The conjusome: a novel structure in Tetrahymena found only during sexual reorganization

A unique structure, the conjusome, has been identified and initially charac terized in Tetrahymena thermophila. The conjusome appears only during a spe cific phase of conjugation. Immunofluorescence microscopy reveals that the conjusome is strongly labeled by antibodies to the protein Pdd1p. Pdd1p is a chromodomain protein and participates in the formation of chromatin-conta ining structures in developing macronuclear anlagen. Recent studies suggest that Pdd1p is physically associated with the elimination of specific germ- line sequences from developing macronuclei (anlagen) and may play a role in heterochromatin assembly. The conjusome contains Pdd1p, but it is devoid o f any detectable DNA, The conjusome appears before DNA elimination begins i n the developing anlagen and after Pdd1p is found in the parental macronucl eus. Transmission electron microscopic observations reveal that the conjuso me is not a membrane-bounded structure. The conjusome ranges in size from a bout I mu m to sizes approaching 7 mu m, depending on its maturity, It is c omposed of a coarse reticulum of a fibrous, electron dense material, inters persed with apparent background cytoplasm, Our initial characterization doe s suggest a number of possible functions for what may he a new, transient o rganelle.