An improved spectrophotometric assay for monoamine oxidase in mitochondria

other Rhizobium species. Most of these plasmids encoding for fixation and nodulation properties are non-self transmissible. Plasmid profiles were determined for 24 strains of R. trifolii. All the strains harboured at least three plasmids and as many as eight were present in some strains. Southern hybridization with cloned R. meliloti nifgenes was used to identify indigenous Nif plasmids. Each strain carried a single Nif plasmid and these plasmids varied in size from 1 12 megadaltons to at least 300 megadaltons. Using a novel system of inserting transposons into these plasmids, they can then be transferred to other species of Rhizobium using a Mob (mobilizing) vector (O’Connell et al., 1983). Tn5 mob is a recombinant transposon containing the origin of transfer of RP4 (R. Simon, unpublished work) cloned in the neomycin-resistant transposon Tn5. It retains the transposition properties of Tn5 and facilitates mobilization of labelled plasmids by a co-resident RP4 plasmid. A 150 megadalton Nif plasmid identified in strain G1008 was labelled with Tn5 mob, a recombinant transposon carrying the origin of transfer of RP4. This facilitated the mobilization of the Nif plasmid to R. leguminosarum 6015 which then acquired the ability to effectively nodulate clover, indicating the presence of both nod and nifgenes on the mobilized plasmid. During subsequent mobilization of this sym plasmid a 55 megadalton deleted derivative was isolated. This plasmid harbours all the genes necessary for effective nodulation and is termed pUG507. Tn5 mob has been used to label and facilitate mobilization of numerous plasmids. The introduction of an asymbiotic plasmid from strain G 1067 to strain G 1006 produced an alteration in the plasmid profile of the recipient. The sym plasmid of G1006 was no longer observed and a larger plasmid, presumably a co-integrate, was generated. Subsequent mobilization resulted in breakdown of the co-integrate and the formation of a novel 140 kilodalton sym plasmid termed pUG506. Mobilization using Tn5 mob has facilitated the investigation of plasmid incompatibility. The sym plasmid of (31008 exhibits incompatibility with the sym plasmid of R. trifolii 1024 and with the novel sym plasmid pUG506. In addition, pUG506 was found to be incompatible with the sym plasmid of R. phaseoli 8002. All these plasmids were thus allocated to the same incompatibility group, termed IncRtrl. An asymbiotic plasmid from R. trifolii G 1015 could also be assigned to this group. Furthermore, the sym plasmid of R. trifolii GI027 was found to be incompatible with pRL6J1, a sym plasmid from R. feguminosarum, and these were allocated to a second incompatibility group, termed IncRtr2, since pRL6J 1 is compatible with plasmids from the IncRtrl group. These results confirm that host-range in a plasmid-borne characteristic in Rhizobium. The results also show that plasmids involved in symbiotic properties in Rhizobium do show certain incompatibilities when transferred into other species. This may limit the generation of multi-host strains of Rhizobium if this phenomenon incompatibility is the rule rather than the exception.