Cell compartmentalisation in planctomycetes: novel types of structural organisation for the bacterial cell

The organisation of cells of the planctomycete species Pirellula marina, Is osphaera pallida, Gemmata obscuriglobus, Planctomyces mat-is and "Candidatu s Brocadia anammoxidans" was investigated based on ultrastructure derived f rom thin-sections of cryosubstituted cells, freeze-fracture replicas, and i n the case of Gemmata obscuriglobus and Pirellllla marina, computer-aided 3 -D reconstructions from serial sections of cryosubstituted cells. All planc tomycete cells display a peripheral ribosome-free region, termed here the p aryphoplasm, surrounding the perimeter of the cell, and an interior region including any nucleoid regions as well as ribosome-like particles, bounded by a single intracytoplasmic membrane (ICM), and termed the pirellulosome i n Pirellula species. Immunogold labelling and RNase-gold cytochemistry indi cates that in planctomycetes all the cell DNA is contained wholly within th e interior region bounded by the ICM, and the paryphoplasm contains no DNA but at least some of the cell's RNA. The ICM in Isosphaera pallida and Plan ctomyces mat-is is invaginated such that the paryphoplasm forms a major por tion of the cell interior in sections, but in other planctomycetes it remai ns as a peripheral zone. In the anaerobic ammonium-oxidising ("anammox" pro cess) chemoautotroph "Candidatus Brocadia anammoxidans" the interior region bounded by ICM contains a further internal single-membrane-bounded region, the anam-moxosome. In Gemmata obscuriglobus. the interior ICM-bounded regi on contains the nuclear body, a double-membrane-bounded region containing t he cell's nucleoid and all genomic DNA in addition to some RNA. Shared feat ures of cell compartmentalisation in different planctomycetes are consisten t with the monophyletic nature of the planctomycetes as a distinct division of the Bacteria. The shared organisational plan for the planctomycete cell constitutes a new type not known in cells of other bacteria.