Summary After the initial screening of more than 200 Frankia strains, the plasmid DNA observed in eight Frankiae was analyzed. In situ lysis was performed to obtain an estimate of their copy number and molecular weight. Four plasmid classes were distinguished, 7-9, 18-20, 30-35 and 50-55 kb. Twelve plasmids were thus analysed with restriction enzymes to determine their plasmid restriction patterns.While some Frankia plasmids with comparable molecular weights were found to be heterologous in their restriction enzyme pattern, an 8 kb plasmid found in both Frankia sp. Arl3, isolated from Alnus rubra and Frankia sp. CpI1 isolated from Comptonia peregrina showed undistinguishable 'fingerprints'. Furthermore, an 18 kb plasmid found in the same two strains, also showed homologous restriction enzyme patterns. However, the copy numbers of the two ArI3 plasmids were higher thart those of the CpI1 plasmids. 9. AG AC AC .Similarly, stratus ACN1, TX41 b-and TX38b---, isolated from Alnus crispa all contained a 50 kb plasmid, and the three plasmids were found upon restriction analysis to be undistinguishable.In one strain, ARgX17c isolated from Alnus rugosa, it was found through restriction enzyme analysis that two plasmids of a similar molecular weight were in fact heterologous.The possible origin of the homologous plasmids and their potential as specific Frankia markers to be used in ecological studies are discussed.

Repeated attempts at isolating the Frankia endophyte of Coriaria spp. have not yielded infective microbial cultures that could fulfil Koch's postulates. In order to circumvent the critical isolation step, nodule endophytes of Coriaria were characterized directly by means of specific amplification of nodule DNA (PCR) followed by sequencing of part of the 16S rDNA gene. Three closely related sequences were obtained from nodules originating from France, Mexico and New Zealand, containing unique sequences different from all other Frankia strains characterized so far. The sequences obtained were closest (with 5 or 6 substitutions) to those of Frankia alni and those of Casuarina-infective Frankia strains, respectively. Two nucleotides unique to the Coriaria endophyte sequences were used to define specific primers, resulting in a hybridization test that could discriminate between Frankia DNAs originating from Coriaria nodules and those recovered from all cultured Frankia strains tested. The endophytes of Coriaria thus appear to form a distinct Frankia lineage.

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