Similarities between large plasmids of Azospirillum lipoferum

Abstract

The nitrogen-fixing bacteria, Azospirillum lipoferum and Azospirillum brasilense, contain several large plasmids. Cryptic plasmids of about 150 MDa have only been detected in A. lipoferum strains. The 150 MDa plasmid of A. lipoferum 4B used as probe hybridized with plasmids of comparable size in all A. lipoferum strains. Total DNA of six A. lipoferum strains were digested and hybridized with the p150 probe. Restriction fragment length polymorphism (RFLP) patterns differ from that of strain 4B except for one strain isolated from the same rhizosphere. However, a high number of hybridizing bands in each strain was close to 150 MDa, showing a high degree of similarity between 150 MDa plasmids. This strongly suggests that A. lipoferum 150 MDa plasmids, though not identical, have evolved from a common ancestor and likely constitute a family of plasmids sharing unidentified common functions.