SELF-TRANSMISSIBLE NIF PLASMID (PEA9) OF ENTEROBACTER-AGGLOMERANS-339- 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 pl asmid pEA9 isolated from Enterobacter agglomerans 339. The cosmid clon e identified to contain the complete nif cluster was used to determine the nif gene organization and the physical map. The restriction patte rn and nif gene organization of this nif cluster showed remarkable sim ilarities to the nif cluster identified on the 110-kb plasmid pEA3 of Enterobacter agglomerans 333. 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 cluster s and no similarity could be detected with the rest of the DNA of thes e plasmids. This reveals the existence of similar nif clusters on diss imilar plasmids, implying the horizontal transfer of the entire nif ge ne cluster. (C) 1995 Academic Press, Inc.