Ribosomal RNA transcription during macronuclear development ofTetrahymena thermophila

Conjugation inTetrahymena, as in other ciliated protozoa is both a necessary tool for genetic studies and a potential model system in development. Conjugation is an ordered sequence of events which involves pair formation between two cells of different mating types followed by a precise sequence of nuclear events leading to the establishment of a new recombinant germinal nucleus (micronucleus) and then to the development of a new somatic nucleus (macronnucleus) from the germinal nucleus. The whole process takes about 20 h at 30°C and can be performed with large volumes of cells.The synthesis of ribosomal RNA during macronuclear development was studied in cultures of conjugatingTetrahymena thermophila by following the incorporation of³H-uridine into whole cells and purified ribosomal and pre-ribosomal RNA as well as by measuring bulk-RNA accumulation. In starving cultures and conjugating cultures refed with growth medium during late conjugation, some (background) ribosomal RNA synthesis was detectable 11-12 h after mixing the cells, which is the time when conjugating cells come apart but the macronnucleus is still developing. However, the major burst of rRNA accumulation occurred 13-18 h in refed conjugants.Observation of the conjugating cells by transmission electron microscopy showed that development of nucleoli took place in the macronuclear analagen concomitantly with the major burst of ribosomal RNA synthesis (13-18 h). A nucleolar organization similar to that found in vegetative cells was attained in the macronuclear anlagen 18 h after mixing of the cells.