Self-transmissible nif Plasmid (pEA9) of Enterobacter agglomerans339: molecular cloning and evidence for the existence of similar nif clusters on dissimilar plasmids in Enterobacter strains

A cosmid library was generated to the 200-kb self-transmissible nif plasmid pEA9 isolated from Enterobacter agglomerans339. The cosmid clone identified to contain the complete nif cluster was used to determine the nif gene organization and the physical map. The restriction pattern and nif gene organization of this nif cluster showed remarkable similarities to the nif cluster identified on the 110-kb plasmid pEA3 of Enterobacter agglomerans333. Nucleotide sequence of several randomly selected regions of the nif cluster of pEA9 showed 96% similarity when compared to the known sequences of the nif cluster of pEA3. However, the homology ended abruptly at the flanking regions of the nif clusters and no similarity could be detected with the rest of the DNA of these plasmids. This reveals the existence of similar nif clusters on dissimilar plasmids, implying the horizontal transfer of the entire nif gene cluster