Several members of the alpha-proteobacteria have a highly complex genome organization (Martin-Didonet et al. 2000; Jumas-Bilak et al. 1998). Some bacteria have more than one chromosome in addition to several plasmids. Agrobacterium tumefaciens, Ochrobactrum intermedium and Rhodobacter sphaeroides have four replicons and the largest DNA molecules were considered to be chromosomes (Jumas-Bilak et al. 1998; Suwanto et al. 1989). One chromosome of A. tumefaciens is linear and another circular (Allardet-Servent 1998). These results altered the definition of the bacterial genome and highlighted its dynamic nature and plasticity. In this study, we examined the presence of multiple replicons in five Azospirillum species using PFGE and hybridization with 16S and 23S rDNA probes. The results obtained suggested the presence of two to five chromosomes in Azospirillum. The restriction profile of A. brasilense FP2 with endonuclease indicated the presence at least five rrn opérons.

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