Restriction enzyme digestion patterns of Frankia plasmids

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 8kb plasmid found in both Frankia sp. ArI3, 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 than those of the Cpll plasmids. Similarly, strains ACN1AG,$TX41_b^{\underline {AC} }$ and $TX38_b^{\underline {AC} }$, 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.