Nif plasmid from Lignobacter

nif genes in alien backgrounds

Since the original construction of diazotrophic Escherichia coli by conjugal transfer of nif genes from Klebsiella pneumoniae in 1972, the manipulation of nif genes into alien prokaryotic backgrounds has become routine: much of the basic genetics of the K. pneumoniae nif cluster was elucidated in an E. coli background. Gene transfers to new species and genera can give new information regarding the stability of nif genes and, particularly, conditions for their expression; recipients in which nif is partly expressed, or not expressed at all, are especially useful. Appropriate examples are discussed. New diazotrophic prokaryotes show little promise for practical exploitation but their construction should give forewarning of problems to be expected in the construction of diazotrophic eukaryotes, as well as hints concerning the ecology and evolution of diazotrophy.

Homology and distribution of CO dehydrogenase structural genes in carboxydotrophic bacteria

The 17 (S), 30 (M) and 87 kDa (L) subunits of CO dehydrogenases from the CO-oxidizing bacteria Pseudomonas carboxydoflava, Pseudomonas carboxydohydrogena and Pseudomonas carboxydovorans OM5 were isolated and purified. The N-terminal sequences of same subunits from different bacteria showed distinct homologies. Dot blot hybridization employing oligonucleotide probes derived from the sequences of the S-subunit of P. carboxydovorans OM5 and the M-subunit of P. carboxydohydrogena and DNA of the plasmid-containing CO-oxidizing bacteria Alcaligenes carboxydus, Azomonas B1, P. carboxydoflava, P. carboxydovorans OM2, OM4 and OM5 indicated that all genes encoding these subunits reside on plasmids. That in P. carboxydovorans OM5 CO dehydrogenase structural genes are located entirely on plasmid pHCG3 was evident from the absence of hybridization employing DNA from the cured mutant strain OM5-12. CO dehydrogenase structural genes could be identified on the chromosome of the plasmid-free bacteria Arthrobacter 11/x, Bacillus schlegelii, P. carboxydohydrogena and P. carboxydovorans OM3. There was no example of a plasmid-harboring carboxydotrophic bacterium that did not carry CO dehydrogenase structural genes on the plasmid. The N-terminal sequences of CO dehydrogenase structural genes were found to be conserved among carboxydotrophic bacteria of distinct taxonomic position, independent of the presence of plasmids. It is discussed whether this might be the consequence of horizontal gene transfer.

Sym plasmid genes of Rhizobium trifolii expressed in Lignobacter and Pseudomonas strains

A 14-kilobase (kb) fragment of Rhizobium trifolii Sym plasmid containing nodulation (nod) genes or the pSym plasmid of R. trifolii cointegrated with a broad-host-range vector R68.45 (pPN1) were transferred to Lignobacter strain K17 and Pseudomonas aeruginosa strain PAO5 by conjugation. Lignobacter transconjugants carrying Sym plasmid pPN1 formed nodules on white, red, and subterranean clover plants. Lignobacter transconjugants containing a 14-kb fragment of nod genes cloned into a multicopy plasmid nodulated only white and subterranean clover plants, whereas transconjugants carrying the same fragment cloned into a low-copy plasmid vector nodulated only white clover plants. All nodules formed by Lignobacter transconjugants showed bacterial release from the infection threads into the host cytoplasm. Pseudomonas transconjugants with plasmid pPN1 formed nodule-like structures on white clover plants. These structures were not invaded by bacteria; however, a few bacteria were found within the intercellular spaces of the outermost cells of the structures. Pseudomonas transconjugants carrying the 14-kb fragment of R. trifolii nod genes did not form nodules on tested clover plants. All clover plants inoculated with either Pseudomonas or Lignobacter transconjugants containing a 14-kb fragment of nod genes (but not entire Sym plasmid) showed the "thick-and-short-root" response when compared to the control plants inoculated with the R. trifolii wild-type strain.

Plasmid visualization and nif gene location in nitrogen-fixing Azospirillum strains

A modified gel electrophoresis technique provided a reproducible way of detecting and isolating plasmids with molecular weights ranging from 12 X 10(6) to 370 X 10(6) for Azospirillum species. Analysis with the nifHD region of Rhizobium trifolii showed that the Azospirillum nif genes were chromosomally located in all eight strains investigated and not on endogenous plasmids.