Genetic variation at theapcAB,cpcAB,gvpA1, andnifH loci and in DNA methylation among N2-fixing cyanobacteria designatedNostoc punctiforme.

Genetic similarity among cyanobacteria of a morphological subgroup ofNostoc was evaluated through a comparison of several specific genes and the extent of DNA methylation. Four of six cyanobacteria were originally cultured from facultative symbioses with higher plants (Gunnera andEncephalartos); these and one free-living isolate had been identified or reputed to beN. punctiforme. No consistent correlation to species or symbiotic history was found from DNA hybridizations to genes coding for phycocyanin (cpcAB), allophycocyanin (apcAB), gas vesicle protein (gvpA1), and dinitrogenase reductase (nifH). One gene (gvpC) was not present, andgvpA1 was a single-copy gene in all strains. The gas vesicle genes were concluded to be potentially useful for broadly characterizingNostoc or at least this subgroup. Incubations ofNostoc genomic DNA with 22 restriction endonucleases indicated a high degree of methylation and similarity of its methylated DNA to that of other heterocystous cyanobacteria. The genetic variation of theNostoc isolates was judged to reflect primarily different soil origins.