RIBOSOMAL-RNA SEQUENCES AND EVOLUTIONARY RELATIONSHIPS AMONG TOXIC AND NONTOXIC CYANOBACTERIA OF THE GENUS MICROCYSTIS

A primary-structure analysis of the 16S rRNA gene was performed with 1 0 strains representing five described and one unidentified species of the genus Microcystis, The phylogenies determined illustrate the evolu tionary affiliations among Microcystis strains, other cyanobacteria, a nd related plastids and bacteria, A cluster of 10 strains that include d hepatotoxic isolates identified as Microcystis aeruginosa formed a m onophyletic group, However, the genus Microcystis appeared to be polyp hyletic and contained two strains that clustered with unicellular cyan obacteria belonging to the genus Synechococcus. The clustering of rela ted Microcystis strains, including strains involved in the production of the cyclic peptide toxin microcystin, was consistent with cell morp hology, gas vacuolation, and the low G+C contents of the genomes. The Microcystis lineage was also distinct from the lineage containing the unicellular genus Synechocystis and the filamentous, heterocyst-formin g genus Nostoc, The secondary structure of a Microcystis 16S rRNA mole cule was determined, and genus-specific sequence signatures were used to design primers that permitted identification of the potentially tox ic cyanobacteria belonging to the genus Microcystis via DNA amplificat ion.