The pleiotropic effects of ftn2 and ftn6 mutations in cyanobacterium Synechococcus sp. PCC 7942An ultrastructural study

Two cell division mutants (Ftn2 and Ftn6) of the cyanobacterium Synechococcus sp. PCC 7942 were studied using scanning electron microscopy and transmission electron microscopy methods. This included negative staining and ultrathin section analysis. Different morphological and ultrastructural features of mutant cells were identified. Ftn2 and Ftn6 mutants exhibited particularly elongated cells characterized by significantly changed shape in comparison with the wild type. There was irregular bending, curving, spiralization, and bulges as well as cell branching. Elongated mutant cells were able to initiate cytokinesis simultaneously in several division sites which were localized irregularly along the cell. Damaged rigidity of the cell wall was typical of many cells for both mutants. Thylakoids of mutants showed modified arrangement and ultrastructural organization. Carboxysome-like structures without a shell and/or without accurate polyhedral packing protein particles were often detected in the mutants. However, in the case of Ftn2 and Ftn6, the average number of carboxysomes per section was less than in the wild type by a factor of 4 and 2, respectively. These multiple morphological and ultrastructural changes in mutant cells evinced pleiotropic responses which were induced by mutations in cell division genes ftn2 and ftn6. Ultrastructural abnormalities of Ftn2 and Ftn6 mutants were consistent with differences in their proteomes. These results could support the significance of FTN2 and FTN6 proteins for both cyanobacterial cell division and cellular physiology.