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

A unique structure, the conjusome, has been identified and initially characterized in Tetrahymena thermophila. The conjusome appears only during a specific 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-containing 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 of any detectable DNA. The conjusome appears before DNA elimination begins in the developing anlagen and after Pdd1p is found in the parental macronucleus. Transmission electron microscopic observations reveal that the conjusome is not a membrane-bounded structure. The conjusome ranges in size from about 1 microm to sizes approaching 7 microm, depending on its maturity. It is composed of a coarse reticulum of a fibrous, electron dense material, interspersed with apparent background cytoplasm. Our initial characterization does suggest a number of possible functions for what may be a new, transient organelle.