Relationships of Frankia isolates among themselves and to other actinomycetes based on DNA homology studies

A molecular genetical approach was used to examine the genetic relatedness of 19 Frankia isolates by measuring the base composition of their deoxyribonucleic acid(DNA), the size of their genomes, the DNA-DNA homology, and the thermal stability of the hybrid DNA molecules. The mole % of guanine plus cytosine of Frankia DNA ranged from 66.6 % to 71.4 % with an average of 68.5 % as measured by the thermal denaturation method, and 69.4% by the buoyant density method. The genome size of two Frankia strains, Arl4 and Full, was 6.2 X 10⁹ and 4.6 X 10⁹ daltons, respectively.

Frankia isolates were divided into two groups based on the results of hybridization tests. Genogroup P, consisting of isolates from Alnus, Myrica and Comptonia host plants, exhibited a high degree of homology (67.4 % - 94.1 %) with Arl4. Genogroup N, consisting of isolates from Elaeagnus, Ceanothus, Purshia, Casuarina and AirI2 from Alnus. did not exhibit significant homology (less than 39 %) with Arl4. None of the strains showed a high degree of homology with Eull (less than 33 Among genogroup N strains, subgroupings seemed to exist evidenced by a very high homology (97 %] between two isolates from Casuarina. D11 and G2, but a low homology (27 %) between other strain in genogroup N and G2 strain. Thermal stability studies of the hybrid DNAs which showed a high degree of homology revealed an average of 3 % mismatch while the low homology duplexes exhibited about 5 % mismatch. Eleven actinomycetes which share a common cell wall type with Frankia were also examined for possible genetic relationships to Frankia by measuring base composition and DNA homology with two frankiae probes, Arl^ and Eull. Although no strain showed more than 30 homology, one strain, Geodermatophilus, showed relatively higher homology with both probe strains.