of plasmid DNA per viable cell with pRK2501 and pGSS15 was about 5 X 10(-2) and 2 X 10(-2), respectively. With RSF1010, transformation frequencies ranged from 3 X 10(-4) to 4 X 10(-2). With each plasmid, the frequency of transformation was independent of the phase of the growth cycle. When concentrations of pRK2501 ranging from 0.1 to 51 micrograms of DNA were tested, the frequency of transformation was directly proportional to the amount of DNA. This linear response indicated that, although the uptake of plasmid DNA with this procedure may be inefficient, there is a high probability that once inside a cell the plasmid will be stably maintained. Cells that have been transformed with pRK2501 did not grow well on transforming medium which lacks iron and contains fixed nitrogen. However, on growth medium which contains iron and lacks fixed nitrogen, transformants produced distinctive colonies larger than those of nontransformed cells. Resistance to kanamycin due to transformation by pRK2501 was stably maintained for at least 10 successive generations in the absence of selective pressure. The present protocol should facilitate the molecular cloning of genes in Azotobacter spp.

Heat sensitivity of Azotobacter vinelandii genetic transformation

Heating competent Azotobacter vinelandii at 37 or 42 degrees C resulted in a total loss of competence with no loss of viability. The transformation process was relatively insensitive to heating at either temperature once DNase-resistant DNA binding was nearly complete. Although competent and 42 degrees C-treated cells bound equivalent amounts of [32P]DNA in a DNase-resistant state, no donor DNA marker (nif) or radioactivity was detected in the envelope-free cell lysate of heated cells, suggesting that DNA transport across the cell envelope was a heat-sensitive event. Competence was reacquired in a 42 degrees C-treated culture after 2 h of incubation at 30 degrees C by a process which required RNA and protein syntheses. The release of a surface glycoprotein, required for competence, from cells treated at 42 degrees C occurred in an insufficient amount to account for the total loss of competence. Recovery of competence in 42 degrees C-treated cells and further transformation of competent cells were prevented by the exposure of cells to saturating amounts of transforming DNA. Further DNase-resistant DNA binding, however, still occurred, suggesting that there were two types of receptors for DNase-resistant DNA binding to competent A. vinelandii. DNase-resistant DNA binding was dependent on magnesium ions, and at least one receptor type did not discriminate against heterologous DNA.

Properties of R1162, a broad-host-range, high-copy-number plasmid

Regions of plasmid DNA encoding characteristic properties of the IncQ (P-4) group plasmid R1162 were identified by mutagenesis and in vitro cloning. Coding sequences sufficient for expression of incompatibility and efficient conjugal mobilization by plasmid R751 were found to be linked to the origin of DNA replication. In contrast, there was a region remote from the origin, and active in trans, that was required for plasmid maintenance. A derivative that was temperature sensitive for stability was isolated. The defect mapped at or near the region required for plasmid maintenance and resulted in far fewer copies of supercoiled plasmid DNA per cell under permissive conditions. A second region required for stability was also identified from the behavior of a deletion derivative of R1162, which did not, however, show an altered number of supercoiled plasmid DNA copies. Finally, a plasmid DNA mutation resulting in a substantially higher copy number was isolated. Plasmid reconstruction experiments suggested that the mutation was linked to the replicative origin.

Specific-purpose plasmid cloning vectors. II. Broad host range, high copy number, RSF1010-derived vectors, and a host-vector system for gene cloning in Pseudomonas

Host-vector systems have been developed for gene cloning in the metabolically versatile bacterial genus Pseudomonas. They comprise restriction-negative host strains of Pseudomonas aeruginosa and P. putida and new cloning vectors derived from the high-copy-number, broad-host-range plasmid RSF1010, which are stably maintained in a wide range of Gram-negative bacteria. These plasmids contain EcoRI, SstI, HindIII, XmaI, XhoI, SalI, BamHI, and ClaI insertion sites. All cloning sites, except for BamHI and ClaI, are located within antibiotic-resistance genes' insertional inactivation of these genes during hybrid plasmid formation provides a readily scored phenotypic change for the rapid identification of bacterial clones carrying such hybrids. One of the new vector plasmids is a cosmid that may be used for the selective cloning of large DNA fragments by in vitro lambda packaging. An analogous series of vectors that are defective in their plasmid-mobilization function, and that exhibit a degree of biological containment comparable to that of current Escherichia coli vector plasmids, are also described.