Endomembrane systems and cell division in planctomycete bacteria

Members of the phylum Planctomycetes in the domain Bacteria challenge our concepts of the prokaryote-eukaryote dichotomy. All members examined so far share a cell plan which involves internal membrane-bounded compartments. Planctomycete cells may form a model for evolution of eukaryote endomembranes which does not involve endosymbiotic engulfment between members of distinct Domains of life. In Gemmata obscuriglobus and related Gemmata-like isolates, the nucleoid is bounded by a nuclear envelope comprising two closely apposed membranes, comparable to a eukaryotic nuclear envelope. High-pressure freezing preparation for electron microscopy of sectioned cells and light and fluorescence microscopy was applied to study of cell structure and cell division in Gemmata obscuriglobus. Fluorescence microscopy involving DAPI and DiOC6 staining of G. obscuriglobus cells demonstrated that nuclear envelopes are demonstrable in live cells, that translocation of the nucleoid and formation of the bud did not occur simultaneously, and that internal membranes could define two nuclear compartments in some cells. Electron microscopy confirmed deductions from light microscopy concerning nucleoid appearance in the bud, and further revealed that nuclear envelope started to form only after the nucleoid was translocated into the bud. The outer nuclear membrane appeared to originate from an intracytoplasmic membrane. Nuclear envelopes appeared to be lined with ribosomes on both nuclear and cytoplasmic sides. The division cycle of the nucleated planctomycete G. obscuriglobus appears to be a complex process in which chromosomal DNA is transported to the daughter cell bud after initial formation of the bud, and this can be performed repeatedly by a single mother cell. Nucleoids appear to be transferred to buds in naked form and are only later enveloped by a nuclear envelope. Planctomycete endomembrane development may form a model for the way in which endogenous membrane development may have occurred during the origin of the eukaryotic nucleus