Identification of nif genes in N₂-fixing bacterial strains isolated from rice fields along the Yangtze River Plain

The aim of this research was to identify nifH and nifHDKYE′ genes in twenty strains of N₂-fixing heterotrophic bacteria isolated from rice fields in the Yangtze River Plain. Southern hybridization of the total DNA from each strain was performed with the Klebsiella pneumoniae nifHDKYE′ gene probe (6.2 kb EcoRI fragment from pSA30) and the Azospirillum brasilense nifH gene probe (0.6 kb EcoRI-HindIII fragment from pHU8). We found that EcoRI fragments of total DNA from Aeromonas hydrophila HY2, Bacillus azotoformans FD, Bacillus licheniformis NCH1, NCH5, WH4, Bacillus brevis NC2, Bacillus pumilus NC12, Bacillus cereus NCH2, Citrobacter freundii HY5, HY9, Derxia gummosa HZ5, Pseudomonas mendocina HZ1 and Pseudomonas pseudoalcaligenes WH3 were positively hybridized with both of the probes. Agrobacterium radiobacter HY17, Corynebacterium sp. HY12, YZ and Pseudomonas sp. HY11 had EcoRI fragments hybridized with the K. pneumoniae nifHDKYE′ gene probe. An EcoRI fragment of total DNA from Bacillus megaterium YY4 was positively hybridized to the A. brasilense nifH gene probe. No hybridization sign was found in the total DNA fragments from Alcaligenes cupidus YY6 and Corynebacterium sp. NC11 hybridized with either of the gene probes. The data provide the number and size of EcoRI fragments of the total DNA hybridized with the nif gene probes for these strains of rarely studied species, suggesting additional evidence for N2 fixing and nif gene diversity of N₂-fixing bacteria in rice fields along the Yangtze River Plain.